Chemicals of life Overview All cells are made up of a variety of substances some of which organic while others are inorganic. Water forms the largest component and is also a medium for all reactions in the cell. The other substance includes acids, bases, salts, vitamins, carbohydrates, lipids and proteins. There are enzymes and nucleic acids which perform a variety of functions.

General objective

By the end of the topic, the learner should be able to describe the composition, structure, properties and importance of inorganic and organic substances to the life of organism.

Acids, bases and salts Specific objectives

The learner should be able to

a. Describe properties of acids and bases b. Explain the role of acids, bases and salts in maintaining a stable internal environment for

physiological processes.

Definition

An acid is a molecule or ion capable of donating a hydrogen (proton or hydrogen ion H+) in solution. They neutralize alkalis, dissolves some metals with liberation of hydrogen, and turns litmus red; typically, a corrosive or sour-tasting liquid of this kind:

A base is a substance that can accept a hydrogen ion (H+) from another substance

PH In chemistry, pH is a numeric scale used to specify the acidity or basicity (alkalinity) of an aqueous solution. It is roughly the negative of the logarithm to base 10 of the concentration, measured in units of moles per liter, of hydrogen ions. More precisely it is the negative of the logarithm to base 10 of the activity of the hydrogen ion.

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Here are some of the places that acids and bases can be found in the human body:

1. DNA is a complex NUCLEIC ACID found in cells that contains four unique nitrogen-based BASES: adenine, cytosine, guanine and thymine. Differing combinations of these four bases determine the genetic characteristics of all human life.

2. AMINO ACIDS are the building blocks of proteins! The majority of amino acids consist of both a carboxylic acid (-COOH) and an amino (-NH2) functional group attached to the same tetrahedral carbon

atom.

3. LACTIC ACID is an acid produced when sugars are processed by the body. The production of too much lactic acid can cause serious health issues and even death.

4. Vitamin C has the chemical name ASCORBIC ACID and Aspirin and other pain relief remedies are also organic acids. Here is the chemical structure of Aspirin:

5. BLOOD is naturally buffered to a pH of 7.35 – 7.45 in healthy humans. While our bodies naturally work to keep a healthy acid-base balance, problems can arise when acidic or basic compounds are too concentrated or not present in high enough concentrations. The blood's acid-base balance is precisely controlled, because even a minor deviation from the normal range can severely affect many organs. The body uses different mechanisms to control the blood's acid-base balance such as the release of CO2 from the lungs, adjusted kidney function for excretion of substances and controlled buffering of blood via concentrations of the bicarbonate ion.

6. Acidosis and alkalosis are the two abnormalities of acid-base balance. In acidosis, the blood has too much acid (or too little base), resulting in a decrease in blood pH. In alkalosis, the blood has too much base (or too little acid), resulting in an increase in blood pH. Acidosis and alkalosis are not diseases, but rather are conditions that result for a variety of reasons.

Effects of unbalanced pH in the body A person with mild metabolic acidosis may have no symptoms but usually experiences nausea, vomiting, and fatigue. Breathing becomes deeper and slightly faster (as the body

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tries to correct the acidosis by expelling more carbon dioxide). As the acidosis worsens, the person begins to feel extremely weak and drowsy and may feel confused and increasingly nauseated. Eventually, blood pressure can fall, leading to shock, coma, and death.

Systems responsible for maintenance of the acid-base balance

Several systems maintain constant pH. The list below is made according to order when they act:

1) Chemical buffering systems

Buffers react immediately – acute regulation. They minimize pH changes that would otherwise occur in their absence. They do not correct pH deviations, but only serve to reduce the extent of the change that would otherwise occur. These buffers include the bicarbonate buffer system, the phosphate buffer system, and the protein buffer system. Capacity of buffers is not indefinite that is why chemical buffers act only in the short-term. Chemical buffering systems deal with pH deviations in common metabolism.

The bicarbonate buffer system is an acid-base homeostatic mechanism involving the balance of carbonic acid (H2CO3), bicarbonate ion (HCO−3), and carbon dioxide (CO2) in order to maintain pH in the blood and duodenum, among other tissues, to support proper metabolic function. Catalyzed by carbonic anhydrase, carbon dioxide (CO2) reacts with water (H2O) to form carbonic acid (H2CO3), which in turn rapidly dissociates to form a bicarbonate ion (HCO− 3 ) and a hydrogen ion (H+) as shown in the following reaction:

CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3-

As with any buffer system, the pH is balanced by the presence of both a weak acid (for example, H2CO3) and its conjugate base (for example, HCO−3) so that any excess acid or base introduced to the system is neutralized

2) Respiratory system

Respiration reacts in 1-3 minutes. Respiratory system regulates carbon dioxide. Respiration is able to change pCO2 by its elimination or retention. Respiratory centre is in brainstem.

3) Kidneys

Kidneys react in hours-days. In the kidney carbon dioxide react with water to form hydrogen ions (H+) and hydrogen carbonate (HCO3

-) 3

CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3-

When the pH is low the collecting duct and distal convoluted tubules secrete hydrogen ions and retain hydrogen carbonate ions. Therefore, acidic, or that is to say urine with low pH, will be excreted in that case.

Conversely, if blood too alkaline, then the collecting duct can secrete bicarbonate into urine and retain H+ lowering the pH of blood; therefore cause the urine to become more alkaline.

4) Liver

Liver is pivotal organ of the energetic metabolism it also has important influence on the acidbase balance. Liver is the most important tissue where ammonium is detoxified in both (1) urea cycle, and (2) glutamine synthesis. Which one of these fates of ammonium is favored closely depends on status of the acid-base balance:

a) NH4+ → urea + 2 H+ → acidification of the body

CO2 + 2 NH4+ → CO(NH2)2 + 2 H+ + H2O

H+ + HCO3- → H2O + CO2 (consumption of bicarbonate-)

b) NH4+ → glutamine synthesis → H+ is not produced, glutamine is taken up by the

kidneys. In the kidney is H+ excreted as NH4+

Water Specific objectives

The learner should be able to

a. Describe the molecular structure of water b. State the function of water c. Explain the importance of water as solvent d. Relate the water properties to its role in the life of organism e. To test for water f. Determine water content in tissue by using dry weight method.

Water

This is the most abundant compound, typically making up of 60-95% fresh mass of an

organism. Structure of water

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Water is made of hydrogen and oxygen.

2H2(g) + O2 (g) → 2H2O(l)

The H2O molecule is electrically neutral, but polar because the positive and negative charges are not distributed uniformly. This lead to apartial positive charges on hydrogen atoms and a partial negative charge on oxygen atom

In liquid and solids these partial charge attract to form hydrogen bonds

The hydrogens are responsible high melting and boiling point and strong surface tension of water. Uses of water

(i) It makes up structures of organism (ii) It is a solvent (iii) It is a reagent in hydrolysis (iv) Provide support for aquatic organism (v) Is a medium of fertilization through which gametes swim. (vi) Medium for removal of waste products (vii) Temperature control (viii) Hearing and balance as endolymph

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Uses of water to the plants

(i) Aid seed dispersal (ii) Provide support to herbaceous plants (iii) Breaks up the testa of a seed during germination (iv) Reagent in photosynthesis (v) Loss of water through the leaves cools the plant. (vi) Medium of fertilization

Uses of water to the animals

(i) A medium of transport (ii) Evaporation cools the animal (iii) Lubricates joints, eyes, lungs (iv) Constituent of protecting fluids such as tears, mucus.

Adaptation of Water to its function

• Water is the universal solvent:

Polar and ionic substances have an electrostatic charge, so they are attracted to the charges on water molecules and dissolve readily. Non-polar substances, such as oil, do not dissolve in water, as they do not have charged molecules. When a salt dissolves in water, the ions separate and a layer of water molecules form around the ions. These layers prevent ions or polar molecules from clumping together, keeping the particles in solution.

• Water has a high surface tension:

At an interface between air and water, a water molecule on the surface forms hydrogen bonds with other molecules around and below it, but not with air molecules above it. The unequal distribution of bonds produces a force called surface tension; this causes the water surface to contract and form a surprisingly tough film or ‘skin’ enabling small animals like insects to walk over. It also protects blood capillaries of gill filaments from bursting.

• Ice floats on water:

Water is at its most dense at 4oC. When water freezes the hydrogen bonds between the molecules forms a rigid lattice, that holds the molecules further apart then in liquid water. Ice, having expanded when freezing, is less dense than its liquid counterpart and so floats on water. This protests water from further freezing because ice insulates the surface of water.

• Water is adhesive and cohesive:

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Water is ‘wet’ because it sticks to things. This is because its molecules can form hydrogen bonds with other polar substances. This is called adhesion. The attraction between molecules of similar substances is called cohesion. In this way water molecules stick together which allows water to enter and move along very narrow spaces, in a process called capillarity. This enables water ascend in the xylem of a tall tree.

• Important thermal properties:

Water has a high specific heat capacity meaning that it needs to gain a lot of energy to raise its temperature. Conversely it also needs to lose a lot of energy to lower its temperature. Water’s specific heat capacity is 4.2 kJ/g/oC. Thus, this minimizes increase in temperature on hot days.

Water has a high latent heat of vaporization which means a lot of energy is required to evaporate it. When it evaporates, water draws thermal energy out of the surface it’s on, which can be observed in sweating. Thus, sweating cools, the body.

Water also has a high latent heat of fusion meaning that at 0oC water must lose a lot of thermal energy before it freezes, thus liquid water can reach temperatures of down to -10oC before it forms ice. This implies too much heat loss is required to freeze water body.

• Other physical properties of water:

It is transparent to sunlight. This allows animals to seen in water also photosynthesis in aquatic plants.

It has a relatively high density compared to air, this support aquatic animals while swimming.

It is difficult to compress. Thus, support aquatic animals while swimming.

It conducts electricity (when it contains dissolved ions) enable conduction of heat, keeping water hot.

Inorganic compounds/mineral salts

(a) Inorganic salts include those needed in large amounts such as salts of Na+, Mg2+, Cl-, K+ and those needed in trace amounts such as manganese, iron, cobalt, copper, zinc, boron, aluminium, silicon, vanadium, molybdenum and iodine

Functions of inorganic salts

(i) They are components of proteins, e.g. nitrogen, phosphorus and Sulphur. (ii) They are components of tissues; e.g. calcium and phosphorus are components of

bones (iii)They are constituents of enzymes; e.g. copper and iron.

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(iv) They are metabolic activators for example, magnesium activates enzymes in phosphate metabolism.

(v) They are constituent of pigments for example, iron in hemoglobin and magnesium in chlorophyll.

(vi) They are determinants of anion-cationic balance in the cell example Na+, Cl-, and K+.

(vii) They are determinants of osmotic pressure, e.g. Na+, Cl-, and K+.

Carbohydrates

Specific objectives

The learner should be able to:

a. Describe the structure and components of various carbohydrates b. Explain the properties of carbohydrates. c. Explain the functions of carbohydrates in organisms. d. Describe the condensation of carbohydrates e. Describe the hydrolysis of carbohydrates f. Identify the various categories of carbohydrates.

Carbohydrates are food substances with a general formula (CH2O)n where n is natural number.

Classification (i) Monosaccharides e.g. glucose, galactose and fructose

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Ring structures of common monosaccharides.

Sources: honey, fruits Properties -They are sweet -Are soluble in water -They reduce blue copper II ions to red precipitates in alkaline medium.

Testing for reducing sugars

When boiled with Benedict’s or Fehling’s solutions the color changes from blue to green to yellow to oranges ppt.

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(ii) Disaccharides They are made of two simple sugars by condensation Illustration

-glucose -glucose maltose

6CH2OH 6CH2OH 6CH2OH 6CH2OH

- Maltose, lactose and sucrose are sweet and are referred to as sugars. Maltose and lactose are reducing sugars whereas sucrose in not and referred to as non-reducing

sugar. Disaccharides Composition Source Maltose glucose + glucose malt Lactose Glucose + galactose milk Sucrose Glucose + fructose Sugar cane

Sugar beets Cellubiose Glucose + glucose wood

Testing for non-reducing sugars 1. When boiled with Benedict’s solution or Fehling’s’ solution, the color remains blue.

2. When boiled with HCl, the solution cooled, neutralized by NaOH, boiled Benedict’s

or Fehling’s solution, the color changes from green, to yellow to orange. HCl hydrolyses non-reducing sugars NaOH neutralizes the excess acid because Benedict’s or Fehling’s solution does not work in acidic medium.

Polysaccharide (CH2O)n 10

These are made of very many mono saccharides per unit molecule e.g. starch and cellulose

Testing for starch

It changes the color of iodine black or blue.

Functions of carbohydrates

1. Glucose, galactose and fructose are oxidized to release energy in he body 2. Glyceraldehyde is an intermediate molecule in photosynthesis. 3. Ribose is component nucleotides. 4. Sucrose is a form in which carbohydrates are transported in plants 5. Lactose is a source of energy in milk 6. Storage of energy (starch in plants, glycogen in animals, inulin is some plants like

Dahlia) 7. Formation of cellular structures (cellulose in plant cell walls, chitin is

Lipids

Specific objectives

The learner should be able to:

a. Describe the structure and components of lipid molecules. b. State the properties of lipids. c. Explain the functions of lipids in organism. d. Describe the structure of steroids e. Explain the effects of lipids and steroids to organisms f. Describe the condensation of fatty acids and glycerol to form lipids g. Describe the hydrolysis of lipids to fatty acids and glycerol h. Compare waxes and lipids i. State importance of cholesterol in organism j. To carry out tests to identify lipids on food and extracts.

(a) Lipids These include natural fats and oil. Fats are solids at room temperature while oils are liquids Structure of lipids Lipids are ester of glycerol and fatty acids

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Table of nature and occurrence of some fatty acid Name of fatty acid formula Saturated/unsaturated occurrence Butyric acid C3H7COOH Saturated Butter fat Linoleic C17CH31COOH unsaturated Linseed oil Oleic C17H33COOH unsaturated All fats Palmitic C15H31COOH Saturated Animal and vegetable

fats Stearic C17H35COOH Saturated Animal and vegetable

fats arachidic C19H39COOH Saturated Peanut Ceritic C25H51COOH Saturated Wool oil

Uses of lipids Structural functions -Make up cell membrane

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-Protection: lipids are constituents of the waxy cuticle of plants and insects - Lipids are water repellant thus prevent water loss from or entry into an animal skin - Their spongy nature protects delicate organs as shock absorbers.

-Being bad conductors, they reduce water loss from the body when deposited beneath the skin for insulation

-Storage ; they are better storage compounds than carbohydrates due high calories value, due to high hydrogen content, they are light, insoluble in water, compact to fit in a small volume and are easily used when required.

Physiological functions

-Source of metabolic water -Store fat soluble vitamins (ADEK) -Source of metabolic water -Raw materials for hormones

Testing for lipids

a. They form a translucent mark on paper that does not disappear when the paper is dried on a flame.

b. Emulsion test When 2cm3 of fats or oil are dissolved in 2cm3 of absolute ethanol followed by water, a white cloudy suspension is formed.

c. Sudan III When a few drops of Sudan III are added to a mixture of 2cm3 of water and 2 cm3 of oil and shaken, a red stained oil layer separates out. Phospholipids. These are lipids in which of the fatty acid is replaced by phosphoric acid. The phosphoric acid attracts water (hydrophilic) whereas the rest of the group repel water (hydrophobic). This property enables them to form a cell membrane. Waxes Waxes are formed by combination with an alcohol other than glycerol. This alcohol is much larger than glycerol, and therefore waxes have a more complex chemical structure. The main role of waxes is waterproofing plants and animals although, they form storage compounds in a few organisms, e.g., castor oil and in fish. Advantage of storing fats over carbohydrate

(i) Has high energy content than carbohydrates (ii) It is lighter (iii) It is compact and requires less space (iv) It is a raw material for hormones

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(v) Insoluble in water that they have low osmotic value

Proteins

Specific objectives

The learner should be able to:

b. Describe the structure and composition of proteins. c. Describe the properties of proteins d. Explain the importance of proteins e. Explain the functions of proteins in organisms f. Describe the condensation of amino acids to form proteins. g. Describe the hydrolysis of proteins to amino acids h. The effect of heat/temperature changes on proteins i. To carry out test and identify proteins on food /extracts.

Proteins These are classified into two groups (i) Structural proteins: insoluble proteins that make up body structures like

bones and muscles. Fibrinogen is a soluble structure protein used in blood clotting.

(ii) Globular proteins are soluble proteins such as enzymes, antibodies, hormones and so on.

Composition of proteins

The basic unit of proteins are amino acids

R =alkyl groupH

There are about 22 different amino acids in the body of which isoleucine, leucine, methionine, phenylamine, proline, threonine and valine cannot be synthesized in human body and they are referred to as essential amino acids. Amino acids unite to form proteins through formation of peptide bonds by a condensation reaction in which a water molecule is eliminated.

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H

OR

H

HCCN

Uses of proteins

(i) Make up structures, e.g., collagen make up connective tissues. (ii) Make up enzymes such as catalyze and amylase. (iii) Are constituent of hormone such as insulin (iv) Are constituents of antibodies that protect the body from foreign particles. (v) Make up muscles such as myosin and actin (vi) They are storage food e.g. egg white (vii) Constitute toxins such as snake venom for protection.

Testing for proteins

a. They coagulate on heating b. They coagulate on addition of Melon’s reagent and on heating they form a pink

precipitate. c. They form a purple color when mixed with equal amount of NaOH followed by 3

drops of copper sulphate solution.

Vitamins Specific objects

The learner should be able to:

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O2H

H

O2R

H

H

HCCN

HO

O1R

H

H

O2HCCN

dnob editpeP H

O2R

H

H

CCN

O1R

H

H

O2HCCN

a. State the type of vitamins i.e., water soluble and fat soluble; essential and nonessential. b. State the importance of vitamins in organism c. Test for vitamin C. d. Demonstrate effect of over boiling vegetables e. Demonstrate the effects of storage on the quality of fresh food.

Vitamins are complex organic compounds present in very small quantities in natural food which are essential for good healthy body and maintains its normal metabolic activities. Some vitamins are fat soluble (ADEK) while others are not.

Vitamin C / ascorbic acid

Sources: citrus fruits, green vegetables, potatoes, tomatoes, etc.

Function of vitamin C

Concerned with the metabolism of connective tissues and the production of strong skin.

Deficient disease : anemia and scurvy: the germs bleed, wounds fail to heal.

Testing for Vitamin C

It decolorizes DCPIP.

Other vitamins and their deficient diseases Vitamin A Night blindness K Delayed clotting E Reduced fertility in rats

Functions of vitamins

-protect the body against diseases - formation of coenzyme that facilitate enzyme reactions -blood clotting -components of visual pigment

Revision questions

1. Lack of iodine in the diet causes cretinism because iodine A. Controls metabolism B. Is essential in the formation of metabolic enzymes C. Influence growth of bones D. Is required for synthesis of thyroxine

2. In the body, proteins combine with acids or bases depending on the

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A. Temperature of the medium B. Hydrogen ion concentration in the medium C. Number of solvent molecules present in the medium D. Number of amino acid molecules in the protein

3. Starch, glycogen and cellulose are all composed of

A. α-glucose B. β-glucose C. monosaccharides D. polysaccharides

4. Some amino acids are known as essential because they are

A. more important in the body metabolism than other B. not made by the body C. contained in first class proteins D. required in larger amounts than others.

5. Which of the following sugars is not reducing?

A. Maltose B. Fructose C. Galactose D. Sucrose

6. Among the following compounds, one cannot be hydrolyzed is

A. Glycogen B. Galactose C. Lactose D. Maltose

7. Which one of the following is the correct formula of a polysaccharide?

A. (C6H10O5)n B. (CH2O)n

C. (C6H12O6)n

D. C12H22O11)n 8. Which one of the following statements is true of essential fatty acids? They

A. They are the most required lipids in the body B. Are required in the body in large quantities C. Cannot be synthesized in the body D. Are most abundant in animal tissues

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9. Which one of the following properties of water facilitates its efficient transportation of glucose?

A. Forms hydrogen bonds with other molecules B. Has high surface tension C. Has low freezing points D. Has high boiling point

10. Which of the following vitamins is water soluble?

A. A B. B C. D D. C

11. Lack of iodine in diet causes cretinism because iodine

A. Controls metabolism B. Is essential for formation of metabolic enzymes C. Influence growth of bones D. Is required for synthesis of thyroxine

12. A property of water that makes it suitable component of a hydrostatic skeleton is it

A. High density B. High surface tension C. Low viscosity D. Incompressibility

13. Aquatic organism survives under solidified water body because

A. Water solidifies from bottom to top of lakes B. Ice is less dense than water at 40C C. Cold water is more dense than hot water and falls to the bottom D. Warm water floats on top of cold water

14. In the body, proteins combine with acids or bases depending on the

A. Temperature of the medium B. Hydrogen ion concentration in the medium C. Number of solvent molecules present in the medium D. Number of amino acid molecules in the protein

15. Which one of the following is not a fibrous protein

A. Keratin B. Globulin

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C. Elastin D. Collagen

16. Sucrose is a non-reducing sugar because it

A. It is not fully digested B. It lacks reducing groups C. Is a disaccharide molecule D. Is a ketose sugar

17. Which one of the following is a fibrous soluble protein?

A. Myosin B. Collagen C. Myoglobin D. Fibrinogen

18. A green plant develops yellow leaves as a result of being deficient in

A. Magnesium B. Manganese C. Nitrogen D. Calcium

19. Which of the following elements is not required by plants?

A. copper B. iodine C. iron D. zinc

20. In the blood plasma, proteins can act as bases or acids depending on the

A. Temperature of the medium B. Hydrogen ion concentration of the medium C. Nature of the protein D. Concentration of the solute in the plasma

21. Which of the following is the function of manganese in the human body?

A. Essential for formation of erythrocytes B. Activate enzymes C. Acts as growth factor in bone development D. Utilized as a component of bone and teeth

22. Which of the following substances consists of globular proteins

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A. Enzymes B. Keratin C. Elastin D. collagen

23. Which one of the following symptoms is likely to be caused by magnesium deficiency in plants?

A. Yellow leaves and stunted growth B. Poor root growth C. Weak stems D. Yell spotted leaves

24. Which one of the following is not a function of globular proteins in the body?

A. Acts as buffers in blood plasma B. Form structural proteins C. Are vital constituents of plasma membrane D. Form enzymes

25. The following structural formula is for an amino acid in solution

H-O

NH3

O H

A substance was added to this solution and the structure of the amino acid molecule changed to

O H

What substance was added and what effect would this have had on the final pH of the solution?

A. Salt added, pH unchanged B. Acid added, pH lowered C. Acid added, pH unchanged D. Base added, pH higher

26. We need to eat iodized salt in order to

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OH

3HN

H

CC

CC

A. Prevent obesity B. Get a balanced diet C. Improve vision D. Avoid goiter

27. Water has comparatively high surface tension and boiling point in relation to other substances of similar sized molecules because its molecules are

A. doubly bonded B. polar C. ionic D. covalent

28. Evaporation of water from the body surface causes cooling because water has a high

A. Latent heat of vaporization B. Latent heat of fusion C. Boiling point D. Specific heat capacity

29. The complexity and variety of organic molecules is due to the ability of the carbon atom to

A. form covalent and ionic bond B. form covalent bonds in three dimensions C. form strong chemical bonds D. bonds with very many other elements

30. When a lipid is combined with a phosphate group, it becomes

A. saturated B. a complex molecule C. water soluble D. amphoteric

31. A property of water which facilitates its efficient transportation of glucose

A. ability to form hydrogen bonds with other molecules B. high surface tension C. low freezing point D. high boiling point

32. When a lipid is combined with a phosphate group, it becomes A. saturated.

B. a complex molecule.

C. water soluble.

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D. amphoteric.

33. Water has a comparatively high surface tension and boiling point in relation to other substances of similar sized molecules because its molecules are

A. doubly bonded. B. polar. C. ionic. D. covalent.

34. Starch and glycogen are suitable storage molecules because they

A. are large in size which makes them less soluble in water

B. are chemistry reactive in the cell

C. can easily be hydrolyzed

D. exert an osmotic pressure in the cell

35. The high heat capacity of water has biological importance of

A. minimizing temperature changes in animal fluids

B. cooling animals

C. Preventing freezing of cell contents

D. controlling heat loss in animals

Paper 1 section 1

36. Fat and glycogen are energy storage compounds in animals (a) Compare the suitability of the two substances as storage compounds (4marks) (b) State advantage of storing fat over glycogen (3marks) (c) Why is glycogen more suitable energy compound in muscle than fat? (3marks)

37. Using the structural formula H

OH

OH

OH

H For glycerol, and molecular formula CH3(CH2)nCOOH for a fatty acid show the formation of triglyceride from fatty acids and glycerol. (2marks)

(b) What properties do lipids possess as storage food substances? (2marks)

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H

H

C

C

C

H

(c) Outline the structural and physiological functions of lipids in living organisms. (i) Structural (3marks) (ii) Physiological (2marks)

38. (2000/1/43) (a) state three ways in which water has similar functions in both plants and

animals. (3marks) (b) Give two ways, in each case, in which flowering plants minimize water loss through

(i) behavioral means (4marks)

(ii) physiological means (4marks)

39. Explain how the structure of proteins enable them to form body tissues and structures

(4marks)

40. (a) Describe the significance of physical properties of water to organism. (12marks) (b) Explain why lipids are better storage material in animals than carbohydrates (8marks)

41. (a) Giving examples, describe the use of nitrogen to plant and animal bodies (b) how is the concentration of nitrogen maintained at constant level in nature?

42. Distinguish between the following

(a) Monosaccharide and polysaccharide (05marks)

(b) Starch and cellulose (04marks)

(c) Saturated and unsaturated fats (05marks)

(d) Globular and fibrous proteins (06marks)

43. Relate the properties of water to its biological importance. (20 marks0 Answers to objective questions

1 D 6 B 11 D 16 D 21 B/C 26 D 31 A 2 B 7 A 12 D 17 D 22 A 27 B 32 C 3 C 8 C 13 B 18 A 23 D 28 A 33 B 4 B 9 A 14 B 19 B 24 B 29 D 34 C 5 D 10 D 15 B 20 B 25 C 30 C 35

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36. Solution

Similarities

- They are both compactly arranged to take up little space.

- they are both less soluble in water and little or none can be lost in solution Differences

Fat Glycogen

-Has a high calorific valve.

-Has a higher hydrogen- oxygen content and

can yield more metabolic water.

Others

- Has less weight and keeps body weight to a minimum which allows buoyancy

-Has a lower calorific valve.

-Has a lower hydrogen- oxygen content and

yield less metabolic water

-Is heavier and can lead to overweight.

Others

Fat is completely insoluble in water and more can be lost in solution. Also. It prevents

desiccation.

Fat forms an insulating layer under the skin that helps in temperature regulation.

Fat has a low density hence provide buoyancy in aquatic animals.

Muscles have a high content of glycolytic enzymes which readily breakdown glycogen to

utilizable glucose.

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The glycolytic enzymes, glycogen phosphorylase, has an allosteric site for binding AMP.

When content is low in muscle, AMP content rises and activates this glycolytic enzyme

which readily break down glycogen to glucose that can be used by the muscle.

Break down of fat to free fatty acids which can be utilized by the muscle is a slow process

because it is hormone- mediated.

41. (a) use of nitrogen

(i) protein synthesis

(ii) production of enzymes

(iii) production of muscles and tissues

(iv) production of nucleic acid

(b) nitrogen cycle

37 (a)

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(c) Properties of fats as storage compounds i. Has high energy content than

carbohydrates ii. It is lighter iii. It is compact and requires less space iv. It is a raw material for hormones v. Insoluble in water that they have low osmotic value

(d) (i) Structural functions -Make up cell membrane -Protection: lipids are constituents of the waxy cuticle of plants and insects - Lipids are water repellant thus prevent water loss from or entry into an animal skin - Their spongy nature protects delicate organs as shock absorbers.

-Being bad conductors, they reduce water loss from the body when deposited beneath the skin for insulation

-Storage ; they are better storage compounds than carbohydrates due high calories value, due to high hydrogen content, they are light, insoluble in water, compact to fit in a small volume and are easily used when required.

(ii) Physiological functions

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-Source of metabolic water -Store fat soluble vitamins (ADEK) -Source of metabolic water -Raw materials for hormones

38 (a) similar functions of water in plants and animals

(v) It is a solvent and medium for transport (vi) It is a medium of fertilization (vii) Evaporation cools the body (viii) Provides support to aquatic organism (ix) Component of the cell (x) A reagent in hydrolytic reaction (xi) A medium in which biological reaction occur.

(b) (i)Minimizing water by behavioral means -Folding or rolling of leaves on a hot day -Reduction of number of stomata -Storage of water in leaves -Sunken stomata

(ii) minimizing water loss by physiological means

-Shedding leaves in hot season -Reversal of normal stomata rhythm -Thickening of waxy cuticle

39. How structure of proteins enable them to form body tissues and structures

(a) Globular - These tend to form ball-like structures where hydrophobic parts are towards the centre and hydrophilic are towards the edges, which makes them water soluble. They usually have metabolic roles, for example: enzymes in all organisms, plasma proteins and antibodies in mammals.

Fibrous - They proteins form long fibres and mostly consist of repeated sequences of amino acids which are insoluble in water. They usually have structural roles, such as: Collagen in bone and cartilage, Keratin in fingernails and hair.

42. (a) Monosaccharide and polysaccharide (05marks) Monosaccharide Polysaccharide Made of 3 to 6 carbon atoms Many carbon atoms Composed of one sugar unit Composed of many sugar units Low molecular mass High molecular mass Soluble Insoluble Used for respiration Used for storage

27

Low energy content High energy content (a) Starch and cellulose (04marks) Starch cellulose Polymer of alpha glucose Polymer of beta glucose

(b) Saturated and unsaturated fats (05marks) Saturated fatty acids lack double bonds between the individual carbon atoms, while in unsaturated fatty acids there is at least one double bond in the fatty acid chain. Saturated fats tend to be solid at room temperature and from animal sources, while unsaturated fats are usually liquid and from plant sources.

(c) Globular and fibrous proteins (06marks)

Fibrous proteins are insoluble in water, weak acids and weak bases but soluble in strong acids and alkalis whereas globular proteins are soluble in water, acids and bases.

Fibrous proteins are highly resistant to digestion by enzymes and are extremely tensile

NATURE OF THE GENE AND PROTEIN SYNTHESIS Specific objectives

(i) Describe the composition of chromosomes and structure of nucleotides (ii) Describe the structure of DNA and RNA (iii) Distinguish between DNA and RNA (iv) Explain the Watson Crick hypothesis of the nature of DNA (v) Explain DNA replication. (vi) Describe the nature of the gene (vii) Describe the structure of the genetic code (viii) Describe formation of RNA (ix) State the role of DNA and RNA in protein synthesis. Describe protein synthesis.

Gene This is a segment of DNA that tell the body how to produce specific proteins – contain the genetic information that is passed from parents to their offspring. These genes are found in structures called chromosomes, which are passed to the embryo during conception. Because the DNA passed from one human to another helps determine gender and physical characteristics, this nucleic acid is necessary for the survival of the species.

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Evidences that DNA is the hereditary material

1. DNA is a stable molecule 2. The amount of DNA in a given species is constant for all cells. 3. Mutation or changes in the composition of DNA alter the organisms‟ characteristics. 4. Purified DNA rather than proteins from dead virulent bacterium Pneumococcus strain which

causes pneumonia was shown to transform the non-virulent form into a virulent form. The ability of DNA to transform nonvirulent form was stopped by addition of an enzyme that breaks DNA in the dead virulent form before DNA was purified.

5. DNA rather than proteins of bacteriophage (T2 phage) that infect E. coli was shown to be the hereditary material because it enabled the E. coli to synthesize the new T2 phages viruses.

Nucleic acids This is a form of genetic material in all living organisms including the simplest viruses. Nucleic acids are polymers made of subunits called nucleotides.

Types

1. Deoxyribonucleic acid (DNA) is found in the nucleus 2. Ribonucleic acid (RNA) is found in both nucleus and cytoplasm. A nucleotide is made up of three molecules:

(i) Phosphate group (ii) Pentose sugar – either Deoxyribose (in DNA) or Ribose (in RNA) (iii) Nitrogen base – any purine (Adenine, Guanine) or pyrimidine (Cytosine and either

Thymine in DNA or Uracil in RNA) Structure of nucleotide

It is made of phosphoric acid, sugar and an organic base.

(a) Phosphoric acid

OH

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OH

(b) Sugar: the pentose sugar in RNA is ribose while that of DNA is deoxyribose sugar. Deoxyribose sugar lacks an oxygen atom on the second carbon atom

5CH2OH 5CH2OH

H OH H OHOH OH OH H

Ribose sugar Deoxyribose sugar

(c) Organic Bases: DNA contains four different organic bases; adenine (A), guanine (G) cytosine (C) and thymine (T). RNA also contains adenine (A), guanine (G), cytosine (C) and Uracil (U). all these bases are ring compounds, made of carbon and nitrogen.(i) Pyrimidines (cytosine, uracil and thymine; CUT) have a six-membered ring.

H

NH2 O

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P

4

3 2

1

H

HH

CC

O

CC 4

3 2

1

H

HH

CC

O

CC

N

C

CC

C

N

O

H3HC

O H

NC

C

CCN

H

H

H

NC

C

CCN

H

OO

H

H

NC

C

CCN

OHO

H H

Cytosine Thymine Uracil

(ii) Purine (Guanine and Adenine (GA) have a two membered ring)

NH2 O

H H

Adenine Guanine Nucleoside forms when a pentose sugar joins an organic base by condensation reaction (a water molecule is lost).

Nucleotide forms when a nucleoside (pentose sugar + organic base) joins a phosphate by loss of second water molecule.

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2HN

HH C

N

N

NC

C

C CN

HH C

N

N

NC

C

C CN

The sugar-phosphate-sugar backbone is formed when the 3‟ carbon on one sugar joins to the 5‟ carbon on the next sugar by phosphodiester bonds repeatedly to form a polynucleotide (long chain of nucleotides) with organic bases protruding sideways from sugars.

Nitrogen base

DNA structure according to Watson and Crick

1. DNA consist of two polynucleotide strands. 2. The polynucleotide stands are antiparallel (face in opposite directions) i.e. one runs from 3‟

to 5‟ direction while another one runs from 5‟ to 3‟ direction. 3. A DNA nucleotide is made up of three molecules:

(i) Phosphate group (ii) Deoxyribose sugar (iii) Nitrogen base - either adenine (A), guanine (G), thymine (T) or cytosine (C).

4. Untwisted DNA is ladder-like, in which the sugar-phosphate backbones represent the handrails while the nitrogen base pairs represent the rungs.

5. Twisted DNA forms a double helix of major and minor grooves. 6. The sugar-phosphate-sugar backbone is held by covalent phosphodiester bonds, while the

nitrogen bases from the two strands form weak hydrogen bonds by complimentary base pairing i.e. A with T, C with G.

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ADAPTATIONS OF DNA

(i) Sugar-phosphate backbone is held together by strong covalent phosphodiester bonds to provide stability.

(ii) The two sugar-phosphate backbones are antiparallel which enables purine and pyrimidine nitrogen bases to project towards each other for complimentary pairing.

(iii) Sugar-phosphate backbones are two / it is double stranded to provide stability. (iv) The two sugar-phosphate backbones form a double helix to protect bases/hydrogen

bonds. (v) Long/large molecule for storage of much information. (vi) Double helical structure makes the molecule compact to fit in the nucleus. (vii) Base sequence allows information to be stored. (viii) Double stranded for replication to occur semi- conservatively/ strands can act as

templates. (ix) There are many hydrogen bonds which increase stability of DNA molecule. (x) There is complementary base pairing / A-T and G-C for accurate replication/identical

copies can be made; (xi) Weak hydrogen bonds enable unzipping /separation of strands to occur readily.

Theories of DNA replication DNA replication is the process by which the parent DNA molecule makes another copy of itself.

1. Fragmentation hypothesis (Dispersive hypothesis) The parent DNA molecule breaks into segments and new nucleotides fill in the gaps precisely.

2. Conservative hypothesis This suggests that the DNA strands remain intact but in some way initiate the synthesis of new but exact copies of DNA to the parent DNA.

3. Semi-conservative hypothesis The parent DNA molecule separates into its two component strands, each of which acts as a template for the formation of a new complementary strand. The two daughter molecules therefore contain half the parent DNA and half new DNA. The semi conservative hypothesis was shown to be the true mechanism by the work of Meselsohn and Stahl (1958) in their experiment on bacterium E.coli using radioactive 15N.

Illustration of three possible theories of DNA replication

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Old strand

New strand

Dispersive conservative Semi-conservative Necessities of DNA replication

1. Free nucleotides to bond with complementary bases on the separated DNA strands. 2. Energy source in form of ATP 3. Complementary DNA strand 4. Enzymes such as DNA polymerase, DNA helicase and DNA lingaze.

Steps of DNA Replication

1. DNA unwinds and then split open by a Helicase enzyme to expose the bases on either strand. The initiation point is a place rich in A-T probably because these are held by two hydrogen bonds as oppose to the three hydrogen bonds between C and G. The structure that is created is known as "Replication Fork".

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Parent DNA

2. An enzyme RNA Primase bind to DNA at the initiation point and attracts the first nucleotide of the 3‟-5‟ strand

3. The elongation process involves the alignment of complementary nucleotides against the

bases on open stands and then joined into new strands by the DNA polymerase. However, it occurs differently for the 5'-3' and 3'-5' template.

a. 5'-3' Template: The 3'-5' proceeding daughter strand -that uses a 5'-3' template- is called leading strand because DNA Polymerase ä can "read" the template and continuously adds nucleotides (complementary to the nucleotides of the template, for example Adenine opposite to Thymine etc.).

b. 3'-5'Template: The 3'-5' template cannot be "read" by DNA Polymerase ä. The

replication of this template is complicated and the new strand is called lagging strand. In the lagging strand the RNA Primase adds more RNA Primers. DNA polymerase å reads the template and lengthens the bursts. The gap between two RNA primers is called "Okazaki Fragments". The RNA Primers are necessary for DNA Polymerase å to bind Nucleotides to the 3'-5‟ end of them. The daughter strand is elongated with the binding of more DNA nucleotides.

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4. In the lagging strand the DNA Pol I -exonuclease- reads the fragments and removes the RNA

Primers. The gaps are closed with the action of DNA Polymerase (adds complementary nucleotides to the gaps) and DNA Ligase (adds phosphate in the remaining gaps of the phosphate - sugar backbone).

Each new double helix is consisted of one old and one new chain. This is what we call semiconservative replication.

5. The last step of DNA Replication is the Termination. This process happens when the DNA

Polymerase reaches to an end of the strands.

6. The DNA Replication is not completed before a mechanism of repair fixes possible errors caused during the replication. Enzymes like nucleases remove the wrong nucleotides and the DNA Polymerase fills the gaps.

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General structure of RNA

a RNA molecules are small/short, single stranded (rRNA and mRNA) but may be coiled around such that bases of the same strand pair with each other. b RNA nucleotide is made up of three molecules:

(i) Phosphate group (ii) Ribose sugar (iii)Nitrogen base - either adenine (A), guanine (G), cytosine (C) or uracil (U) c The

sugar-phosphate-sugar backbone is held by covalent phosphodiester bonds. d RNA occurs in three types whose sizes, shapes, amounts/abundance and roles vary:

Types of RNA

1. Ribosomal RNA (rRNA)

Makes the highest percentage of RNA. It is a single strand with a double helical region.

Its function are:

a. integral component to ribosome structure i.e when removed from ribosome, ribosomes structure collapses

b. it serve as a temperate partner for synthesis of ribosomal proteins

2. Messenger RNA (mRNA) Forms 3-5% of the total RNA in a cell. mRNA carries coded information from DNA to ribosomes in the cytoplasm

3. tRNA is an adaptor molecule composed of RNA, typically 76 to 90 nucleotides in length,[2] that serves as the physical link between the mRNA and the amino acid sequence of proteins. tRNA does this by carrying an amino acid to the protein synthetic machinery of a cell (ribosome) as directed by a three-nucleotide sequence (codon) in a messenger RNA (mRNA). As such, tRNAs are a necessary component of translation, the biological synthesis of new proteins in accordance with the genetic code.

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Comparison of DNA and RNA Similarities Both: (1) are polymers of nucleotides (2) carry genetic information (3) have same purine bases adenine and guanine plus pyrimidine bases cytosine (4) originate from the nucleus Differences

Aspect Deoxyribonucleic Acid (DNA) Ribonucleic Acid (RNA)

Function Stores genetic information for a long time and transmits it.

Transfers genetic code needed for the creation of proteins from the nucleus to the ribosome.

Structure

(i) Double-stranded. (ii) High molecular mass (iii) Pentose sugar is deoxyribose (iv) Quantity is fixed in a cell

(i) Single-stranded. (ii) Low molecular mass (iii) Pentose sugar is ribose (iv) Quantity is variable

Base Pairing

(v) Organic bases are guanine, adenine, cytosine, and thymine

(v) Organic bases are guanine, adenine, cytosine, and uracil

Location (vi) Much of DNA is in the nucleus of a cell,

little in mitochondria and chloroplasts. (vi) Much of RNA is in the cytoplasm, little in

the nucleus.

Stability (vii) stable (vii) less stable

Propagation (xiii) DNA is self-replicating. (xiii) RNA is synthesized from DNA when

needed.

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Unique Features

(xiv) DNA is protected in the nucleus, as it is tightly packed.

(xiv) RNA strands are continually made, broken down and reused.

Types (xvii) Only two types: intra nuclear and extra nuclear DNA

(xviii) Three different types: mRNA, tRNA and rRNA

The central dogma of molecular biology It states that DNA makes RNA makes proteins PROTEIN SYNTHESIS

Protein synthesis is the process by which individual cells construct proteins. Protein synthesis occurs in separate but interrelated steps as follows:

1. Transcription Transcription is the process whereby the DNA code of a gene is copied to make messenger RNA (mRNA).

Importance of transcription (i) DNA is too large to fit through the nuclear pores, yet mRNA being small can readily exit

the nucleus. (ii) DNA contains many codes that aren‟t always needed at a given time, so m-RNA only

carries that code needed to make specific proteins out of the nucleus to the ribosome. Stages in transcription

1. A specific region of DNA called a cistron unwinds to expose bases on each strand. 2. Each base on one strand attracts its complementary RNA nucleotide, for example. a dree

guanine base on DNA attracts an RNA nucleotide with cytosine and adenine attracts uracil. 3. The enzyme RNA polymerase moves along the DNA adding one complementary RNA

nucleotide at a time to a newly unwound portion of DNA. 4. The region of base pairing between the DNA and RNA is only around 12 base pairs at a time

as the DNA helix reforms behind the RNA polymerase. 5. The DNA acts as a template against which mRNA is constructed.

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Translation This describes how the genetic code is converted into amino acids.

It occurs in three stages

The beginning step is called initiation, a middle step, called elongation, and a final step, called termination.

Initiation

In initiation, mRNA is attached to tRNA, which is attached to the specified amino acid.

During initiation, the mRNA, the tRNA, and the first amino acid all come together within the ribosome. The mRNA strand remains continuous, but the true initiation point is the start codon, AUG that specifies amino acid methionine. So, methionine is first the amino acid that is brought into the ribosome.

40

Elongation The next step makes up the bulk of translation. It's called elongation, and it's the addition of amino acids by the formation of peptide bonds. Elongation is just what it sounds like: a chain of amino acids grows longer and longer as more amino acids are added on. This will eventually create the polypeptide.

The mRNA shifts a little through the ribosome so that the next codon exposed to attract the next tRNA with a matching anticodon. A peptide bond forms between the two amino acids on the tRNA . one codon is exposed a time to attract activated tRNA with appropriate anticodon until all the mRNA has moved through the ribosome and each case a peptide bond is formed between the amino acid on the arriving tRNA and the peptide chain already formed.

Termination occurs when all the codons on mRNA are read. The ribosome reaches one or more stop codons on mRNA (UAA, UAG, UGA). The ribosome detaches from the mRNA and splits into its small and large sub-units, while the new protein floats away Note:

1. Several ribosomes can attach to a molecule of mRNA (to form polysomes/polyribosomes) one after another so that several proteins of the same type can be made from one mRNA at the same time.

2. Newly synthesized proteins are packaged and sent to Golgi complex for modification/processing. This is called post translation processing of the protein.

Comparison between DNA replication and transcription Similarities Both: (1) involve unwinding the helix (2) involve separating the two strands (3) involve breaking hydrogen bonds between bases (4) involve complementary base pairing

41

(5) involve C pairing with G (6) work in a 5` to 3` direction (7) involve linking/ polymerization of nucleotides (8) DNA or RNA polymerase require a start signal Differences

DNA replication Transcription Involves DNA nucleotides, where the pentose sugar is deoxyribose, and the base adenine pairs with thymine

Involves RNA nucleotides where the pentose sugar is ribose, and the base adenine pairs with uracil

Both strands are copied Only one strand copied not both

Ligase enzyme / no Okazaki fragments are involved No ligase enzyme / no Okazaki fragments

Has multiple starting points Has only one starting point

replication gives two DNA molecules whilst transcription gives mRNA Compare DNA transcription with translation Both: (1) Occur in 5‟ to 3‟ direction (2) Require ATP

Differences (i) DNA is transcribed while mRNA is translated (ii) Transcription produces RNA while translation produces polypeptides/ protein (iii) RNA polymerase for transcription while ribosomes for translation/ ribosomes in translation

only (iv) Transcription occurs in the nucleus (of eukaryotes) while translation occurs in the

cytoplasm/ at ER (v) tRNA is needed for translation but not transcription Explain briefly the advantages and disadvantages of the universality of the genetic code to humans. (i) Genetic material can be transferred between species/ between humans (ii) One species could use a useful gene from another species (iii) Bacteria/ yeasts can be genetically engineered to make a useful product (1) Viruses can invade cells and take over their genetic apparatus e.g. HIV (2) Viruses cause disease

GENETIC CODE

The genetic code is the set of rules by which information encoded in genetic material (DNA or RNA sequences) is translated into proteins (amino acid sequences) by living cells.

42

THE GENETIC CODE CHART / TABLE

MOST IMPORTANT PROPERTIES OF GENETIC CODE

1. The code is a triplet codon

The nucleotides of mRNA are arranged as a linear sequence of codons, each codon consisting of three successive nitrogenous bases, i.e., the code is a triplet codon.

2. The code is non-overlapping In translating mRNA molecules, the codons do not

overlap but are “read” sequentially.

3. The code is comma less

This means that no codon is reserved for punctuations. After one amino acid is coded, the second amino acid will be automatically, coded by the next three letters and that no letters are wasted as the punctuation marks.

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4. The code is non-ambiguous

A particular codon will always code for the same amino acid. The same codon shall never code for two different amino acids.

5. The code has polarity The code is always read in a fixed direction, i.e., in the 5‟→3 direction. ′

6. The code is degenerate More than one codon may specify the same amino acid; For example, except for tryptophan and methionine, which have a single codon each, all other 18 amino acids have more than one codon.

Biological advantages of degeneracy (i) It permits essentially the same complement of enzymes and other proteins to be specified

by microorganisms varying widely in their DNA base composition. (ii) It provides a mechanism of minimizing mutational lethality. E.g. Substitution of the third

base-U in GUU (for valine) with C/A/G does not change the amino acid coded for. 7. Some codes are start codons

In most organisms, AUG codon is the start or initiation codon, i.e., the polypeptide chain starts either with methionine (eukaryotes) or N-formylmethionine (prokaryotes).

8. Some codes are stop codons

Three codons UAG, UAA and UGA are the chain stop or termination codons. They do not code for any of the amino acids. These codons are also called nonsense codons, since they do not specify any amino acid.

9. The code is universal Same genetic code is found valid for all organisms ranging from bacteria to man.

Sample questions 1. If a messenger RNA has a base sequence of CUGACGAGU, which one of the following would

be the possible maximum number of amino acids coded for, if the code is overlapping? A. 7 C.4

B.6 D.3

2. The two strands of DNA easily separate during replication because of the

44

A. helical nature of the nucleotide B. the closeness of the base pairs C. weak hydrogen bonds between the base pairs D. the week hydrogen bonds between phosphate and sugars.

3. Which one of the following is the mRNA strand that corresponds to the DNA strand TAGGCT?

A. AUCCGA B. UUCCGU C. CGAAUC D.UAGGCU

4. Which one of the following bas triplet pair with ACG triplet base? A. TGC B. AAT C. GTG D. ACC

5. Which one of the following is confined within the nucleus?

A. DNA molecules B. Ribosome C. Messenger RNA D. Transfer RNA

6. Which of the following are purines? A. Adenine and cytosine B. Thymine and Adenine C. Thymine and cytosine D Adenine and guanine

7. Which one of the following statement correctly describes the transcription of DNA? A. It produces amino acids B. it results in and increased DNA synthesis C. It produces messenger RNA D. It occurs at the surface of the ribosome

8. Which of the following is found in both DNA and messenger RNA? A. double helix structure B. ribose C. sugar-phosphate chain D Thymine

9. Which base sequence below pair with AGU? A. TGC B. TCA C. GCC D ATT

10. What is the maximum number of triplets of nucleotides that could code for 20 amino acids? A. 3 B. 6 C. 48 D. 64

45

11. Which one of the following tRNA anti-codons will correspond to the mRNA base triplets CAT?

A. GUA B.GUC C. GTC D. GTA

12. The synthesis of mRNA may be described as a

A. Replication B. Transcription C. translocation D. transduction

13. Which of the following is the correct sequence of combination forming a double helix of

DNA? A. Phosphate- sugar- guanine-hydrogen bond- cytosine-sugar - phosphate B. Thymine-sugar-phosphate-hydrogen bond-adenine-sugar-phosphate C. Sugar- phosphate-cytosine-hydrogen bond-guanine-sugar- phosphate D. Phosphate-sugar-guanine-hydrogen bond-thymine-sugar-phosphate

14. Hyroxylamine, a mutagen, converts cytosine to a compound which pairs with adenine. If DNA is treated with hydroxylamine, the resulting mutation is A. a deletion B. a substitution C. an insertion D. an inversion

15. Name the bases that may be synthesized of the t-RNA from strand (ii) of the DNA indicated in the diagram below

16. If the bases on t-RNA are ACU, what would be the corresponding bases on original DNA coding strand during protein synthesis? A. ACT B. UGA C. TGA D. ATG

17. When DNA replicate, it thought to unwind and „unzip” along A. bonds between the deoxyribose and phosphate units B. Phosphate to phosphate bonds

46

C. hydrogen bonds between base pairs D. ribose to deoxyribose sugars.

18. Which of the following carries the triplet nucleotide code?

A. ribosome RNA (r-RNA) B. Transfer RNA (t-RNA) C. Messenger (mRNA) D. Nuclear (n-RNA)

19. The nucleotide is attached to another in DNA strand by bridge

A. Base to Base B. Sugar to Base C. Phosphate to Base D. phosphate to Sugar

20. Which one of the following show the correct coding sequence during protein synthesis? A. DNA, mRNA, tRNA, rRNA, amino acids B. rRNA, tRNA, mRNA, polypeptide C. DNA, mRNA, tRNA, amino acids D. DNA, mRNA, rRNA, tRNA, amino acids

21. Transfer RNA of function in

A. Carrying RNA from the ribosome to mRNA B. Attaching RNA to ribosome C. Carrying amino acids to the correct site on mRNA D. Carrying nucleotide to mRNA on the ribosome

22.The genetic code is most directly related to the sequence of

A. Ribose unit B. Deoxyribose unit C. Nitrogen bases D. Phosphates and pentose sugar

23. Any change in the sequence of bases of a DNA molecule by addition, deletion or substation always result in a

A. Lethal gene B. mutation of the gene C. visible change in offspring D. variation in the new individual making it more capable of adaptation

24. In the Watson-Crick model of double helix, the “rugs” of the “twisted ladder” are

composed of A. Sugar B. A purine and a pyrimidine

47

C. Two purines D. Two pyrimidine

25. The figure below shows a stage in protein synthesis. A mutation in DNA template strand

involving substitution of the original base G on the mRNA codon was transcribed with match with a complementary anticodon having the following base triplet code

A. UUA B. TCU C. GCA D. UCA

26. Analysis of a sample of DNA showed that 33% of the bases were adenine. The percentage

of guanine bases in the sample was A. 34 B. 33 C. 17 D. 28

27. The figure below represents the structure of

A. Amino acid B. Glucose C. Ribose D. Fatty acid

28. Some bacteria when infected with microphages, may make a particular amino acid they

could make before. This is due to A. Transformation B. Mutation

48

U A G

C. Transduction D. conversion

29. In a non-dividing cell, the percentage of guanine is 40%. The percentage of adenine in the

cell is. A. 30 B. 20 C. 40 D. 10

30. In an outline for protein synthesis, DNA mRNA polypeptide

Stage 1 stage 2

Stage 2 represents

A. Transcription B. Translation C. Transduction D. transformation

31. If the triplet of mRNA is AAG, what is the complementary triplet of the base on tRNA

molecule? A. TTC B. UUC C. CCT D. CCU

32. Which of th following molecules is represented in figure below

A. Fattay acid B. Deoxyribose C. Glucose D. ribose

33. Which two of the following strands of nucleotide would pair with strand X in the figure? A

49

A

C

U

G

U

U

C

A

C

A

A

C

T

G

T

T

G

A

C

A

C

A

G

T

50

X (i) (ii) (iii) (iv)

A. (i) and (iii) B. (ii) and (iv) C. (i) and (ii) D. (iii) and (iv)

34. A biochemical analysis of a DNA sample showed that 34% of the bases were guanine. The

percentage of adenine bases in the sample is A. 32 B. 16 C. 17 D. 34

35. The process of changing the information on mRNA into formation of polypeptides is known as A. Transcription B. Translation C. Transduction D. transformation

36. Which one of the following shows he correct coding sequence during protein synthesis? A. DNA → mRNA → tRNA → rRNA → amino acid B. rRNA → tRNA → mRNA → polypeptide C. RNA → mRNA → tRNA → proteins D. DNA → mRNA → rRNA → tRNA → amino acid

37. During protein synthesis, the anticodon base of tRNA is AUG. What is th base sequence on the

template DNA strand? A. UAU B. ATG C. AUG D. TAC

38. Analysis of a sample of DNA showed that 33% of the bases were adenine. The percentage of guanine bases in the sample was

A. 34 B. 33 C. 17 D. 28

39. (a) Biochemical analysis of sample of DNA showed that 33% of the nitrogenous base were

guanine. Calculate the percentage of the bases in the sample which would be adenine. Explain how you arrived at your answer

(b) What name is given to the triplet of bases which designate individual amino acid? (c) If the triplet of mRNA base which designate the amino acid lysine is AAG, what is the

complementary triplet of bases on the tRNA

51

40. (a) state where each of the following is found in a cell

DNA ……………………………………………… RNA …………………………………………….. (b) give three structural differences between DNA and RNA (3marks) (c) What is the genetic significance of DNA replication? (2marks) (d) Give two ways that suggest that DNA is hereditary material (4marks)

41. Describe the process of protein synthesis (20marks) 42. (a) Describe the biological function of amino acids (05marks)

(b) Describe how amino acids form a polypetide (09marks) (c) How do inhibitors change the rate of enzyme controlled reaction? (06marks)

43. (a) Describe the structure of DNA (11marks) (b) Using examples, explain an effect of gene in humans (6marks) (c) What is the signifince of mutation in crop hasbandary? 06marks)

44. (a)Compare DNA and RNA (10marks)

(b) Describe the role of mRNA protein synthesis in a cell(5marks) (c) How does molecular structure of proteins relate to their functions? (05marks)

Suggested answers

1 B 6 D 11 B 16 A 21 C 26 C 31 B 36 D 2 D 7 C 12 B 17 C 22 B 27 C 32 D 37 B 3 A 8 C 13 A 18 B 23 B 28 C 33 B 4 A 9 D 14 B 19 A 24 B 29 D 34 B 5 A 10 D 15 A 20 D 25 D 30 B 35 B 39. (a) Percentage of adenine = 17% (b) codon (c) UUC 40. (a)(i) DNA is found in the cell nucleus

RNA is found in both nucleus and cytoplasm

(b)

DNA RNA

52

Double helix Has thymine Has no uracil Has hydrogen bonds between base pairs High molecular weight

Single Has no thymine Contains uracil Had no hydrogen bond Low molecular weight

(c)

a. it allows maintenance of a constant amount of genetic information within organisms of a population

b. it allows passing over of genetic information from parents to offsprings in constant amounts, generation after generation

(d)

c. ability to replicate d. it the major component of chromosomes believed to transmit genetic material 41. Process of protein synthesis

(i) Protein synthesis occurs in the cells of living organism controlled by DNA which reside in the nucleus

(ii) (ii) In the nucleus, DNA unwinds to form two separate DNA strands of which one acts as a template for synthesis of mRNA strand in the process called transcription.

(iii) mRNA molecule leaves the nucleus through nuclear pores to the cytoplasm where it attaches to ribosome which consists of rRNA and proteins on the rough endoplasmic reticulum.

(iv) The ribosome reads the sequence of codons in mRNA, and molecules of tRNA bring amino acids to the ribosome in the correct sequence.

(v) rRNA helps bonds peptide bonds form between the amino acids starting with methionine until the entire genetic code on mRNA is read to form a polypeptide.

(vi) The polypeptide molecule then peels off from the ribosome and released into the cytoplasm (vii)After a polypeptide chain is synthesized, it may undergo additional processing to form the

finished protein

42. Solution:

(a) (i) Amino acids are used to build up proteins, which have functional roles such as enzymes and

hormones, and structure roles such as keratin and collagen.

(ii) The amino and carboxylic group of amino acids act as buffers, resisting changes in pH in the

body fluids.

(iii) Amino acids can be used substrate in cell respiration to provide energy in cases of starvation.

(iv) They are also used to synthesize neurotransmitters. For example, tyrosine is used to

synthesize adrenaline and norepinephrine.

53

(v) Amino acids are regulators of gene expression. They do so by modulating the translation of

mRNA

(b) The order and nature of amino acids used in the assembly of polypeptide chain is dictated by the

sequence of bases in the mRNA molecule, in a process called translation, it occurs in ribosomes.

- First, the mRNA attaches to the ribosome.

- The amino acids are then activated and attached to their specific tRNA molecules in a reaction

catalyzed by the enzymes aminoacyl –RNA synthetase.

- The first mRNA codon then binds the tRNA molecule having the complementary anticodon and

carrying the first amino acid (usually methionine)

- The second mRNA codon then also binds the second tRNA molecules bearing the

complementary anticodon and carrying a specific amino acid.

- The ribosome holds the mRNA, tRNA and associated enzymes controlling the process until a

peptide bond forms between adjacent amino acids.

- Once the new amino acid has added to the growing polypeptide chain, the ribosome moves

one codon along the mRNA.

- The tRNA previously attached to the polypeptide chain leaves the ribosome and returns to the

RNA pool in the cytoplasm. another tRNA molecule binds to the new codon.

- This process continues until the ribosome comes to a terminating codon (no sense codon),

signaling „stop‟

- At this point, the polypeptide chain detaches from the ribosome and is released into the

cytoplasm.

- The necessary energy for the synthesis of the polypeptide chain comes from ATP.

- Note: translation is the mechanism by which the sequence of bases in a mRNA molecule is

converted into a sequence of amino acids in a polypeptide chain. It is the second steps in

protein synthesis

- The first is transcription where the base sequence of a section of DNA representing a gene

converted into the complementary base sequence of mRNA.

this occurs in the nucleus and is influenced by the enzymes RNA polymerase.

- The last step of protein synthesis involves modification in the protein structure.

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This may occur in the Golgi apparatus in the cytoplasm.

(c) Inhibitors reduces the rate of enzyme controlled reactions.

They do so two mechanisms competitive inhibition and non-competitive inhibition

Competitive inhibition

- Substance inhibitors‟ which are structurally similar to the substrate compete with the substrate

for the active site on the enzyme.

- This result in fewer enzyme-substrate complex formation and therefore the rate of reaction

reduces.

- This type of inhibition is reversible.

Non- competitive inhibitions.

- Substance (inhibitors) bearing no structural resemblance to the substrate bind with the

enzymes or it cannot be released from it.

- This types of inhibitions can be reversible or irreversible.

Note: in competitive inhibition, an increase in substrate concentration increases the rate of the

reaction. This is because the chances of the substrate molecules binding to the active site are then

higher.

However, in non-competitive inhibition, increasing the substrate concentration has no effect on the

rate of reaction since the inhibitors does not bind to the active site. The degree of inhibition only

depends on the concentration of the inhibitor.

43. Solution

(a) In this question you should outline the structural components of DNA and show how they are

linked to form the DNA molecule.

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• A DNA molecule is composed of two antiparallel polynucleotide chains as shown above.

• Each chain consists of repeated units of deoxyribonucleotides formed by a deoxyribose sugar

phosphate backbone with nitrogenous bases as side branches.

• The two polynucleotide chains are twisted around a common axis to form a right handed

double helix.

• These two chains are held together by hydrogen bonds between the nitrogenous bases from

either chain. Adenine in one chain is always pairs with thymine of the other by two hydrogen

bonds while cytosine pairs with Guanine by three hydrogen bonds.

• The arrangement of these nitrogen bases gives DNA its identity.

• DNA in eukaryotes is wound around histone proteins to form chromosomes.

• In prokaryotes, it occurs in a circular shape.

(b) In this questions, first show your understanding of the term gene mutation. Then using suitable

examples outline the effect of this process in humans.

A gene mutation is a sudden irreversible change in the structure of a gene. It is also called point

mutation. It results in loss or change of function of the gene in question. Gene mutation is the cause

of some of the disease called genetic disease. Example is sickle cell anemia;

This is caused by a mutation in the gene responsible for synthesis of β – polypeptide chains in

hemoglobin.

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This results in replacement of glutamine with valine in position 6 of the chains, leading to formation

of hemoglobin S.

The condition has the following effects.

• Hemoglobin S crystallizes at low oxygen tensions, making the red blood cells fragile and to

adopt a sickle shape. They can hardly pass through capillaries and the spleen sinusoids.

• The red blood cells become more easily hemolyzed in the liver and spleen. This reduces the

number of red blood cells causing anaemia.

• Bilirubin, the bi-product of hemoglobin breakdown, accumulates in the body and causes

yellowing of the body surface membranes especially the sclera (jaundice)

Note: other conditions that occur due to gene mutation include;

• Alkaptonuria

• Phenylketonuria

• Albinism

(c). In this question you should point out how the effect of a mutation is of importance in crop

husbandry.

Mutations can induce polyploidy in plants causing hybrid vigor. This leads to;

• Increased crop yield

• Increased pest and disease resistance Increase size of fruits, leaves and flowers.

• Faster rates of growth.

43. Solution:

(a) Similarities between DNA and RNA

• Both contain purines adenine and guanine and the pyrimidine base cytosine Both

contain phosphate groups.

• They both have a basic sugar- phosphate backbone in their chains Both contain

polynucleotide chains.

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Differences between DNA and RNA

DNA RNA

• Double stranded structure.

• Found only in the nucleus.

• The sugar molecule is deoxyribose.

• Contains a pyrimidine base thymine

• Giant molecules with very high

molecular weight

• Has hydrogen bonds in its structure

• Single stranded structure

• Found in both nucleus

and cytoplasm

• Found both sugar molecular is

ribose

• Contains a pyrimidine base uracil

• Short, small with low molecular weight

• Has no hydrogen bonds in its

structure

(b) mRNA caries genetic information in form of nitrogenous bases from chromosomal DNA in the

nucleus to the ribosomal RNA in the in the cytoplasm. It acts as a template for protein

synthesis. It has complementary DNA base sequence to part of DNA from which it is

transcribed. As a result, mRNA directs the synthesis of proteins as directed by the base

sequence on the segment of DNA from it is copied, a process called translation

(c) According to molecular structure, proteins can be classified as follows in relation to function;

• Fibrous proteins from a long, tough, fibre insoluble in water. They therefore function as

structural and supporting proteins and include; keratin, collagen, elastin, e.t.c.

• Globular proteins have a globular shape but lack contractile properties. They contain

polypeptide chains coiled about them. They therefore act as regulatory molecules and as

food storage molecules in plants. Soluble globular proteins are amphipathic and therefore

act as buffers of body fluid pH.

• Conjugated proteins consist of a simple protein united with some non-protein sub-stance.

Examples include; glycoproteins which form mucin of saliva that helps in softening food;

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chromo proteins combined with a pigment such as hemoglobin, an oxygen carrying

molecule; metallo-proteins which act as enzymes.

Enzymes

Specific objectives

The learner should be able to:

a. Describe the criteria for naming enzymes using type of substrate and type of reaction b. Explain the characteristics/properties of enzymes c. State factors that affect enzyme action (pH, temperature, inhibitor, substrate concentration) d. Explain the mechanism of enzyme action using the lock and key mechanism and induced

fit e. The role of enzyme in the organism’s life.

Practical f. Demonstrate properties of enzymes action in specific temperature, pH range, substrate

concentrations g. Identify enzymes in different parts of the gut based on their different actions on different food

substances

The chemical reaction that occur in cells constitute metabolism and the participating molecules are called metabolites.

Types of metabolism 1. Anabolism [or synthetic reaction]. Here single molecules are linked together by chemicals bonds to

form more complex compounds.

A + B → AB

A and B are substrate molecules or reaction and AB represent the product.

Generally anabolic reactions require [absorb] energy and are therefore endogenic reaction.

These reactions are concerned with building up structures, storage compounds and complex metabolites in the cell. Starch, glycogen, lipids and proteins are all products of anabolic pathway.

Plants and other autotrophic organism synthesize these complex organs c molecules from simple inorganic sources such carbon dioxide and water. They have much greater synthetic power, and their metabolic pathways and therefore more extensive than heterotrophs.

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2. Catabolism [or breakdown reaction] The complex compounds are splint into simple molecules

AB → A + B

In this case AB is the substrate and A and B are products.

Generally catabolic reaction release energy and are therefore exergonic reaction. These reactions are mainly concerned with mobilizing food stores and making energy available in cells. Energy is require for 3 main purpose.

(a) For synthesis e.g. the synthesis of proteins, storage compounds (b) For work e.g., contraction of muscles (c) for maintenance; such as the constant body temperature.

Small steps gentle reactions

Metabolites are not converted into products in single large reactions. Instead they are converted gradually, step by step, through a series of small reactions which together comprise a metabolic pathway, each reaction though small, brings the raw materials closer to the end product.

There are five main reaction why metabolism proceeds in small steps.

(i) Large catabolic reaction would create unfavorable conditions, such as very high temperature, that would be incompatible with life.

(ii) Energy can be obtained from small catabolic reaction in a usable form. (iii) Substance can be partially broken down so as to provide raw materials for other. (iv) It is not possible to synthesize, in one step complex organic compound from simple small

raw material in the gentle condition prevailing in cell. (v) Having small steps in an anabolic pathway increase the cell’s ability to control what

product are made.

Living cells have the unique ability to perform numerous individual reaction in dilute aqueous solution at low temperatures and with a narrow range of pH.

However, the sheer number of reaction require a fantastic degree of organization in the cell. Much of this organization is achieved by enzyme which catalysis individual reactions.

Energy

All living organisms may be regarded as working machines which require a continuous supply of energy in order to keep working and so to stay alive.

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Energy is defined as the ability [ capacity] to do work. The main source of metabolic energy is the cell in the oxidative break down of glucose [ respiration] given by the following over simplified reaction

C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy

Activation energy

The speed of biological reaction.

It’s affected the following factors

- The concentration of the substrate molecule. The higher the concentration the higher the rate

and reaction due to high rate of collision. -

- The temperature of the reaction mixture: Temperature speeds up biological reaction by supplying the activation energy for the reaction and increase the rate of collision of the substrate molecules.

- Enzymes are biological catalyst i.e. a catalyst is a substance that increase the rate of the chemical reaction without taking part in a chemical reaction through lowering the activation energy.

Difference between enzyme and catalysts.

Enzymes Catalysts 1. proteins in nature inorganic chemicals e.g. Pt 2. catalyze specific reactions Such as

hydrolysis of starch

may catalyze more than one reaction e.g. platinum catalyze decomposition of H2O2 and reduction of benzene

3. Denatured by heat above 450C Usually are not affected by heat 4. Very sensitive to pH Not sensitive to pH 5. Initiate reaction Do not initiate reaction Enzyme classification

Enzymes are placed into six groups according to the general type of reaction which they catalyze Each enzyme is given a systematic name, accurately describing the reaction it catalysis. However, since many of these names are very long and complicated, each enzyme as allocated a trivial name of the substrate acted on by the enzyme (i) the name of the substrate acted on by the enzyme (ii) the type of reaction catalyzed.

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(iii) the suffix-ase.

The categories of enzyme are;

1. Oxidoreductase; are involved in biological oxidation and reduction reaction. They include dehydrogenase which catalyze removal of hydrogen atoms from a substrate and oxidase which formation of water e.g., in respiration.

2. Hydrolase; catalyze the addition of water to or the removal from a substrate.eg protease. 3. Transferase; These catalyze transfer of chemical groups or atoms from one substrate to

another. Those that transfer amino groups [NH2] are called transaminase. 4. Lyase; break chemical bonds other than hydrolysis this creating double bond. They include

carboxylase which remove carboxyl group [ COOH] from intermediates in respiratory pathways. 5. Isomerase; These enzymes catalyze the transfer of an atom from one part of a molecule to

another.

6. Ligase or synthetase; catalyze joining together of two molecules coupled with the breakdown of ATP e.g., phosphokinase which catalyze the addition of phosphate group to a compound.

The properties of enzymes.

1. They catalyze the rate of biological reactions. 2. 2. They are not destroyed by the reaction in which they catalyze. 3. They work in either direction i.e., catalyze both forward and backward reaction. 4. They are inactivated by high temperatures 5. They are sensitive to pH changes 6. They are usually specific to particular reactions

FACTORS AFFECTING THE RATE OF ENZYME REACTION

(a) Enzymes concentration; provided that the substrate concentration is maintained at a high level, and other conditions such as pH and temperature are kept constant, the rate of reaction is proportional to enzyme concentration.

A graph showing relationship between enzyme concentration and the rate of enzyme- controlled reaction

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Question; How would you design an experiment to show this?

(b) Substrate concentration.

For a given enzyme concentration, the rate of an enzyme reaction increases with increasing substrate concentration. The theoretical maximum rate [Vmax] is never quite obtained, but there comes a point when any further increase in substrate concentration produces no signification change in reaction rate. This is because at high substrate concentration the active sites of the enzyme molecules at any given moment are virtually saturated with substrate. Thus, any extra substrate has to wait until the enzyme/ substrate complex has dissociate into product and free enzyme before it may itself complex with the enzyme.

(c) Temperature

Up to 400C, the rate of enzyme -controlled reaction increase smoothly with temperature, a ten degree rise in temperature being accompanied by approximately doubling the of reaction. Above that 400C the rate begins to fall off and then decline rapidly, ceasing at about 600C, because the enzyme is denatured.

Graph showing the effect of temperature on the activating of enzyme as salivary amylase.

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Enzyme concentration

catalyzed reaction

Rate of enzyme

Substrate concentration

catalyzed reaction

Rate of enzyme

An experiment to investigate the rate of action of amylase Materials.

- 1% starch solution - Test tubes - 40 water bath -_Glass rod - White tile - Iodine solution Method

1.Prepare 3 test tube each with 2ml of starch solution and 2ml of 1% amylase solution.

Test tube 1; in beaker containing ice

Test tube2; in a water bath at 40 c water bath.

Test tube3; in a beaker of boiling water And simultaneously start the stop clock.

2.Every after 1 minute withdraw a few drops from every test tube, place them on a white tile and add a drop of iodine solution.

3.Note the time taken for starch to disappear from each test tube.

NB; The rate of enzyme reaction is inversely proportion to this time.

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Temperature 060 045 037

catalyzed reaction

Rate of enzyme

(b) pH

Under conditions of constant temperature, every enzyme function most efficiently over a narrow pH range e.g. pepsin at pH= 2, Amylase (salivary) at pH 6.8.

Mechanism of enzyme action.

The action of enzymes are specific to a given substrate and this specificity can be explained by two hypothesis.

1. Lock and key hypothesis

Enzyme are very specific to the substrate they act on because they have particular shape / configuration into which substrate with complementary shape fit in exactly as the key fit into the lock, thus the lock (enzyme) and key (substrate) hypothesis.

When an enzyme / substrate complex is formed, the substrate activated into forming the product of the reaction. Once formed, the product no longer fit into the active site and escape into the surrounding medium leaving the active site free to receive other substrate molecule.

2. Induced fit hypothesis. This hypothesis claim that enzyme and their active site are physically rather more flexible structure than this described by the lock and key hypothesis, and that the active site of the enzyme is molded into a precise configuration in presence of a substrate to enable it perform its catalytic functions more effectively.

Inhibitors of enzymes Certain substrate inhibit enzyme, thereby slowing down or stopping enzyme – controlled reactions.

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in the lockEnzyme active site like a key

The Substrate fit exactly into

Complex Substrate-Enzyme

Substrate Enzyme

These enzyme inhibitors are of specific interest for the following reasons.

a. They give us important information about the shape and properties of active site of enzyme. b. They can be used to block particular reactions thereby enabling biochemist to reconstruct

metabolic pathways. c. They have important medical and agricultural use as for example drugs and pesticides.

Type of inhibitors (i) Competitive reversible inhibitor;

Here a compound is structurally similar to that of the usual substrate associates with the enzymes active site but it’s unable to react with it. While it remains there, it prevents access of any molecules of the substrate. This type of inhibition depends on the concentration of the substrate and that of an inhibitor. At high concentration of the substrate, inhibition is overcome.

This knowledge of competitive inhibitors has been utilized in chemotherapy. Sulphonamides drugs and antibiotic such as penicillin are competitive inhibitors of essential metabolites for enzymes in bacteria.

(ii) Noncompetitive reversible inhibitors.

This is a type of inhibition in which the inhibitors attach themselves outside the active site thereby preventing the enzyme normal catalytic reaction by changing the shape of the enzyme or allosteric effect. It may be revisable when the inhibitor forms loose attachment to the enzyme that may be detached when circumstances permit e.g. cyanide or irreversible noncompetitive inhibition when the inhibitor permanently disorganizes the structure of the enzyme that it may no longer react with the substrate, e.g., mercury.

Allosteric enzymes and their inhibition

Allosteric enzymes are enzymes which exist in two different form, one active and the other inactive. The activity of these enzyme is regulated by compounds which are not their substrate and these substances (allosteric effectors) binds the enzyme at specific sites well away from the active site. They modify enzyme activity by causing a reversible change in the structure of the enzyme active site. Allosteric activators speed up enzyme catalyze reaction whereas or allosteric inhibitor slow down the action of allosteric enzymes.

Enzyme cofactors.

These are non-protein components required by enzymes to function efficiently. Cofactor may vary from simple inorganic ions to complex organic molecules, and may either remain unchanged to the end of a

66

reaction or be regenerated by the later processes. The enzyme-cofactor complex is called a halo enzyme whilst the enzyme portion without its cofactor is called an apoenzyme.

There are three recognized cofactors.

(i) Inorganic ions (activator)

These are thought to modify either the enzyme or the substrate such that an enzyme-substrate complex can be formed, hence increasing the rate of reaction catalyzed by that particular enzyme, salivary amylase activity is increased in the presence of chloride ions.

(ii) Prosthetic group (for example FAD, Biotin, Haem)

The organic molecule is integrated in such a way that it effectively assists the catalytic function of its enzyme, as in Flavin adenine dinucleotide [FAD]. This contain riboflavin [vitamin B2] which is the hydrogen accepting part of FAD. It’s concerned with cell oxidative pathways such as part of the respiratory chain in respiration.

Net effect; 2H are transferred from A to B. one halo enzyme acts as a link between A and B.

(iii) Co enzyme (NAD, NADP, Co enzyme A, ATP]

Nicotinamide adenine dinucleotide (NAD) is derived from the vitamin nicotinic acid and can exist in both a reduced and an oxidized form. In the oxidized state, it functions in catalysis as a hydrogen acceptor.

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Reduced substrate 2BH

Hydrogen acceptor B 2FADH-me Enzy

FAD-Enzyme

substrateOxidized

A

Hydrogen donor2AH

e2 e1 Reduced substrate

2BH

Hydrogen acceptor B 2NADH

NAD

substrateOxidized

A

Hydrogen donor2AH

Where e1 and e2 are two different dehydrogenase enzyme.

Exercise

1. Chloride ions are vital for efficient functioning of salivary amylase because ion

A. Activator B. Are coenzyme C. Are co factor D. For alkaline medium

2. Enzyme that catalyze the removal of water molecules from a substrate are known as A. Reductase B. Dehydrase C. Hydrolase D. hydrase

3. What name is given to a chemical reaction in which two or more hexose sugars combine to form larger units

A. condensation B. hydrolysis C. dehydrogenation

D. isomerization Energy from ATP

4. C6H12O6 + C6H12O6 (Enzyme) C12H22O11 + H2O

Glucose Fructose

What is the above called?

A. Dehydration synthesis B. Hydrolysis C. Dehydration process D. Condensation reaction

5. The activity of an enzyme in a

A. Molecukar weight of the enzyme B. Protein nature of the enzyme C. Activation energy of the enzyme D. Surface configuration of the enzyme

6. The reaction rate of salivary amylase with starch decrease as the concentration of chloride ions. A. Co enzymes B. Competitive inhibitor

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C. Co factor D. Allosteric inhibitor

7. Which one of the following characteristic of enzyme distinguishes than from inorganic catalyst

A. Initiation and speed up the rate of reaction B. Remain the same at the end of the reaction C. May promote reversible reaction D. Exert their effect when present even in smal quanties

8. Use the figure below, the part of reaction of the Krebs cycle to answer 8 and 9

Succinic acid acculates when mallonic acid is added to the reaction medium. Which one of the following statement best describes the role of mallonic acid?

A. Malonic acid is an inhibitor of enzyme 1 B. Malonic acid reacts with α-keto glutaric acidto form succinic acid C. Malonic acid is an inhibitor of enzyme 2 D. Molonic acid acts as coenzyme of enzyme 1

9. Enzyme 2 is a

A. Dehydrogenase B. Decarboxylase C. Dehydrase D. Reductase

10. An enzyme which catalyze the conversion of dipeptide into separate amino acid is example of A. Dehydrogenase B. Hydrolase C. Transferase D. Oxidase

11. An enzymatic reaction of the type ATP + hexose ADP+ hexose-6-phosphate is an example of

A. A hydrolysis B. An isomerase C. Transfer D. A synthesis

12. When a piece of liver is dropped into containing H2O2, there is vigorous reaction. this is due to enzyme

A. Catalyze

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2NADH +NAD Enzyme 3

Enzyme 2

Maleic acid

acid Fumaric

acid Succinic Enzyme 1

glutaric acid keto-α

B. Amylase C. Trypsin D. Carbonic anhydrase

13. In the lock and key hypothesis for the mechanism of enzyme the mechanism of enzyme action,

how does inhibitor substance stop enzyme action? By A. Raising activation energy B. Distorting substrate molecule C. Destroying coenzyme D. Occupying active sites on substrate and enzyme

14. When the extent of inhibition in an enzyme controlled reaction depends entirely on the concentration of the inhibitor, it indicates that the inhibition is

A. Competitive B. Reversible C. Non-competitive D. Irreversible.

15. Which one of the following is not true about the lock and key theory in enzyme-catalyzed

reaction? A. A small change in the active site alter the enzyme B. The substrate and active site are complementary C. Enzyme catalyzed action go through the enzyme-substrate complex stage D. A molecule which fits in the active site is a substrate

16. Which one of the following does not have an effect on a non-competitive inhibition? A. Temperature change B. pH change C. enzyme concentration D. substrate concentration

17. Which one of the following describes the turnover number of an enzyme? A. Number of molecules affected by the enzyme B. Number of substrate molecules turned into its product per minute C. Number of product molecules formed D. Number of substrate molecules catalyzed per minute

18. Which of the following is true about non-competitive inhibition of enzyme catalyzed reaction? A. The degree of inhibition decreases with increase in substrate concentration B. The inhibitor has a similar structure and chemical composition with the substrate C. The degree of inhibition is independent of the substrate concentration D. The shape of the

enzyme is not affected by the inhibitor 19. When the extent of inhibition of an enzyme-controlled reaction depends entirely on the

concentration of the inhibitor, it indicates that the inhibition is A. Competitive B. Reversible C. Non-competitive

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D. irreversible 20. Chloride ion are vital for efficient functioning of salivary amylase because the ions

A. are activator B. are co-enzyme C. are co-factors D. form alkaline medium

21. The activity of an enzyme in a chemical reaction depends on the A. Molecular weight of the enzyme B. Protein nature of the enzyme C. Activation energy of the enzyme D. Surface configuration of the enzyme

22. Which one of the following correctly represents the effect of increasing substrate concentration on the degree of inhibition in a competitive and noncompetitive inhibition reaction?

competitive noncompetitive A Decrease increased B Decrease No change C increased Decrease D No change

23.

A B C XEnzyme 1 Enzyme 2 Enzyme 3

If an excess X controls the metabolic pathway of the reaction, the control mechanism is known as,

A. Multi-enzyme control B. Excess inhibition C. End product inhibition D. Negative feedback

24. An adaptation by plant to obtain nitrogen include all the following except.

A. Mycorrhiza on plant roots B. Bacteria in root nodules C. Possession of aerial roots D. Being insectivorous

25. Which one of the following environmental factors has a direct effect on all organisms? A. Light.

B. Humidity.

C. Temperature.

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D. Rainfall.

26. Figure 4 show the effect of increasing the concentration of a substrate on the rate of an enzyme

controlled reaction in presence of inhibitors A and B, in relation to the control experiment without

an inhibitor.

concentration

(a) Describe the effect of each inhibitor on the rate of reaction.

(i) Inhibitor A

(ii) Inhibitor B

(b) Explain the difference in the effect of inhibitors A and B on the rate of reaction

27. Explain the following

(i) Competitive inhibition

(b) In what way do enzymes differ from catalyst? (c) Briefly describe the lock and key hypothesis of enzyme action

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reactionRate of

Substrate

BEnzyme + inhibitor

Ar Enzyme + inhibito

)control(Enzyme only

28. The figure below shows the effect of varying substrate concentration on an enzyme catalyzed reaction, in absence and presence of compound A

(a) Explain the relationship between the reaction rate and substrate concentration (i) In absence of compound A (3marks)

(ii) In presence of compound A(4marks)

(b) State two factors which would have kept constant in this experiment (1mark) (c) What would be the effect of increasing the concentration of compound A in this

experiment? (2marks)

29. (a) Explain the difference between coenzyme and activators (b) Giving example explain what is

meant by enzyme inhibitors (c) How do enzyme bring about their effects.

30. (a) Describe the working of an enzyme using the Lock and Key hypothesis (11mark)

(b) Explain the

(i) Effect of excessive heat on the enzyme action (3marks) (ii) Non-competitive inhibition in an enzyme controlled reaction

(6mark)

31. (a) Classifies enzymes basing on the type of reaction they catalyze. (06marks)

(b) Explain how competitive and noncompetitive inhibitions of enzymes occur. (10marks) (c) What

is the importance of enzyme in enzyme catalyzed reaction?

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absent Compound A

1)-Substrate concentration (moll

1)-

Amount of product (s

absent Compound A

Suggested answers

1 A 6 C 11 D 16 D 21 D

2 C 7 A 12 A 17 B 22 B

3 A 8 C 13 D 18 C 23 D

4 D 9 D 14 B 19 A 24 C

5 D 10 B 15 D 20 A 25 C

26. Solution:

(a) Inhibitors A reduced of reaction initially but as substrate concentration increase the rate of

reaction also increase at a lower rate than the control. It attains a lower maximum but at a higher

substrate concentration than the control.

Inhibitors B greatly reduces rate of reaction, however, rate increase gradually with substrate

concentration to a very low maximum at a lowest substrate concentration than all.

(b) Inhibitor A is reversible competitive inhibitors of the enzyme. It competes with the substrate to

bind to the active site of the enzyme, being structurally similar to the substrate.

At low substrate concentration, the inhibitors out compete the substrate for the active site. Fewer

than normal substrate molecules combine with the enzyme and the reaction is slower than the

control.

As substrate concentration increase, the substrate competes more favorable for the active site and the

reaction increase with the substrate concentration up to saturation.

Saturation is slightly lower than control since some enzyme active sites still occupied by inhibitors A

molecules.

Inhibitor B is a non-competitive inhibitor. It binds with the enzyme permanently and prevents the

substrate from binding to the enzyme. This reduces effective enzyme molecules and the reaction

proceeds as it would occur if the enzyme concentration was lower than the control.

27. (a)(i) competitive inhibition

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- Substances structurally similar to enzyme substrate compete with enzyme substrate for active site on

the enzyme molecule where both reversibly bind.

- fewer true substrate bind to the enzyme and so the rate is reduced or inhibited. - this is a competitive inhibition whose degree depends on the relative amount of the substrate and

inhibitor.

(ii) Noncompetitive inhibitor

- A substance with the structure different from that of the substrate irreversibly binds on the

enzyme at a point different from the active site stops or reduces the action of the enzyme. The

degree of inhibition depends on the concentration of inhibitor and not the substrate.

(b) Differences between enzyme and catalysts

Enzymes Catalysts 1. proteins in nature inorganic chemicals e.g. Pt 2. catalyze specific reactions Such as

hydrolysis of starch

may catalyze more than one reaction e.g. platinum catalyze decomposition of H2O2 and reduction of benzene

3. Denatured by heat above 450C Usually are not affected by heat 4. Very sensitive to pH Not sensitive to pH 5. Initiate reaction Do not initiate reaction

(c) Lock and key hypothesis

- The enzyme’s active side and its substrate have complementary structure.

- The enzyme randomly binds with the substrate to form enzyme-substrate complex like a key

fits in the lock.

- The enzyme acts on the substrate to form products.

- The products do not fit in the active site, leave the active site for another substrate.

28. Solution

(a) (i) Reaction rate increase rapidly with substance concentration. Rate then increase gradually

before it becomes constant at higher substrate concentration

Explanation

Initially, the number of enzymes molecules available for reaction is higher than the number of

substrate molecules; any available substance will react to form a product. As the substrate

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concentration increase, the number of enzymes molecules occupied by substrate at a given

time increase, hence more amount of product is formed per unit time.

This continues until all the enzymes molecules are occupied so that a substrate has to wait for

the active site of the enzymes to be freed before it can bind the enzymes, in order to react. the

rate of formation of products therefore becomes constant.

(ii). Relationship

Rate of reaction increase slowly with substrate concentration and reaches a lower maximum at a higher

substrate concentration.

Explanation:

Compound A competes with the substrate for the active sites of the enzyme molecules.

As a result, the number of enzymes molecules available for binding the substrate at a given time is

lower.

As the substrate concentration increase, the substrate (out-competes compound A and the reaction rate

increase, through at a lower rate. He enzymes molecules become saturated at a higher substrate

concentration. However, at this point, the output is lower because some enzyme molecules are

occupied by compound A and cannot react with substrate.

(b)

Temperature of the reaction medium

PH of the reaction medium Others:

The amount of the enzyme.

(c) The rate of reaction would decrease with increase in amount of compound A added until the reaction

stops altogether at higher concentrations of compound A.

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29. (a) Cofactors/activators serve the same purpose as coenzymes, as they regulate, control,

and adjust how fast these chemical reactions would respond and take

effect in our body. The big difference is that coenzymes are organic substances, while

cofactors/activators are inorganic.

(b) Enzyme inhibitors are molecules that interact in some way with the enzyme to

prevent it from working in the normal manner. There are a variety of types of

inhibitors including: nonspecific, irreversible, reversible - competitive and

noncompetitive. Poisons and drugs are examples of enzyme inhibitors.

(i) Competitive reversible inhibitor; Here a compound is structurally similar to that of the usual substrate associates with the enzymes active site but it’s unable to react with it. While it remains there, it prevents access of any molecules of the substrate. This type of inhibition depends on the concentration of the substrate and that of an inhibitor. At high concentration of the substrate, inhibition is overcome.

This knowledge of competitive inhibitors has been utilized in chemotherapy. Sulphonamides drugs and antibiotic such as penicillin are competitive inhibitors of essential metabolites for enzymes in bacteria.

(ii) Noncompetitive reversible inhibitors.

This is a type of inhibition in which the inhibitors attach themselves outside the active site thereby preventing the enzyme normal catalytic reaction by changing the shape of the enzyme or allosteric effect. It may be revisable when the inhibitor forms loose attachment to the enzyme that may be detached when circumstances permit e.g. cyanide or irreversible noncompetitive inhibition when the inhibitor permanently disorganizes the structure of the enzyme that it may no longer react with the substrate, e.g., mercury.

Chemical or substance added or applied to another substance, to slowdown a reaction or to

prevent an unwanted chemical change. For example, anti-oxidants are added as inhibitors

to food to retard its spoilage from exposure to air (oxygen

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