● Earth

● Biosphere

● Biome

● Ecosystem

● Community

● Population

● Organism

● Systems

● Organs

● Tissues

● Cells

● Organelles

● Molecules

● Atoms

2The chemical composition of cells

C H A P T E R

29

Chapter 2 The chemical composition of cells

Key knowledge● General role of the enzymes in biochemical activities of cells

● Composition of cells: major groups of organic or inorganic substances, including carbohydrates,

proteins, lipids, nucleic acids, water, minerals, vitamins; their general role in cell structure and function

If you have ever ventured into the Australian bush, you may have come across an intriguing sight: a wriggling knot of what seems like hundreds of caterpillars hanging onto a eucalypt branch. These are the larvae of the sawfly wasp. They are better known as spitfires and for a very good reason. If they are distracted in any way from their writhings, they spit out a horrible, smelly mixture of green slime. This is made up of the juice of the eucalypt leaf on which it feeds. On a good day, this can travel as far as 20 cm.

Spitfires, like all other living things, are made up of cells. If you investigate their cell structure you will find that it is very like your own cell structure. As you have seen in Chapter 1, animal cells share a lot of common organelles. The way that spitfire cells function is also very similar to your own cells.

If we were able to zoom in on the cell of the spitfire and its organelles, and investigate the types of chemicals that make up and are used in those structures, we would see even more similarities between different types of cells.

The chemicals in cells

Organic and inorganic moleculesMany of the molecules found in living matter are larger and more complex than those in non-living material. With the exception of water, most of the molecules in living organisms contain the element carbon. These complex carbon-containing molecules are known as organic compounds. In the majority of cases, carbon (C) is joined with hydrogen (H), oxygen (O), and in some cases, nitrogen (N) and phosphorus (P). The organic compounds most commonly found in cells are carbohydrates, lipids, proteins, and nucleic acids.

Other compounds are classified as inorganic. Of the inorganic compounds, water and minerals are among the most important. Carbon dioxide is termed ‘inorganic’ as it is not a complex molecule and does not contain hydrogen. Living things contain both organic and inorganic compounds.

Figure 2.1 A wriggling knot of sawfly wasp larvae.

bioTERMSorganicderived from living organisms; complex carbon containing compounds

inorganicall other compounds that are not organic

30 Unit 1

cell wall – peptidoglycan (polysaccharide and protein)

cytoplasm – 80% water

plasmid – DNA

plasma membrane – − phospholipid bilayer

chromosome – DNA

capsule – polysaccharide and protein

ribosomes – protein and RNA

aa

Investigating the chemicals in cellsCells contain a large variety of organic and inorganic substances. It is possible to identify which organic substances occur in which part of the cell by using biochemical tests. The solutions that are used in these tests react with certain organic substances and undergo a colour change described in Table 2.1.

Table 2.1 Biochemical tests for specific organic substances.

Substance tested for Biochemical test used Expected colour change if chemical present

Monosaccharide (glucose) Benedict’s solution Green – orange – red

Starch Iodine solution Dark blue

Lipid Sudan IV indicator Pink – red

Protein Biuret reagent Pink – violet – purple

As shown in Table 2.1, Benedict’s solution is an indicator for monosaccharides. Table 2.2 summarises the expected colour changes of Benedict’s solution when reacted with simple sugars.

By using the tests outlined in Table 2.1 and other biochemical tests, it is possible to create an image of a cell and the chemicals that it contains. In Chapter 1 you saw that bacterial cells do not contain membrane-bound organelles, but plant and animal cells contain many similar organelles and some different organelles. You would therefore expect the chemicals within bacterial, plant and animal cells to show similarities and differences (see Figure 2.2(a), (b) and (c)).

Colour of Benedict’s reagent

Approximate sugar concentration

Blue Nil

Light green 0.5–1.0%

Green to yellow 1.0–1.5%

Orange 1.5–2.0%

Red to red–brown >2.0%

Table 2.2 Changes in the colours of Benedict’s solution when reacted with simple sugars.

Figure 2.2 (a) (above) Bacterial cell showing where inorganic and organic substances occur (b) (opposite) Plant cell showing where inorganic and organic substances occur (c) (p.32) Animal cell showing where inorganic and organic substances occur.

PRA

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AL ACTIVITY 2.1

Chapter 2 The chemical composition of cells

31

bchloroplast

DNA carbohydrate – glucose

nucleolusprotein – enzymes

cytoplasmwatercarbohydrateslipidprotein – enzymeRNA

cell wallcellulose

nucleusDNARNAprotein – enzymes

mitochondrioncarbohydrate – glucose DNAprotein – enzymes

large permanent vacuolewater

plasma membranelipid – phospholipids

– steroidsprotein – receptor

– transport

32 Unit 1

c

endoplastic reticulumproteins

plasma membranelipid – phospholipids

– steroidsprotein – receptor

– transportcytoplasmwatercarbohydrateslipidprotein – enzymeRNA

mitochondrioncarbohydrate – glucoseDNAprotein – enzyme

ribosomesRNAprotein – enzyme

nucleolusprotein – enzymes

cell nucleusDNARNAprotein– enzyme

33

Chapter 2 The chemical composition of cells

Table 2.3 Organic compounds in cells.

Organic compound Elements Examples Where found in cell Functions

Carbohydrates Carbon

Hydrogen

Oxygen

Monosaccharides– glucose

Disaccharides– sucrose

ChloroplastMitochondrionCytoplasm

Energy source and storage

Strengthens plant cell walls

Provides tough exterior covering

Polysaccharides– starch– glycogen– cellulose

PlastidCytoplasmCell wall

Lipid Carbon

Hydrogen

Oxygen

Phosphorus (in phospholipids)

Fats and oils Cytoplasm Energy storage

Cell membrane component

Chemical messengers

Protection

Phospholipids Cell membrane

Steroids Cell membrane

Waxes Cell surface

Protein Carbon

Hydrogen

Oxygen

Nitrogen

Sometimes sulfur and phosphorus

Messenger proteins and antibodies

Ribosome, endoplasmic reticulum, Golgi apparatus, vesicles

Immunity

Communication

Transport materials

Support cell and body structures

Speed up chemical reactions

Receptor proteinsTransport proteins

Cell membrane

Structural proteins Cell surface

Enzymes Throughout the cell

Nucleic acid Carbon

Hydrogen

Oxygen

Nitrogen

Phosphorus

DNA NucleusChloroplastMitochondrion

Store hereditary information (DNA)

Decode hereditary information (RNA)

RNA NucleusCytoplasmRibosome

The plasma membraneThe cells in spitfires, like your own cells, are animal cells. The cells in the eucalypt leaves that the spitfires eat are plant cells. The bacteria in the digestive system of the spitfire that assist in digesting the eucalypt leaves are prokaryotic cells. As you saw in Chapter 1, all these types of cells are enclosed by a plasma membrane. This membrane forms the boundary between the inside of the cell and the environment on the outside of the cell. In order for the cell to survive, it must be able to allow substances to pass across it.

In order to maintain cellular respiration, substances such as oxygen and glucose move into the living cell, and waste products such as carbon dioxide and excess water move out into the surrounding environment where they are disposed. Carbon dioxide and water must be able to move into the plant cell for use in photosynthesis,

R E V I E W

34 Unit 1

hydrophilic head containing phosphate

hydrophobic tail made of fatty acid side chains

and glucose and oxygen must be able to move out. Other cellular processes require different inputs and produce different wastes and products. As you learnt in Chapter 1, other substances, such as proteins, enzymes and hormones, must also be able to move across the plasma membrane.

The plasma membrane has other interesting features. It needs to be a flexible structure so the cell can change shape easily. It also needs to be able to grow and expand as the cell contents increase, especially during cell division. If the plasma membrane is punctured, some of the cytoplasm will leak out, but the hole will be sealed quickly.

What makes up the plasma membrane to allow it to act as a regulatory boundary between the inside of the cell and the outside? How is material selected to move across the membrane? How does it reseal a puncture? To answer these questions, we need to look at the property of the chemicals that make up the plasma membrane.

One remarkable property of plasma membranes is their ability to change shape, expand and contract. During cell division and vesicle formation, membranes can break and reassemble themselves. This is because membranes are actually two-dimensional fluids, constantly flowing and changing shape.

The plasma membrane, and in fact all membranes surrounding organelles within the cell, are made up of a double layer of phospholipid molecules, the phospholipid bilayer, as shown in Figure 2.3. Phospholipids are part of a larger group of molecules known as lipids (see below). Phospholipids alone would not allow this required flexibility, as strong inflexible bonds naturally form between the lipid tails.

Another type of lipid called cholesterol is interspersed among the phospholipid molecules. Cholesterol prevents the lipid tails forming strong bonds and therefore prevents inflexibility. Therefore, it is cholesterol that is responsible for the flexibility of membranes.

Cholesterol belongs to a special group of lipids called steroids. Cells can remodel cholesterol to form other steroids such as the sex hormones progesterone, oestrogen and testosterone and bile salts that assist with fat digestion in the small intestine. Vitamin D (see page 36) is another steroid derived from cholesterol.

Figure 2.3 A phospholipid molecule. The hydrophilic heads are attracted to water whereas the hydrophobic tails repel water.

1 Organise the following list of substances under either the heading

of organic or inorganic:

water, lipid, protein, minerals, carbon dioxide, carbohydrate,

nucleic acid

2 How do scientists determine which chemicals occur within cells?

3 Explain why the plasma membrane needs to be able to allow some

substances to pass through it.

4 Explain what chemicals are responsible for each of the following

features of the plasma membrane:

a ability to change shape

b flexibility

c ability to reseal a puncture

BIOLINK

Plasma membrane

35

Chapter 2 The chemical composition of cells

Examples:beefporkcheese

coconut oilbutter

Examples:olive oilpeanut oilalmondssunflower oil

corn oilfishmayonnaisemost margarines

H

H

CH

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C CO

OH

saturated fatty acid

H

H

CH

H

H

C

H

C

H

C

H

H

C

H

C

H

C

H

H

C

H

C

H

C

H

H

C

H

H

C CO

OH

unsaturated fatty acid

a

b

bio B Y T EFats are an essential part of our diet.

Fats that originate from plants and seafood (such as soy, nuts and vegetable oils) are the healthiest type of fat you can eat. Fats from animals such as steak and chicken are healthy in moderation, and the worst fat that you can eat is fat from processed foods (hydrogenated oil).

LipidsLipids form a larger class of compounds containing fats and oils. Fats are solid at room temperature and oils are liquid. They all contain the elements carbon, hydrogen and oxygen and are insoluble in water. You have probably noticed that if you mix oil and water together, the oil will float on top of the water. As lipids are complex molecules and contain carbon, they are an example of an organic compound.

The most common type of lipid is the triglyceride. As its name (tri-) suggests it is composed of three fatty acids and one glycerol (see Figure 2.5). If the fatty acid contains only single bonds between the carbon atoms, it is described as a saturated fat. Single bonds tend to be difficult to break down by the cells in your body. As such they can also withstand much higher temperatures. This is why you will find fast-food outlets cooking with saturated animal fat such as lard instead of the more unstable unsaturated fats such as peanut oil and sunflower oil.

If there are double bonds between some carbon atoms then the fat is described as unsaturated. If there are many double bonds between the carbon atoms then it is described as polyunsaturated. Double bonds are more easily broken down by the cells in your body than are single bonds.

Phospholipids differ to triglycerides because one of the fatty acids is replaced by a phosphate group. This causes one end of the phospholipid molecule to be ‘water-loving’ (hydrophilic) and the other end to be ‘water-hating’ (hydrophobic) (see Chapter 3).

Figure 2.4 Saturated fats are used for cooking at high temperatures; unsaturated fats will break down at these temperatures.

Figure 2.5 Structural diagram of (a) saturated fat showing single bonds between carbon atoms (b) unsaturated fat showing double bonds between some carbon atoms.

bioTERMSfatty acida type of organic acid that combines with glycerol to form fat

glycerola molecule that combines with three fatty acids to form fat

36 Unit 1

VitaminsFats are often regarded to be an unhealthy inclusion in the diet. They are, however, essential in the absorption of many important vitamins. Vitamins are organic molecules that are needed by the body in minute amounts. Vitamins help your body grow, assist in the normal functioning of many metabolic processes and change food into energy. Vitamins are either water-soluble (vitamins B group and C) or fat-soluble (vitamins A, D, E and K). Your body cannot store water-soluble vitamins and as such they must be eaten every day. If these vitamins are not used immediately by your body they are lost in the urine. Fat-soluble vitamins are stored within your fatty tissue, but if you eat too much of certain vitamins they can accumulate and eventually become harmful. Table 2.4 shows some of the essential vitamins that need to be included in your diet to ensure normal healthy cell functioning.

bio B Y T EControlled supplements of vitamin K

may help to retain calcium in the bones during menopause. Newborn babies are given an injection of vitamin K to help prevent haemorrhagic disease. (Haemorrhagic disease is a condition where excess bleeding occurs either internally, in any part of the body, or from an open wound.)

Vitamin A toxicity results from an excessive intake of the vitamin. The amounts ingested need to be

well above the recommended daily intake and occur over several months. As vitamin A is fat-soluble, it

is not excreted in the urine but, instead, is stored in the liver, where it may reach toxic levels.

The signs and symptoms of vitamin A toxicity include nausea, loss of hair, drying and scaling of

skin, bone pain, fatigue and drowsiness. Blurred vision and headaches along with growth failure and

enlargement of the liver can also be observed.

Xavier Mertz accompanied Douglas Mawson on the 1911 Australasian Antarctic Expedition.

When food supplies ran out, Xavier killed and ate the husky dogs who had been pulling the sleds.

Consumption of too many husky livers led to vitamin A toxicity and eventually to Xavier’s death.

Today, over-consumption of vitamin A can result from the ingestion of unsuitable amounts

of synthetic vitamin preparations. This can lead to a yellowing of the skin, a condition known as

carotenaemia.

Define a vitamin excess.

Explain why a large intake of vitamin A can lead to it building up to toxic levels in the body.

List the symptoms of vitamin A toxicity.

BIOBOX 2.1MERTZ OF THE ANTARCTIC

BIOLINK

Icy explorers

37

Chapter 2 The chemical composition of cells

Table 2.4 Vitamins – their source and functions.

Vitamin Source Function

Fat-soluble vitaminsA (retinol)

Fish-liver oils, butter and margarine, green and yellow vegetables, carrots, yellow-fleshed fruits, tomatoes, egg yolk, liver and kidneys, whole milk

Epithelial tissues – skin, linings of nose, mouth, digestive and urinary tracts; vision in dim light – forms visual purple in retina of the eye

D (calciferol) Liver, eggs, and can be made by the body

Stimulates absorption of calcium and phosphorus in bone and teeth formation

E (tocopherols) Wheat germ, butter and margarine, bread, green leafy vegetables, whole-grain products

Prevents damage to cell membranes; protects fats and vitamin A from destruction by oxidation

K (phytomenadione) Green vegetables, tomatoes, cabbage, cauliflower, potatoes, cereal, eggs

Formation of prothrombin, essential for blood clotting

Water-soluble vitaminsB1 (thiamine)

Seafood, meat, whole-grain products

The release of chemical energy from carbohydrates

B2 (riboflavin) Meat, eggs, green vegetables, mushrooms, whole-grain products, pasta

Protein metabolism; helps maintain healthy skin and eyes; essential for growth of new body tissue

B3 (niacin) Leafy vegetables, whole-grain products, peanut butter, potatoes, tuna, eggs

Enzyme systems that convert carbohydrates, proteins and fats into energy; aids synthesis of hormones

B6 (pyridoxine) Yeast, wheat germ, cereal, whole-grain products, liver, meat, soybeans, peanuts, egg yolk

Many enzyme reactions, including protein and fat metabolism, the conversion of tryptophan to niacin, and for development of red blood cells

B12 (cyanobalamin) Liver, meat, eggs, oysters, sardines Development of red and white blood cells; also involved in protein, fat and carbohydrate metabolism

Folic acid Liver, kidneys, yeast, mushrooms, leafy green vegetables

Development of red blood cells and metabolism of protein; linked with vitamin B12

Biotin Liver, meat, fish, egg yolk, milk, smaller amounts in grains and vegetables

Metabolism of fat and protein; growth and function of nerve cells

Pantothenic acid Liver, meat, fish, egg yolk, yeast, cereals, peanuts and vegetables

Metabolism of carbohydrate, fat and protein

C (ascorbic acid) Citrus and other fruits, tomatoes, leafy vegetables, potatoes, rosehips

Connective tissues, bones, teeth; promotes wound healing and absorption of iron

38 Unit 1

MineralsMinerals are inorganic compounds present in the food we eat and incorporated into many structures of the body, such as bones, teeth and blood. Over 20 minerals are required by the human body (see Table 2.5).

Pellagra is a nutritional disorder caused by a deficiency of vitamin B3 (niacin). Symptoms of the

disease include dermatitis, diarrhoea and dementia. Dermatitis is the first symptom seen. Skin lesions

occur as a result of an increased sensitivity to sunlight. Gastrointestinal symptoms usually consist of

alternate constipation and diarrhoea. These symptoms can be accompanied by inflammation of the

mouth and tongue. The dementia or neurological symptoms are the last to appear and may include

nervousness, confusion, depression, apathy and delirium. Niacin deficiency is treated effectively with

vitamin supplements and recovery is quick. Pellagra is usually a disease seen in poorer nations, and,

in more affluent societies, the disease may result from chronic alcoholism. In developed nations all

commercial breads are fortified with niacin to assist in preventing this disease.

List the symptoms of pellagra.

How can pellagra be prevented?

BIOBOX 2.2PELLAGRA

BIOBOX 2.3TYPES OF MINERALS

bio B Y T EChronic iron toxicity can occur in

people who readily consume alcoholic beverages brewed in unlined iron vessels, such as beer. The iron in the vessel dissolves in the beverage and, when ingested, is absorbed and deposited in the liver. The condition is known as haemosiderosis. It may also occur as a result of cooking in iron pots.

The term ‘mineral’ was used in the 19th century to describe the inorganic nutrients left in ash after

plant or animal matter was burned. Now we know that minerals are really inorganic ions needed for

life. Organisms need some minerals in relatively large amounts. This is the case for calcium (Ca2+),

iron (Fe2+ or Fe3+), magnesium (Mg2+), potassium (K+), sodium (Na+), chloride (Cl–), nitrate (NO3–),

phosphate (H2PO4–) and sulfate (SO

42–). These ions are referred to as major mineral elements.

Some minerals are needed in smaller amounts and are called trace elements or micronutrients.

They include cobalt (Co2+), copper (Cu2+), manganese (Mn2+), molybdenum (generally found as

MoO4

2–) and zinc (Zn2+). However, there is no hard and fast dividing line between major mineral

elements and trace elements. Further, some trace elements needed by animals are not required

by plants, and vice versa. For instance, iodine (I–) is an essential trace element for many animals,

including ourselves, but does not seem to be required by plants.

39

Chapter 2 The chemical composition of cells

Table 2.5 Mineral elements required by humans.

Element Functions Effects of deficiency Sources

Macro elements

Sodium Major extracellular ion, maintains body fluid concentrations, carries nerve impulses

Cramp in muscles, loss of appetite, impeded brain function

All foods, table salt

Potassium Major intracellular ion, maintains body fluid concentrations, carries nerve impulses

Lethargy, muscular weakness Fruits, vegetables, legumes, whole grains, meat

Chlorine Maintains body fluid concentration, aids digestive juice formation

Cramp in muscles, loss of appetite Table salt, all foods

Calcium Bone and tooth formation, role in muscle contraction and cell function

Stunted growth, rickets, convulsions Milk and other dairy products, whole grains, legumes, nuts, whole fish (sardines)

Phosphorus Bone and tooth formation, part of many cellular compounds

Loss of calcium from bone, weakness, poor growth

Milk and other dairy products, meat, whole grains, legumes, nuts

Magnesium Involved in many cellular functions Reduced growth, mental depression, muscular weakness, sometimes convulsions

Milk and other dairy products, meat, seafood, whole grains, legumes, nuts

Sulfur Involved in cellular functions, part of the essential amino acid methionine

Reduced growth Animal and plant proteins

Micro elements

Iron Component of haemoglobin and some respiratory enzymes

Anaemia (lack of haemoglobin in blood), reduced resistance to infection

Liver, meat, eggs, whole grains*, dark green vegetables*, legumes

Iodine Component of thyroid hormones Goitre, cretinism Seafood, iodised salt

Copper Component of melanin and haemoglobin

Anaemia, bone disorders Seafood, liver, meat, legumes, whole grains

Zinc Component of digestive enzymes Retarded growth, reduced resistance to infection, skin disorders

Seafood, oysters, liver, meat, eggs, whole grains, legumes, nuts

Selenium Component of selenium enzyme Reduced growth in young animals, liver and muscle damage

Seafood, whole grains*, legumes*, meat

Chromium Helps maintain glucose concentration in the body

Excess glucose in blood Whole grains, brewer’s yeast, meats

Nickel Involved in membrane structure, helps in absorption of iron

Reduced growth, anaemia Legumes, whole grains

Silicon Cross-linking agent in matrix of early bone

Reduced growth and skeletal development

Whole grains, nuts, eggs

Manganese Activation of enzymes Reduced growth and fertility, bone and neurological disorders

Whole grains, legumes, leafy vegetables

Molybdenum Component of molybdenum enzymes Reduced growth and uric acid synthesis

Legumes, whole grains, milk, organ meats, leafy vegetables

Fluorine Accumulates in tooth enamel and bones Increased tooth decay Fish, tea, fluoridated water

* Vary with soil.Source: Australian Academy of Science, The Common Threads, Part 1, Canberra: AAS, 1990: 198.

bio B Y T EChronic iron toxicity can occur in

people who readily consume alcoholic beverages brewed in unlined iron vessels, such as beer. The iron in the vessel dissolves in the beverage and, when ingested, is absorbed and deposited in the liver. The condition is known as haemosiderosis. It may also occur as a result of cooking in iron pots.

R E V I E W5 How do you tell a fat from an oil?

6 Why are lipids also called triglycerides?

7 If you wanted to cook chips at a high temperature, would it be

best to use lard (a saturated fat) or sunflower oil (an unsaturated

fat)? Explain your answer.

8 Explain why it is important to include some fat in a healthy diet.

9 Why do vitamins need to be eaten in small amounts?

10 If a person was suffering from anaemia, which mineral(s) would

they be lacking in and what would be the effects?

11 List all the vitamins and minerals you would obtain by eating

whole grains.

40 Unit 1

C

H

O

CO H

NH

H

R

Proteins in cellsYour body produces over 1 million new red blood cells each second. A range of different proteins is found embedded within the membranes of these cells. Channel proteins, for example, penetrate through the membrane and select substances to pass through the membrane. This means that your existing cells need to produce proteins for 1 million plasma membranes and organelle membranes each second.

Like lipids, proteins are made up of the elements carbon, hydrogen and oxygen, but they are different to these compounds in that they always contain nitrogen. In addition, sulfur is often present, and sometimes phosphorus and other elements.

These elements combine to form building blocks called amino acids. There are over 20 different types of amino acids and they join together to form a protein or polypeptide. It is the order and number of amino acids that make different types of protein. The ordering of amino acids in proteins is determined by the genes in our chromosomes.

Your body produces most of the amino acids required for protein production, but you must include eight essential amino acids in your diet. Failure to do this will result in several protein-deficiency diseases.

Essential Non-essential

IsoleucineLeucineLysineMethioninePhenylalanineThreonineTryptophanValineHistidine (for children)

CysteineTyrosineGlycineArginineProlineGlutamic acidAspartic acidSerineAlanineAsparagineGlutamine

Table 2.6 Essential and non-essential amino acids.

bioTERMSproteinslarge organic molecules, containing nitrogen, essential to the structure and function of living things

amino acidsnitrogen-containing compounds that are the building blocks of all proteins

Figure 2.6 Chemical diagram of an amino acid where the R group can represent a number of different chemicals. Many amino acids join together to form a polypeptide.

41

Chapter 2 The chemical composition of cells

channel protein open channel protein closed

high concentration

concentration gradient

low concentration

Regulating movement – transport proteinsMembrane proteins control movement of substances into and out of organelles and between the cell and its external environment.

Many chemicals are unable to move directly through the plasma membrane. Glucose, for instance, is a soluble compound and, as such, fails to pass through the phospholipid layer of a membrane directly. How do substances such as water get through the membrane? The answer lies in special channels made of transport proteins that act like gates, facilitating movement across the membrane.

BioBox 2.4PKU – a Protein disease

Figure 2.7 Channel protein.

Phenylketonuria (PKU) is a genetic disease of metabolism. It is detectable during the first days of life with appropriate

blood testing (every newborn in Australia is screened for PKU). The disease is characterised by the absence or deficiency

of an enzyme that is responsible for processing the essential amino acid, phenylalanine. In people not suffering from

PKU, phenylalanine is converted to another amino acid (tyrosine), which is then used by the body. However, when the

enzyme that breaks down phenylalanine (hydroxylase) is absent or deficient, phenylalanine accumulates in the blood

and can rise to such a level that it is toxic to brain tissue.

Without treatment, most infants with PKU develop mental retardation. To prevent this happening, treatment

consists of a carefully controlled diet restricting foods that contain phenylalanine. This diet is begun during the

first days or weeks of life. Most experts suggest that this diet should continue lifelong. Foods that are high in

phenylalanine and therefore need to be excluded from the diet include cheese, eggs, fish, meat, almonds, peanuts,

chocolate and any food containing wheat flour.

42 Unit 1

Structural proteinsOther proteins, called structural proteins, link membranes, cytoplasm and the nucleus, allowing for communication. Structural proteins such as the insoluble fibrous protein keratin are found in hairs, feathers, nails, hooves and horns. Another fibrous protein, collagen, is the most abundant protein in vertebrates, making up a third of their total protein mass. Humans are largely held together by collagen as it is found in bones, cartilage, tendons, ligaments, connective tissue and skin. Collagen is even found in the cornea of the eye. Collagen fibres have a tensile strength greater than that of steel.

Controlling metabolism – enzymesA cell is essentially a busy chemical factory. Because of the membrane-bound organelles, many different types of reactions can happen in different parts of the cell at the same time. Different types of reactions happen in different types of cells. What is it that controls the type and duration of these reactions?

Enzymes are one of the most important groups of proteins. Without enzymes the reactions that occur in living organisms would be so slow as to hardly proceed at all; this would be incompatible with the maintenance of life.

Enzymes do more than merely speed up the reactions; they also control them. Over 1000 different reactions can take place in an individual cell. The functional organisation that this demands is achieved by a specific enzyme in a particular place within the cell acting as a catalyst for each individual reaction.

There are as many enzymes in living organisms as there are types of chemical reactions. They are divided into two broad groups: intracellular and extracellular. Intracellular enzymes occur inside cells, where they speed up and control metabolic reactions. Extracellular enzymes are produced by cells but achieve their effects outside the cell; they include digestive enzymes, which break down food in the gut (see Chapter 5).

Figure 2.8 Nails, horns and hooves are all made of the protein keratin.

BIOLINK

Enzyme activity

43

Chapter 2 The chemical composition of cells

The properties of enzymesEnzymes are nearly always proteins. The main properties of enzymes are given below:1 Enzymes generally work very rapidly. One of the fastest enzymes is catalase (the

name of an enzyme generally ends in –ase). This enzyme is found in several organs and tissues, including the liver, where its job is to speed up the breakdown of hydrogen peroxide (H

2O

2) into oxygen and water:

2H2O

2 2H

2O + O

2

Hydrogen peroxide is a waste of cellular metabolism. If it is allowed to accumulate it can be toxic. Its rapid conversion to water and oxygen is, therefore, important.

2 Enzymes are not destroyed or altered by the reactions that they catalyse, so they can be used again. This is not to say that a given molecule of an enzyme can be used indefinitely, for the action of an enzyme depends critically on its shape, which is readily affected by changes in temperature, acidity and so on.

3 Enzymes can work in either direction as metabolic reactions are generally reversible. The direction in which the reaction proceeds at any given time depends on the relative amounts of substrates and products present. For example:

A + B C + DSubstrate Products

If there is a lot of A and B compared with C and D, the reaction will go from left to right until an equilibrium between the substrates and products is reached.

4 Enzymes are affected by temperature and have an optimal range in which they operate. In mammals this is 35–40°C. Below this range, they do not function efficiently. Above this range, they are denatured; the enzyme changes shape so that it can no longer function efficiently.

The effect of temperature on the rate of an enzyme-controlled reaction is shown in Figure 2.9. Up to about 40°C the rate increases smoothly, a 10°C rise in temperature being accompanied by an approximate doubling of the rate of reaction. Above this temperature the rate begins to fall off and then declines rapidly, ceasing altogether at 60°C. The reason for this is that enzymes, being proteins, are denatured at high temperatures. Because of the susceptibility of enzymes to heating, few cells can tolerate temperatures higher than approximately 45°C. Organisms that live in environments where the temperature exceeds 45°C either have heat-resistant enzymes (Figure 2.10) or are able to regulate their body temperature. Figure 2.9 The effect of temperature

on the rate of an enzyme-controlled reaction. All other variables were kept constant.

Figure 2.10 The sooty grunter fish in Central Australia has enzymes that can function at high temperatures, thus enabling it to live in hot springs.

PRA

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AL ACTIVITY 2.2

bioTERMSsubstratessubstances that enter a reaction; also called reactants or precursors

productssubstances at the end of a metabolic reaction

denatureddescribes a protein, the structure of which has been altered so that it no longer functions in the way it was meant to

Temperature (°C)

Rate

of r

eact

ion

(mg

of p

rodu

cts

per

unit

tim

e)

1.0

2.0

3.0

4.0

10 20 30 40 50

STUDENT CD

Enzymes

R E V I E W12 Draw a concept map to illustrate what a polypeptide is made of,

and why it is an important part of a healthy diet.

13 Why are enzymes important for the maintenance of life?

14 Explain why there are thousands of different types of enzymes in

the human body.

15 What is the difference between an intracellular enzyme and an

extracellular enzyme?

16 List the main properties of enzymes. To what extent can each

property be explained by the lock-and-key mechanism?

17 Explain why doctors get worried if their patient develops a

temperature in excess of 42°C.

product molecules

substrate molecule

active site

enzyme molecule

enzyme-substrate complex

44 Unit 1

enzyme molecule

substrate molecule

substrate molecule enters the active site

enzyme molecule closes round the substrate molecule

5 Enzymes are sensitive to pH. Every enzyme has its own range of pH in which it functions best. Most intracellular enzymes function optimally around neutral (pH 7). Excessive acidity (less than pH 7) or alkalinity (more than pH 7) denatures them and makes them inactive. Digestive enzymes behave differently. The protease enzyme pepsin functions more effectively in an acidic environment, such as that found in the stomach, while trypsin functions effectively in an alkaline environment, such as that found in the duodenum (see Chapter 5).

6 Enzymes are usually specific to particular reactions. Normally, a given enzyme will catalyse only one reaction. This is generally the case with intracellular enzymes, which work on one particular substrate. Extracellular enzymes, such as pancreatic lipase, will digest a variety of fats.

Enzyme specificity also explains how organisms manage to digest proteins within a container made of protein. For example, the various proteases in the stomach digest the proteins in the food but not the proteins in the stomach wall.

We can explain the properties of enzymes by suggesting that, when an enzyme-controlled reaction takes place, the enzyme and substrate molecules become joined together for a short time to form an enzyme–substrate complex. The substrate molecules then react together and the end product leaves the enzyme. The enzyme, unchanged by the reaction, can be used again.

enzyme + substrate(s) enzyme–substrate complex enzyme + end product(s)

It is thought that each enzyme molecule has a precise place on its surface to which the substrate molecules become attached. This is called the active site. This model of enzyme action is known as the lock-and-key mechanism (see Figure 2.11).

The lock-and-key mechanism suggests that there is an exact fit between the substrate and the active site of the enzyme. However, recent research has

suggested that the active site may not necessarily be exactly the right shape to begin with. It is believed that when the substrate combines with the enzyme, it causes a small change to occur in the shape of the enzyme molecule, thereby enabling the substrate to fit closely into the active site. This is called the induced-fit hypothesis (see Figure 2.12).

Because of its specific shape, an enzyme will act only on one type of chemical reaction, making it proceed much faster than it normally would. It is the type of enzymes in a cell that ultimately control what functions the cell can carry out. Enzymes are said to be organic catalysts.

Figure 2.11 With the lock-and-key mechanism, the substrate fits into a specific active site on the surface of the enzyme, where the reaction takes place.

Figure 2.12 The induced fit hypothesis. The substrate molecule enters the enzyme’s active site, causing the enzyme molecule to change its shape so that the two molecules fit together more closely.

bioTERMSpH a measure of how acidic or alkaline a solution is

lock-and-key mechanisma model suggesting that the shape of a substrate molecule is an exact fit to the shape of an enzyme’s active site

organic catalystsenzymes

R E V I E W12 Draw a concept map to illustrate what a polypeptide is made of,

and why it is an important part of a healthy diet.

13 Why are enzymes important for the maintenance of life?

14 Explain why there are thousands of different types of enzymes in

the human body.

15 What is the difference between an intracellular enzyme and an

extracellular enzyme?

16 List the main properties of enzymes. To what extent can each

property be explained by the lock-and-key mechanism?

17 Explain why doctors get worried if their patient develops a

temperature in excess of 42°C.

45

Chapter 2 The chemical composition of cells

DenaturationWhen cells are exposed to high temperatures or an environment that is more or less acidic (outside its pH range) than normal, they often cease to function. This is because proteins within the cell change their three-dimensional shape. This change in shape, caused by the environmental conditions, is called denaturation. If the shape has changed enough to break bonds between the connecting units of amino acids, proteins cannot return to their original shape when conditions revert to normal. In this case, the protein is destroyed.

This has repercussions, both dangerous and useful, for us. If our body temperature rises too much during an infection, critical enzymes in our brain could denature, leading to seizures and possible death. On the other hand, we are able to chew and digest meat more easily after cooking. Raw meat is difficult to chew because of fibrous proteins contained in the muscle cells. By heating the meat, proteins are denatured, making it easier to chew and digest.

The cell wallPlant cells have an extra coating on their cells to provide the cell with protection and strength. This is the cell wall and it is made from cellulose. Cellulose belongs to a larger group of molecules known as carbohydrates.

CarbohydratesCarbohydrates are members of a large group of molecules that contain the elements carbon, hydrogen and oxygen. The carbohydrate glucose is the most common single sugar molecule and is known as a monosaccharide. It contains carbon, hydrogen and oxygen in the ratio 1:2:1. It has six atoms of carbon, 12 atoms of hydrogen and six atoms of oxygen; hence its chemical formula is C

6H

12O

6.

The original source of all glucose molecules is photosynthesis. As you saw in Chapter 1 photosynthesis occurs in the chloroplasts of plant cells. During this process, the inorganic molecules, carbon dioxide and water, are broken apart using light energy from the Sun and enzymes found in the thylakoid membrane system and stoma (see Figure 1.25) of the chloroplast. The atoms are rejoined to form glucose, water and oxygen.

Figure 2.13 Sugar, flour, rice and celery all contain large amounts of carbohydrates.

R E V I E W18 Draw a concept map to illustrate the chemical composition, production, building up and

breaking down of carbohydrates.

19 Where, and in what form, is excess carbohydrate stored in animals? Plants?

20 What chemical substances are found in the prokaryotic boundary?

bio B Y T ESugar that people buy for use in cooking is almost pure sucrose. Fructose, a

monosaccharide, is sweeter than sucrose and is often used in the manufacture of sweets and diet foods, as the same sweetness can be obtained for fewer calories. However, sugars are not the only compounds that are sweet. Saccharin is 500 times sweeter than sucrose, although chemically it is quite distinct from sugars. Certain proteins taste even sweeter than saccharin. One such protein is obtained from serendipity berries, the fruit of a West African plant. This protein is 2500 times as sweet as sucrose. New low-calorie sweeteners are being developed by the food industry. One is aspartame. This compound is 200 times sweeter than sucrose and lacks the slightly bitter aftertaste often found with saccharin.

46 Unit 1

Cellulose

If two monosaccharide molecules join together, they form a disaccharide (‘two sugars’). Sucrose or common table sugar is an example of a disaccharide.

If many monosaccharides join together, or two or more disaccharides join together, or two or more polysaccharides join together, they form a polysaccharide (‘many sugars’). Cellulose, found in plant cell walls, is composed of many glucose molecules joined together. The different linking patterns between the glucose molecules mean that cellulose fibres are tough, insoluble and resistant to being crushed and bent. Hence, they are ideal as the structural components of plant cell walls. The efficient functioning of plants is very much bound up with the properties of their tough, cellulose cell walls.

This toughness also means that when plant material is eaten by humans, the cellulose cannot be broken down easily. Humans do not produce the enzymes necessary to break down cellulose. It passes through our digestive system virtually unchanged. It is important to us in digestion though, as a source of fibre or ‘roughage’. Fibre assists by holding water in the large intestine, thereby aiding in defecation.

Starch is another example of a plant polysaccharide made up of amylose and amylopectin, two other polysaccharide molecules. They are linked together to form long chains. Some starches contain over 6000 glucose molecules and hence are perfect molecules to act as glucose storage. Most plants store excess starch in their roots and break it down into glucose when they require it for respiration. Potatoes, for example, are rich in starch.

Sugars move around in the phloem of the plant in the form of sucrose (a disaccharide) rather than glucose. Phloem, being made up of living cells, would use any available glucose for respiration. Being in the less reactive form of sucrose ensures that the sugar will reach other cells that need it.

Animals tend to store excess carbohydrate in the form of glycogen, a polysaccharide. Glycogen is very similar to starch except for some slight differences in the way that the polysaccharide molecule is bonded together (see Figure 2.15). Glycogen is stored in the liver and muscles and is converted back into glucose as the concentration of glucose in the blood begins to drop.

Figure 2.14 Cellulose is a polysaccharide. It is composed of many glucose molecules joined together.

+ +glucose glucose glucose

R E V I E W18 Draw a concept map to illustrate the chemical composition, production, building up and

breaking down of carbohydrates.

19 Where, and in what form, is excess carbohydrate stored in animals? Plants?

20 What chemical substances are found in the prokaryotic boundary?

47

Chapter 2 The chemical composition of cells

The prokaryotic boundaryAll prokaryotic cells are surrounded by a plasma membrane. On the outside of this is the cell wall. As in plant cells, the main function of the cell wall is to protect and provide strength. The prokaryotic cell wall is composed of peptidoglycan, which is composed of polysaccharides and protein. The peptide links between the proteins provide tremendous strength. When the bacterial cell divides by binary fission, the size of the cell wall must also grow. This is done when the links between the peptidoglycan are broken and new building blocks that are synthesised in the cytosol are inserted.

Around the cell wall is a polysaccharide, and sometimes protein, jelly-like capsule. This is often seen in disease-causing bacteria and offers protection to the cell. It is thought to stop phagocytosis by white blood cells.

Figure 2.16 Bacterial cell dividing by binary fission, causing more cell wall to be produced.

Figure 2.15 (a) Amylose is a polysaccharide starch (b) Glycogen molecule. Note the difference in branching to amylose.

a

b

R E V I E W21 If the water content of a cell drops below a certain point, the cell will die. Explain why this happens.

22 List the dissolved and suspended substances in the cytoplasm of a cell.

23 Give two reasons why water is necessary for efficient enzyme functioning.

48 Unit 1

CytoplasmThe mitochondria in actively respiring cells are in constant need of oxygen. The carbon dioxide that is produced needs to be removed from the area so that it does not build up and inhibit its diffusion.

The cytoplasm provides the fluid medium for substances to move about within the cell. Cytoplasmic streaming ensures that organelles are constantly provided with their requirements and any wastes or products that they are producing are removed. The cytoplasm is made up mainly of the inorganic molecule water (H

2O) in which other

molecules are either dissolved or suspended. Your cells are approximately 80% water. Cells will die if they drop below a certain percentage of water.

Water as a solventWater is unusual among small molecules by being a liquid at room temperature. It is a good solvent for most salts in that they dissolve in it readily. However, water is a poor solvent for proteins, lipids and other large organic molecules in living cells. This means that liquid water can transport nutrient elements to and within living cells without dissolving and destroying the organic molecules of which cells are made. Water also

transports waste elements away from cells through the extracellular fluid that bathes all cells.

Increased water content leads to increased chemical activity within the cell because all the chemical reactions that take place in cells do so in aqueous solution. Many chemical reactions require water as part of their reactants.

Temperature-stabilising effects of waterThe range of temperatures in which biochemical processes can proceed

is quite narrow, and most cells cannot tolerate wide variations in temperature. The presence of water helps keep the temperature of cells

fairly stable.

Water’s cohesionSwimming on a hot summer night can be refreshing, if you do not mind all the night-flying insects that hit the water’s surface and float about on it. Because of the attractive forces holding water molecules together, liquid water shows cohesive behaviour. It

resists rupturing when it is placed under tension. Surface tension is the resulting force that causes the surface of water to act as if it has an elastic skin. At ordinary temperatures, water has the highest surface tension of any known liquid except mercury, and this is of considerable biological significance.

Due to the strong cohesive forces, water is incompressible. Cells filled with water become swollen and distended and, because of the resulting rigidity, plants can support themselves.

Figure 2.17 Light micrograph of a section through cartilage.

Figure 2.18 A water strider supported by surface tension.

bioTERMSsolventa substance in which other substances can be dissolved, the most common being water

extracellular fluidfluid surrounding and bathing a cell

cohesivedescribes behaviour that causes resistance to rupturing when placed under tension

surface tensionbonds between surface molecules that stop or slow down molecules leaving or objects penetrating the surface

R E V I E W21 If the water content of a cell drops below a certain point, the cell will die. Explain why this happens.

22 List the dissolved and suspended substances in the cytoplasm of a cell.

23 Give two reasons why water is necessary for efficient enzyme functioning.

49

Chapter 2 The chemical composition of cells

The strong cohesive forces between water molecules play an important part in the movement of water up the xylem in the stems of plants. Were these forces much weaker, trees could not be so tall. Surface tension also allows the surface film of still water to support – and provide a habitat for – certain aquatic organisms such as the water strider pictured in Figure 2.18.

Freezing properties of waterMost liquids increase in density, and so decrease in volume, on solidifying. Water is most unusual in that the reverse is the case. Cells that freeze increase in volume so much they normally burst. So does the environment have to be higher than freezing point for a cell to survive?

It seems not, as nearly all the Antarctic continent is permanently covered with ice but quite a lot of ‘lower’ plant groups – like moss, fungi, liverworts – and lichens live there. Many marine invertebrates live in the below-freezing temperatures of the Antarctic marine environment. How is it that these organisms can survive the extreme cold of Antarctica?

To help answer this, we can look at how cars are designed to prevent damage to engines in freezing conditions. An automobile’s engine is cooled by jackets of water. In winter, water in these channels can freeze and expand. The expansion is so forceful it can crack an engine block. Antifreeze is a liquid solute (ethylene glycol) that dissolves easily in water. In solution and added to a car’s engine, it lowers the freezing point of water, preventing it from freezing and ruining the engine.

Glucose, like the antifreeze ethylene glycol, dissolves in water and depresses the freezing point of water. In cells, lowering the freezing point of the liquid protects it from freezing and ripping open with fatal results. Antarctic marine invertebrates deal with their situation by accumulating salts and organic compounds, such as glucose and amino acids, which lower the freezing point of the body fluids.

Figure 2.19 Moss growing at Bailey Peninsula, Antarctica.

50 Unit 1

R E V I E W24 The cytoskeleton is made up of the protein, actin. Explain why actin is a suitable molecule for

the cytoskeleton to be made of. (Hint: Think of what the function of the cytoskeleton is.)

25 What happens to the energy in glucose before it is utilised by the cell?

26 How does the structure of the mitochondrion increase enzyme efficiency?

27 List the molecules that you would expect to find in the endoplasmic reticulum.

CytoskeletonRed blood cells carry oxygen from our lungs to our cells. As they move through our smallest blood vessels, the capillaries, they are squashed and bent as they squeeze through the narrow tubes. When they reach a larger blood vessel, they return to their regular shape.

It is the cytoskeleton that provides the cell with structure and strength. But it cannot be so rigid that the cell cannot change its shape. The cytoskeleton must also allow for flexibility in cell shape.

The cytoskeleton is made up of microfilaments and microtubules. They are made of the protein actin. Actin is a globular protein that is joined together to form long filaments or tubes. There are capping proteins at either end of the rod and, depending on which end they occur, they either stabilise the filament or assist in its disassembly.

Powering the power supplyThe mitochondrion is a membrane-bound organelle and commonly known as the power supply of the cell. Inside the mitochondrion occurs a series of chemical reactions where glucose molecules combine with oxygen and are broken down into carbon dioxide and water. Along the way, energy is released from the chemical bonds in the glucose molecule. This energy is stored temporarily in a chemical called ATP (adenosine triphosphate). When a cell requires instant energy, it is the ATP molecule that is broken apart to supply it.

The rate of cellular respiration is dependent upon the specific enzymes present and the temperature in the cell. These enzymes are found on the infolded projections of the mitochondria called cristae (see Figure 1.14). These infoldings significantly increase the mitochondria’s surface area and hence the space for the enzyme to work. More enzymes working means a greater rate of cellular respiration and more energy is released for use by the cell.

In the case of animals and fungi the sugar molecules that are broken down by the mitochondria come from the food that the organism ingests. In the case of plants, the sugar is produced during the chemical process of photosynthesis. Glycogen or starch, both complex carbohydrates, store the sugar source (usually glucose) in animals and plants respectively.

Glucose is not the only source of energy in cellular respiration. Fats and proteins as well as other types of carbohydrates can be used. For some cells in our body, glucose is the only starting material. A good example is the brain. A significant drop in blood sugar levels can be fatal to the brain because the brain does not store carbohydrate and cannot fall back on either lipids or amino acids as energy sources.

Endoplasmic reticulumThe endoplasmic reticulum is a series of interconnected canals that transport material throughout the cytoplasm. The canals are composed of parallel membranes that are made up of phospholipid bilayers (see page 14). Rough endoplasmic reticulum is studded

bioTERMSATP (adenosine triphosphate)the main energy-carrier molecule in cells, produced during cellular respiration

PRA

CTIC

AL ACTIVITY 2.3

51

Chapter 2 The chemical composition of cells

R E V I E W24 The cytoskeleton is made up of the protein, actin. Explain why actin is a suitable molecule for

the cytoskeleton to be made of. (Hint: Think of what the function of the cytoskeleton is.)

25 What happens to the energy in glucose before it is utilised by the cell?

26 How does the structure of the mitochondrion increase enzyme efficiency?

27 List the molecules that you would expect to find in the endoplasmic reticulum.

with ribosomes, which produce proteins. The endoplasmic reticulum in the cells of the pancreas, for example, transport digestive enzymes. Smooth endoplasmic reticulum has no ribosomes, and produces lipids and fats.

Golgi apparatus and lysosomesThe Golgi apparatus looks like a series of flattened balloons, with the outer membrane being made up of a phospholipid bilayer (see page 15). As materials move from the endoplasmic reticulum into this structure, the ends pinch off to form smaller vesicles filled with proteins, mainly enzymes. These vesicles move towards the plasma membrane and, through the process of exocytosis, join with the plasma membrane to release their contents to the outside.

Nucleus and nucleic acids Many television programs now feature crime scenes where the victim is identified, or the killer is found, through the process of DNA analysis. DNA is deoxyribonucleic acid, a compound found mainly in the nucleus of eukaryotic cells. In prokaryotic cells, the DNA is found floating freely in the cytoplasm or in small rings called plasmids.

DNA belongs to a larger class of organic molecules called nucleic acids. Also found in this class is RNA (ribonucleic acid).

DNA is made up of sugars, phosphate groups and four bases containing nitrogen – adenine, guanine, cytosine and thymine. A sugar, a phosphate group and a base are bonded together to form a subunit called a nucleotide. But how can this help forensic scientists identify one person from another?

The nucleotides are joined together by the sugar and phosphate groups to form a long string (see Figure 2.20). Two strings are twisted around each other and held together by the hydrogen bonding of the bases – adenine always bonds with thymine and cytosine always bonds with guanine (see Figure 2.20). This is called a double helix. It is the order in which these bases occur that is important. This order is unique to individual people, unless they are one of identical siblings.

A segment of DNA is called a gene, and a gene is a chemical code for a protein, usually an enzyme. It is the enzymes that determine which, and at what rate, specific chemical reactions occur in cells. An example of this is the segment of DNA which forms the gene that codes for the enzyme which produces brown pigmentation in skin cells. This results in an organism having brown skin.

DNA is a large molecule and unable to move through the nuclear membrane. How then do the instructions for protein production coded into DNA get to the

bioTERMSplasmidssmall, circular molecules of extra bacterial DNA

RNA (ribonucleic acid)a single-stranded nucleic acid that functions in transcribing and translating information from DNA into proteins

BIOLINK

Golgi apparatus

52 Unit 1

Figure 2.21 Two colourful chameleons.

ribosomes where the protein is produced? The answer lies in the RNA. RNA also comprises chains of the nucleotides adenine, cytosine and guanine, but contains uracil instead of thymine. Unlike DNA, RNA is usually a single chain of unpaired nucleotides, and therefore a much smaller molecule. RNA is able to move through the nuclear membrane and is found in both the nucleus and cytoplasm. It has the job of taking information encoded in DNA to the protein synthesising organelles, the ribosomes. You will learn more about DNA and RNA in Chapter 6.

Colouring our world – pigmentsIt is the pigments in animal cells and plant cells that provide us with the colourful world we see around us. The beautiful green of the leaves is due to the structure of the plant pigment, chlorophyll. Chlorophyll contains the elements carbon, hydrogen, oxygen, nitrogen and magnesium. It is very similar in structure to the haem group found in haemoglobin in red blood cells (see Chapter 5).

Chlorophyll absorbs the red and violet wavelengths of the electromagnetic spectrum most strongly. Green light is absorbed poorly and thus reflected back to our eyes.

Figure 2.20 A length of the double helix of a DNA molecule. The bases are bonded together by hydrogen bonds.

bioTERMSnucleotidesorganic compounds composed of a sugar, a phosphate group and a nitrogenous base. Subunits of DNA and RNA

electromagnetic spectrumconsists of electromagnetic waves ranging from a long wavelength such as radio waves to a short wavelength such as X-rays and gamma rays; visible light is a small part of this spectrum

R E V I E W

53

Chapter 2 The chemical composition of cells

Chlorophyll also contains the carotenoid pigment. Carotenoids absorb the blue wavelengths, therefore allowing chlorophyll to utilise a larger range of the light spectrum. In leaves, the carotenoids are usually masked by chlorophyll, but in autumn the chlorophyll begins to break down and the red and yellow of the carotenoids shows through as autumn foliage.

Biochromes are microscopic pigments that are found in animal cells. Their chemical make-up determines which wavelengths of light they absorb and which they reflect. The colour of the pigment is determined by the combination of reflected and absorbed wavelengths.

Non-cellular materialLiving things are made up not only of cells, but also of materials produced by cells that are themselves non-cellular.

Hair is not made up of cells, although it is produced from the cells in the hair follicle. Hair is made of the protein keratin, as are toenails, fingernails, horns and hooves.

One of the main problems faced by terrestrial plants is water loss. To combat this, many plants have leaves that are coated in wax. Wax is made up of lipid (see page 35), and as such is insoluble in water. This makes it a suitable material to stop water loss through the leaf surface.

Figure 2.22 Plant leaves are coated in wax, which is insoluble in water.

28 Where in a cell would you expect to find DNA and RNA?

29 Explain why DNA is a suitable molecule to contain a chemical code.

30 How do chlorophyll and carotenoid pigments combine to enhance photosynthesis in leaves?

31 ‘Living things are made up of cells and the products of cells.’ Explain what this means.

54 Unit 1

Visual summary

hooves

Cellular Structures

nucleic acids

polysaccharides

starch

storage in plant cells

nucleus

wax

found in cell walls

storage in animal cells

cellulose

glucose

sucrose glycogen

DNAtransport proteins monosaccharides

disaccharides

RNAenzymes

proteins

carbohydrates

water solublestructural proteins amino acids

fat solublesaturated

Golgi apparatusfats and oils

Chemical Composition

of Cellsendoplasmic reticulumunsaturated

triglycerides

mitochondrionminerals

cytoskeletonwater

Inorganic Compounds

cytoplasm

plasma membrane

fingernails and toenails hair horns

Non-cellular Structures

lipids

vitamins

Organic Compounds

products of cells

55

Chapter 2 The chemical composition of cells

Key termsactin glycogen polysaccharide

active site hydrophilic polyunsaturated

ATP hydrophobic products

amino acid inorganic protein

biochromes intracellular enzymes RNA

carbohydrates keratin saturated

cholesterol lipid solvent

cohesive lock-and-key mechanism starch

collagen minerals steroids

denatured monosaccharide substrates

disaccharide nucleotides surface tension

electromagnetic spectrum organic triglyceride

enzyme–substrate complex organic catalysts unsaturated

extracellular enzymes pH vitamins

extracellular fluid phagocytosis

fatty acids phospholipid bilayer

glycerol plasmids

Apply understandings Match up list A (simple molecule) with list B

(complex molecule) and list C (examples).

List A List B List C

Fatty acids and glycerol Polypeptide Keratin

Amino acids Polysaccharide Steroids

Glucose Lipids Cellulose

Explain the difference between the concepts in the following groups:a monosaccharide, disaccharide,

polysaccharideb substrate, product, enzymec organic, inorganicd saturated fat, unsaturated fat.

Give three reasons to explain how a potato and a liver are similar both in structure and function.

A friend of yours decides to cut all fats out of her diet. What advice would you give her?

The pH of human blood and body fluids (excluding the gastric juices) is around 6.8–7.0. Explain why maintaining this level of pH is important.

Figure 2.23 shows the effect of substrate concentration on the rate of an enzyme-controlled reaction. The temperature was kept constant. a Use the lock and key theory to explain

why the rate of reaction levels out although the substrate concentration increases.

56 Unit 1

b Draw in another graph to show what would happen if you doubled the amount of enzyme present.

Figure 2.23 The effect of substrate concentration on the rate of an enzyme-controlled reaction.

Investigate and inquire The effect of anaesthetics on cells is very

quick. Find out whether anaesthetics are water-soluble or lipid-soluble chemicals. Suggest how they would best move through the plasma membrane.

Organisms such as the bacterium Thermophilus can survive in hot springs at about 80°C. Use reference material to find out why some enzymes are more heat stable than others.

Rate

of r

eact

ion

Concentration of substrate