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Unit 1 – Biology and disease

3.1.2 The digestive system provides an interface with the environment. Digestion involves enzymic hydrolysis producing smaller molecules that can be absorbed and assimilated.By the end of this topic I will know:-

The structure of carbohydrates

The biochemical tests for carbohydrates

The structure of protein

The biochemical test for protein

Role of enzymes

The gross structure of the human digestive system limited to oesophagus, stomach, small and large intestines

and rectum. The glands associated with this system limited to the salivary glands and the pancreas.

Digestion is the process in which large molecules are hydrolysed by enzymes to produce smaller molecules

that can be absorbed and assimilated.

Organic Molecules – Carbohydrate and ProteinMany organic molecules including proteins and carbohydrates, are made up of a chain of

individual molecules. Biological molecules contain few chemical elements, mainly Carbon,

Hydrogen, oxygen and nitrogen.

Atoms of these elements are joined together to form molecules. A simple molecule is called a

monomer and when monomers are joined together the chain is called a polymer (poly =

many)

Two monomers join together in a condensation reaction – this removes a molecule of water

To separate a monomer from a polymer chain a hydrolysis reaction occurs – a molecule of

water is used (hydro = water, lysis = splitting)

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MonomersPolymer


CarbohydratesCarbohydrates are all made from Carbon, Hydrogen and Oxygen. Carbohydrates are carbon

molecules (carbo) joined with water (hydrate).

There are 3 main groups of carbohydrates

1. Monosaccharides - made of 1 molecule e.g. glucose, fructose

2. Disaccharides - made of 2 monosaccharides joined together e.g. sucrose

3. Polysaccharides - made of many monosaccharides joined together e.g. starch,

cellulose

1 MonosaccharidesThese are simple sugars that have a general formula (CH2O) n

Monosaccharides are grouped according to the n value

If n=3 are called triose sugars; If n=5 are called pentose sugars; If n = 6 are called hexose

sugars

Hexose Sugars (n=6) ( C H 0 )These include Glucose and Fructose

Glucose can be found in 2 forms - glucose and glucose (found in plants- not needed in this

module)

Alpha () glucose has the -OH group BELOW C1

Simple version

Alpha () glucose is used to form starch

Glucose is used in the production of energy in respiration and is also the building block of

substance e.g. starch, glycogen

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Other hexose sugars include Fructose and galactose. These are isomers of glucose i.e. it has

the same molecular formula as glucose but are structurally different.

2 Disaccharides

When two monosaccharides join together they form a disaccharide. The type of reaction

involved is a __________________ reaction. What molecule is removed during this reaction?

The bond that holds the two monosaccharides together is called a glycosidic bond

Maltose – made from two glucose molecules.

The glycosidic bond is formed between carbon 1 of one glucose and carbon 4 of the other

glucose

Formation of Maltose

What is the formula of maltose?

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When water is added to a disaccharide under suitable onditions it breaks the glycosidic bond

releasing the monosaccharides. This is called hydrolysisThe hydrolysis of maltose

Other disccharides include:

1 glucose + 1 Fructose =

1 galactose + 1 glucose =

3 Polysaccharides

Polysaccharides are long chains (polymers) of many monosaccharides joined together by

glycosidic bonds in _________________________ reactions. Polysaccharides are very long

molecules and are insoluble. This makes them suitable for storage – starch is found in plants

as small granules or grains, glycogen is found in the liver of animals.

Starch is one of the most important sources of energy in the human diet. It makes up to 30%

of what we eat. Starch is a mixture of two substances, amylose and amylopectin. Both of

these are polymers made from a large number of -glucose molecules joined together

in a condensation reaction

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Amylose is a long chain of -glucose molecules and the chain is coiled into a spiral – its coils

are held in place by hydrogen bonds.

Amylopectin is also a polymer of -glucose molecules but its molecules are branched .

When they are hydrolysed they are broken down to disaccharides or monosaccharides.

(Copy Fig 2 Page 23)

Some polysaccharides are not used for storage – cellulose is a polysaccharide that is used in

plant cell to give structural support (in cell wall)

(More on polysaccharide structure in Modules 2)

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Biochemical tests for carbohydrates:

Identification of Reducing SugarsAll monosaccharide sugars and certain disaccharides will reduce soluble blue copper (II) sulphate,

containing copper (II) ions (Cu2+) to produce an insoluble red precipitate of Copper (I) oxide on

heating. Reduction is a chemical reaction involving the gain of electron. Reducing sugars are sugars

that donate electrons or reduce another chemical – in this case Benedict’s solution. Such sugars are

known as reducing sugars and a suitable reagent with which to test them is Benedict’s reagent. This

contains copper (II) sulphate containing copper (II) ions (Cu2+) which will change to insoluble red -

brown copper oxide containing copper (I). The latter is seen as a precipitate.

EquipmentSolutions to test :-

Apple – cube of apple.

Benedict’s solution

Bunsen burner, test tubes, beakers

Method:

Place 5cm3 of testing solution into a test tube Add an equal amount of Benedict’s solutions

and bring to the boil by heating the test tube in a water bath.

Record your results for each solution

The colour and density of the precipitate gives a rough indication of the amount of sugar

present. A green precipitate means relatively little sugar: a brown or red precipitate means

more sugar.

Grind up the cube of apple in a mortar with a small quantity of water. Transfer the material to

a test tube, add water to make the volume of liquid approximately quarter of a test tube. Add 5

cm3 of Benedict’s solution and heat in the water bath

Record your results

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Table 1 below shows the relationship between the concentration of reducing sugar and the

colour of the solution and precipitate formed during the Benedict’s test. The difference in

colour mean that the Benedict’s test is semi-quantitative, that is it can e used to estimate the

amount of reducing sugar in the sample.

The Benedict’s test was carried out on 5 food samples. The results are shown in the table 2

Table1 Table2

Concentration of reducing sugar

Colour of solution and precipitate

Sample Colour of solution

None Blue A Yellowish brown

Very low Green B Green

Low Yellow C Red

Medium Brown D Dark brown

high red E Yellowish green

Place the letters in sequence of the increasing amount of reducing sugar in each sample

___________________________________________________________________________

Suggest a way, other than comparing colour changes, in which different concentrations of

reducing sugar could be estimated.

___________________________________________________________________________

___________________________________________________________________________

Sucrose is a disaccharide which does not reduce copper sulphate. It is called a non-reducing sugar Method

1. Add 2 cm3 of sucrose solution to a boiling tube.

2. Add 1 cm3 dilute Hydrochloric acid. Boil for 1 minute.

3. Carefully Neutralise the solution by gently adding small amounts of solid sodium

hydrogen carbonate until it stops fizzing (Care is required because effervescence

occurs)

4. Carry out Benedict’s test as before.

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Basis of test:

Disaccharide can be hydrolysed to it’s monosaccharide constituents by boiling with dilute

hydrochloric acid. Sucrose is hydrolysed to glucose and fructose, both of which are reducing

sugars and give the reducing sugar result with the Benedict’s test.

Starch (iodine test). To approximately 2 cm³ of test solution add two drops of

iodine/potassium iodide solution. A blue-black colour indicates the presence of starch as a

starch-polyiodide complex is formed. Starch is only slightly soluble in water, but the test works

well in a suspension or as a solid.

You need to know the three tests for carbohydrates

ProteinA protein is a polymer made up of amino acids – there are 20 different amino acids found in

proteins and the order that they are arranged determines the type of protein that is formed and

the function that the protein will have.

Amino acids

Amino acids are composed of a central carbon atom attached to

An amino group (-NH3) ( a basic group)

A carboxyl group (-COOH) (an acidic group)

An R group which is different in each of the 20 amino acids that commonly occur in

natural proteins

Hydrogen atom (-H)

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DipeptideWhen two amino acids are joined together a dipeptide is produced. This is a condensation

reaction and the bond connecting the two amino acids is called a peptide bond.

Polypeptides and proteinsLarge numbers of amino acids can join together to long chains called polypeptides and

proteins. This is known as polymerisationA polypeptide is a long chain of amino acids joined together by peptide bonds

Proteins contain one or more polypeptides that are folded into a complex shape

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Proteins – make notes on each of the 4 stages (include relevant diagrams) Protein structure has four stages:

Primary structure

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Secondary structure

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Tertiary structure

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Quaternary structure

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____________________________________

____________________________________

____________________________________

The final three-dimensional shape of a protein

can be classified as :- globular or fibrous

10 Globular fibrous


The vast majority of proteins are globular, including enzymes, membrane proteins, receptors,

storage proteins, etc.

Fibrous proteins look like ropes and tend to have structural roles such as collagen (bone),

keratin (hair), tubulin (cytoskeleton) and actin (muscle). They are usually composed of many

polypeptide chains.

Protein testTo about 2 cm³ of test solution add an equal volume of biuret solution, down the side of the

test tube. A blue ring forms at the surface of the solution, which disappears on shaking, and

the solution turns lilac-purple, indicating protein.

Questions1 Describe how you would perform a test to check for the presence of starch (2)

2 The diagram below shows a molecule of maltose.

This molecule can be broken into two glucose molecules by a chemical reaction

(a)(i)What type of reaction would this be? (1)

(ii)What substance would need to be added for this reaction to proceed? (1)

b) Draw one of the glucose molecules that would be formed by this reaction (2)

c) Describe how you would perform a test to indicate that a substance contained sucrose and not

glucose (2)

3 The diagram below shows two amino acids

+

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a) Draw the result of these two amino acids forming a dipeptide

b) Name the chemical reaction used to form the dipeptide

c) Name the type of bond which links the two amino acids in the dipeptide

d) State what the R shown in the amino acid diagrams refers to

e) Describe how you perform a biochemical test to identify proteins

Enzymes

Enzymes are globular proteins that catalyse chemical reactions in living organisms.

Each enzyme is specific – this is due to the 3-D shape of the enzyme which is complementary

to the reacting molecules or substrates

The position on the enzyme that the substrate binds to is called the active site.

Enzyme molecules and substrate molecules in a solution, as in a cell, are constantly moving

around. They must collide with each other before they will react. Collisions are random. The

more collisions that take place the greater the rate of reaction

Enzymes remain unchanged at the end of a reaction and therefore can be used again

In a reaction energy is needed to break chemical bonds so that new ones can be formed.

Read page 30 and 31

Q What is meant by the term activation energy?

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Q How does supplying an enzyme to a reaction help increase the rate of a reaction (include

graph diagrams to explain)Fig 1 Page 30

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___________________________________________________________________________

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How enzymes work

The active site is the part of the enzyme into which a substrate fit, forming an enzyme-substrate complex. The shape of the active site is different between enzymes, so each

enzyme is specific to a particular substrate.,

When the complex has formed, a reaction takes place and the products are released from the

active site leaving the enzyme free to combine with another substrate molecules

Two models have been put forward to explain how enzymes work

1. The lock and key hypothesis

2. The induced fit hypothesis

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Describe both hypothesis and use diagrams to explain each (page 60/61)

Enzyme reactions

The enzyme combines reversibly with the substrate to form an enzyme-substrate complex

The enzyme substrate complex then breaks down to give the product and releases the

enzyme in an unchanged form

Factors that affect enzyme reactions

An enzyme-controlled reaction depends on substrate molecules fitting into the active site of the enzyme. Environmental factors which affect the ability of the active site to combine with the substrate will alter the rate of reaction These factors are

temperature pH Concentration of enzyme concentration of substrate inhibitors

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It is important that you do not say that the substrate has the same shape as the active site. It is more precise to say that the shape of the substrate is complementary to that of the

active site.


1 TemperatureCopy graph (page 34) and make notes on what happens when the temperature increases

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Not all enzymes have an optimum temperature of 40oC. Make sure you read the information given in the question or look at the data carefully. When giving a description of the changes shown in a graph always use the information on the axes. Never say only that it is increasing or decreasing When giving an explanation give the detailed biology of why it is happening If given the name of the enzyme or substrate or both, use them in your answer Never say the enzyme is killed Denaturation is only one word == 1 mark. Usually a question will have 2 or 3 marks – so some detail of what is happening during denaturation is required


2 Effect of pH on enzyme activity

What is pH?_____________________________________________________________________Enzymes usually function in a very narrow pH range. Altering the pH beyond the narrow range in which an enzyme functions results in denaturation.Any change from the optimum pH disrupts the ionic bonding which is holding the tertiary structure of the protein in the correct shape.

3 Effect of Enzyme concentration Draw graph Describe what happens as the concentration of enzyme increases and why this happens.

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___________________________________________________________________________

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4 Effect of substrate concentration – draw graph (Read Page 35)Describe what happens as the substrate concentration increases

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5 Inhibitors All inhibitors slow down the rate of an enzyme-controlled reactionThere are two types of inhibitors:- Competitive Inhibitors - Non- competitive inhibitors

1 Competitive inhibitorsThe competitive inhibitor has a similar shape to the substrateThe inhibitor fits into the active site, forming an enzyme – inhibitor complexThis stops the substrate from entering the active site of the enzymesFewer enzyme substrate complexes are formedThe rate of reaction decreases

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The extent to which a competitive inhibitor reduces enzyme activity depends on the relative number of substrate molecules- remember substrate, enzymes and inhibitors collide by chance – if there are more substrate molecules than inhibitor molecules then the chance of a substrate colliding with an enzyme is greater but if there are more inhibitor molecules than substrate then there is less chance of a substrate molecule colliding with an enzyme so the rate of reaction will decrease

2 Non-Competitive Inhibitors

The non-competitive inhibitor is a different shape to the substrateIt will never fit into the active site The inhibitor fits into a site other than the active siteThis distorts the active site so it is no longer a complementary shape for the substrateNo enzyme-substrate complexes formThe rate of reaction decrease

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The effect of non-competitive inhibitors is largely independent of the substrate concentration. As long as there are enough inhibitor molecules to bind with all the enzymes there will be 100% inhibition. If there is only enough inhibitor molecules for 80% of the enzymes then there will only be 80% inhibition.

- Make notes on end-product inhibition Page 37 Q 1 to 4 page 7

Enzyme Questions1) Enzymes are catalysts which catalyse specific reactions by lowering their activation energy. The

lock and key and induced fit models have been used to explain the way in which enzymes work a) Explain what is meant by activation energy. (1)b) Describe how the lock and key model can be used to explain how an enzyme breaks down a

substrate molecule (3)2) The diagram below shows the rate of an enzyme controlled reaction. The solid line indicates the

normal relationship between rate and substrate concentration and the dotted line indicates the relationship when a competitive inhibitor is added

a) Explain how a competitive inhibitor acts (2)

b) Explain why in the graph above the inhibitor is a competitive inhibitor? (2)

c) The graph below shows the relationship between rate of reaction and temperature of most

enzyme reaction.

Explain the relationship shown on the graph. (5)

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3 Amylase is an enzyme which hydrolyses starch to maltose. Some amylase and starch were mixed and the mixture incubated at 37 °C until the reaction was complete.(i) Sketch a curve on the axes below to show the progress of this reaction.

(ii) Explain why the rate of the reaction decreases as the reaction progresses. (2 marks)The effect of temperature on the rate of reaction of an enzyme was investigated. A test tube containing the enzyme and a test tube containing the substrate were incubated separately at each of the temperatures being investigated.After 5 minutes, they were mixed and the rate of reaction was determined. The experiment was repeated but, this time, the enzyme and the substrate were left for 60 minutes before they were mixed.The results of the investigation are shown in the graph.

(b) The enzyme solution used in this investigation was made by dissolving a known mass of enzyme in a

buffer solution. Explain why a buffer solution was used.

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...............................................................................................................................................................(1)

(c) (i) Use the graph to describe how incubation time affects the rate of the reaction.

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(2)

(ii) The maximum rate of reaction with an incubation time of 60 minutes is less than the maximum rate

of reaction with an incubation time of 5 minutes. Explain why.

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(3)

(d) Explain how inhibitors affect the rate of enzyme-controlled reactions.

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6 marks)(0104)

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The digestive system – how enzymes help in digesting our food.The digestive system is made of a long muscular tube and its associated glands

The glands produce enzymes that break down large, insoluble molecules to small, soluble

molecules ready for absorption.

Parts of the digestive system

1. In the mouth salivary glands, which are near the mouth, pass there secretions via a duct

into the mouth. These contain enzymes to digest starch to maltose.

2. The oesophagus carries food from the mouth to the stomach. It is made up of a thick

muscular wall

3. The stomach is a muscular sac with the inner membrane producing enzymes. It stores

and digests food, especially proteins. – Glands in the stomach produce enzymes which

digest protein and other glands produce mucus- this prevents the stomach being digested

by its own enzymes.

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Oesophagus


4. The small intestine is a long muscular tube.

Food is further digested by enzymes produced by the inner walls of the small intestine and from

associated glands e.g. pancreas

The lining of the small intestine is folded into villi –

each villi is folded with millions of tiny projections

called microvilli on the epithelial cells of each villus –

this helps absorption of food into the bloodstream.

5. The pancreas is a gland that produces pancreatic juice containing enzymes – protease,

lipase and amylase

6. The large intestine absorbs water.

The rectum is the final section of the intestines – faeces is stored here until it is removed via the

anus in the process of egestion

What is digestion? Two parts to digestion: Physical breakdown and chemical digestion

Physical digestionThis is the breaking down of large pieces to smaller pieces – e.g. using teeth. Food is also

churned by the muscles in the stomach wall.

Chemical digestionChemical digestion breaks down large insoluble molecules to small soluble molecules using

enzymes. All digestive enzymes function by hydrolysis. Each enzyme is specific in the

breaking down of a particular molecule

1. Carbohydrases break down carbohydrates

2. Lipases break down lipids to glycerol and fatty acid

3. Proteases break down protein to amino acids

Once the large molecules have been hydrolysed to the smaller molecules they are able to be

absorbed from the small intestine into the blood. They are then carried in the blood to different

parts of the body were they may e incorporated into the body – this is called assimilation.

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Carbohydrate digestionEnzymes are specific so more than one enzyme is needed to completely break down the

large molecules. The enzymes that act on the digestion of carbohydrates are produced in

different parts of the digestive system. This is because each enzyme works fastest at different

pH and are produced in the correct sequence.

The process that occurs:

Food taken into mouth and chewed by teeth producing a larger surface area

Saliva enters the mouth form salivary glands and is mixed with the food.

Saliva contains ____________________________________ which start hydrolysing starch to

maltose. Optimum pH for salivary amylase is neutral

Food swallowed and enters the stomach which is acidic – this denatures the amylase and

prevents further digestion. No digestion of carbohydrates occur in the stomach

Food enters the small intestine – pancreatic juices are added which contains pancreatic

amylase to continue hydrolysing starch to maltose.

Alkaline salts produced by pancreas and intestine wall help to maintain a neutral pH.

The epithelial lining of the small intestine releases enzyme maltase which is used in the

digestion of maltose to glucose.

Disaccharides digestionSucroseSucrose found within cells of food and these are physically released by chewing of teeth.

Sucrose passes through the stomach to the small intestine into the small intestine. The

epithelial lining of the small intestine releases the enzyme sucrase which hydrolyses sucrose

to its monosaccharides _______________________ and _________________________

LactoseLactose is found in milk and is digested in small intestine in which the small intestine epithelial

lining produces the enzyme lactase. Lactase hydrolyses lactose to its monosaccharides

_______________________ and _____________________________

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SummaryStarch digestion

Disaccharide digestion

Lactose intoleranceA babies diet is mainly milk so they produce large amount of the enzymes lactase. As milk is

a smaller part of the diet of adults the production of lactase is reduced. In some people this

reduction is so great that they end up producing no or little lactase.

If no lactase is produced the lactose is not digested. When the undigested lactose reaches

the large intestine, microorganisms break it down, giving rise to a large amount of gas. This

may also result in bloating, nausea, diarrhoea, and cramps. Lactose intolerance is not life

threatening in adults and can be managed by avoiding foods containing lactose. The main

difficultly is taking in sufficient calcium in the absence of milk. This can be resolved by taking

in foods rich in calcium or by adding the enzyme lactase to the milk before drinking it.

Questions1. The diagram shows 2 amino acids.

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a) Indicate the amino group on each amino acid (1)

b) Draw another diagram to show how these two amino acids could form a dipeptide(2)

c) Name both the process involved in the formation of the dipeptide and the type of bond formed(2)

d) What is the secondary structure of a protein? (2)

2 Five solution labelled P, Q, R, S and T are known to be

Amylase (an enzyme that digests starch to maltose)

Albumen (a protein)

Starch

Sucrose

Glucose

The following tests are carried out

- Each solution is tested with iodine solution. This allows identification of solution S

- The remaining four solutions are tested with Benedict’s solution. This slows the identification

of solution P

- The remaining three solution are tested with Biuret reagent. Solutions R and T both turn

purple. Solution Q can now be identified.

A) Identify giving reasons solutions P, Q and S (3)

b) How could solutions R and T be distinguished (4)

3 A molecule of lactose is formed by the condensation of a molecule of β glucose and galactose . both β

glucose and galactose have the formula C6H12O6.

a) Explain how both have the same formula but are different substances (2)

b) How many oxygen atoms are there in a molecule of lactose. Explain why this is the case (3)

c) Galactose is a reducing sugar. What does this mean? (2)

4a) Describe how you would test a solution to see if it contained a protein (2)

b)The diagram shows the structure of a protein molecule

Identify the region that represents

i) An α-helix

ii) A β-pleated sheet (2)

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iii) Explain why the final shape of a protein

molecule is determined by its primary structure

(4)

5 Lactose intolerance can cause sufferers quite serious abdominal pain. It is rare in children under the

age of 4years but in certain populations it is common in adults. Scientists believe that lactose intolerance

in adults was the normal condition and that lactose tolerance arose as a result of a mutation

a) Give 2 other symptoms of lactose intolerance (2)

b) Explain the cause of the abdominal pain (2)

c) Explain why some people become lactose intolerant as they grow older (2)

d) Suggest why scientists believe that being lactose intolerant was the normal condition for the

majority of people (3)

7 The diagram shows how, during digestion a molecule of maltose is converted into two

molecules of α-glucose

a) Name the process taking place (1)

b) Explain the role of the enzyme maltase in this reaction (3)

8The rate of enzyme action depends on a number of factors, including the concentration of the substrate.

The graph below shows the rate of reaction of a human enzyme at different substrate concentrations.

The investigation was carried out a 25oC

Explain the shape of the graph in terms of kinetic theory and enzyme substrate complex formation:

i) From substrate concentration 0.1% to 0.5%

ii) From substrate concentration 1.0% to 2.0% (4)

Sketch on the graph the curve you would expect if the experiment had been carried out at 35oC rather

than 25oC (1)

8 A metabolic pathway iin which each stage is controlled by enzymes is represented as

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A . e1 B e2 C e3 D e4 EUse this example to explain what is meant by end product inhibition (3)

Explain the induced fit model of enzyme action (3)

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