CCEA GCSE Biology - Denmour Boyd, James Napier 2017

Unit 1
Digestion

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Specification points

This chapter covers specification points 1.4.1 to 1.4.5. It covers enzymes as proteins that are biological catalysts, the effects of temperature, pH, enzyme concentration and inhibitors on the action of enzymes, their role in digestion and commercial uses, and how the structure of the ileum is adapted for absorption of digested food molecules.

Enzymes

All enzymes are proteins that function as biological catalysts, speeding up the rate of reactions such as photosynthesis, respiration, digestion and many others in living organisms.

Some of these reactions, such as respiration and digestion, involve breaking down large molecules, while others, such as photosynthesis, join small molecules together into larger ones. This chapter looks at the enzymes of digestion.

Tip

A catalyst is a substance which increases the rate of a chemical reaction without being changed or consumed during the reaction.

How enzymes work

The molecule on which an enzyme acts is the substrate, while the molecule which is formed by the reaction is the product. Enzymes work because the shape of the substrate matches exactly the shape of a special region in the enzyme molecule, called the active site. The shape of the substrate and the active site are complementary, so when they collide the substrate fits snugly into the active site of the enzyme. This tight fit then enables the enzyme to catalyse the reaction and, in a reaction such as digestion, split the substrate into its products. The enzyme molecule is not changed or used up by the reaction and so is available to repeat the reaction every time it collides with a substrate molecule.

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The action of enzymes as described in Figure 4.1 is referred to as the ’lock and key model’ due to the importance of the tight fit between the enzyme’s active site and the substrate. This lock and key model can explain the principle of enzyme specificity — each enzyme is specific in that it will only work on its normal substrate. For example, only starch will fit into the active site of amylase and be broken down — other molecules such as proteins cannot fit into the active site. Table 4.1 shows the substrates and products produced by some enzymes.

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Tip

The name of most enzymes begins with the name of the substrate and ends with ’ase’.

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Inhibitors

However, there are molecules that fit loosely or partially into the active site of some enzymes. Such molecules are called inhibitors because, while they occupy the active site of the enzyme, substrate molecules cannot enter and be broken down, which leads to a reduced (inhibited) rate of reaction.

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Show you can

Explain the effect of an inhibitor on the action of an enzyme.

Test yourself

1 What is an enzyme?

2 What does it mean when the shape of a substrate and the shape of an enzyme’s active site are described as complementary?

3 Explain substrate specificity.

4 Name the type of enzyme which digests proteins.

The effects of temperature and pH on the action of amylase

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Temperature and pH affect the activity of all enzymes. Figures 4.3 and 4.4 show how changing the temperature and pH affect the activity of amylase.

At low temperatures, the enzyme and substrate molecules have reduced kinetic energy and move slowly, resulting in few collisions between them and a low rate of enzyme activity. Increasing the temperature increases the kinetic energy, the number of collisions and the rate of activity. The temperature (or pH) which causes the maximum rate of enzyme activity is the optimum.

Show you can

Describe and explain the rate of enzyme action at points X and Y in Figure 4.5.

Increasing temperatures above the optimum (or changing the pH away from the pH optimum) causes a decrease in the activity of the enzyme. This is due to an irreversible change to the shape of the enzyme’s active site, known as denaturation. The further away from the optimum the temperature or pH are, the lower the enzyme activity.

The effect of enzyme concentration on enzyme activity

Figure 4.5 shows that the more enzymes there are, the faster the enzyme reaction (X). This is because there are more active sites for substrates to attach to. This applies up to a limit, when the rate levels off (Y) because there are not enough substrate molecules to react with the extra enzymes.

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Commercial enzymes

Many enzymes have commercial uses. For example, many biological washing powders have enzymes for breaking down difficult-to-remove stains. These enzymes are thermostable — they can work at a wide range of temperatures and they break the complex, large and insoluble stains down into small, soluble molecules that dissolve in the water.

Tip

Other commercial uses of enzymes include pre-digesting food for babies, extracting juice from fruit, making lactose-free products and softening the centre of chocolates.

Test yourself

5 Why do enzymes react slowly at low temperatures?

6 What is the optimum pH of an enzyme?

7 Describe how denaturation changes an enzyme and what effect this has on the enzyme action.

8 Why do biological washing powders need thermostable enzymes?

Prescribed practical

Biology Practical 1.4 Double Award Science B3: Investigating the effect of temperature on the action of an enzyme

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Procedure

1 Set up five water baths at 10 °C, 20 °C, 30 °C, 40 °C and 50 °C.

2 Label five test tubes with ’starch’ and the temperature of one of the water baths.

3 Label five test tubes with ’amylase’ and the temperature of one of the water baths.

4 Use a syringe to measure 5 cm3 of 1% starch solution into each of the test tubes labelled starch.

5 Use another syringe to measure 5 cm3 of 1% amylase solution into each of the test tubes labelled amylase.

6 Place one starch and one amylase labelled test tube in each water bath for at least five minutes.

7 Prepare a spotting tile for each of the temperatures by placing one drop of iodine solution in each of the dimples as shown in Figure 4.6.

8 Starting with the 10 °C water bath, pour the amylase solution into the starch solution. Use a clean dropping pipette to sample the mixed solution and start a timer.

Tip

It will be necessary to prepare several spotting tiles as the reactions often last more than three minutes.

9 Add one drop of the sample to the iodine solution in the first dimple of the spotting tile and note any colour change. Return the remainder of the sample into the test tube.

10 Repeat the sampling (steps 8 and 9) every minute until the iodine shows no colour change. Record the time taken for the starch to be digested in minutes.

11 Repeat steps 8, 9 and 10 with each of the water baths in turn.

12 Copy and complete the results table.

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13 Draw a graph of the time taken for the starch to be digested at different temperatures on a grid, as in Figure 4.7. When drawing this graph, you should use a best fit line (not a point to point graph).

Sample results and questions

1 Explain why a tube of amylase and a tube of starch were placed in the water bath for five minutes before being mixed.

2 What effect will the sample drop have on the iodine solution at zero minutes?

3 Explain why the iodine will show no colour change after a period of time.

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4 Explain why it is appropriate to use a line of best fit on a graph of these results.

5 Explain the results between 10 °C and 20 °C.

6 Between which temperatures is the optimum temperature for this amylase enzyme?

7 Suggest how the optimum temperature could be determined more accurately.

8 Explain what happens to the starch after 50 °C.

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Digestion

Most of the food we eat is in the form of large, complex, insoluble molecules. We need enzymes to digest and break down these molecules so that they are soluble and small enough to be absorbed into the blood system, where they can be used by the body.

Digestion can be defined as the breakdown of large, complex, insoluble molecules into small, simple, soluble ones.

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The ileum (small intestine)

Although the final stages in the digestion of our food take place in the ileum where proteases, lipases and amylases break down protein, fats and starch respectively, the main function of this section of the small intestine is the absorption of the digested food products. It is adapted for the process of absorption in many ways, including its great length (approximately 3 metres) and the presence of many folds (or twists) along its length that greatly increase the surface area for the absorption of food. The process is aided by the good blood supply and the thin and permeable membranes.

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Villi

The presence of millions of microscopic outgrowths called villi on the inner surface of the ileum also increase the surface area. Figure 4.8 shows a section through a villus.

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Villi that line the inner surface of the ileum (the part in contact with the food) aid absorption due to the presence of:

the excellent blood supply — each villus has an extensive capillary network, which means that the whole small intestine is well supplied with blood to transport the absorbed products of digestion. The products of digestion, once absorbed, are quickly transported away and replaced with blood low in digested food molecules, thus maintaining a high concentration gradient. Note: the lacteal is part of a system of tubes that absorb the breakdown products of fat before returning them to the blood.

the thin and permeable surface lining each villus consisting of a single layer of surface epithelium cells — this reduces the number of cells between the lumen of the gut and the capillaries and lacteals.

The villi are further adapted for the efficient absorption of digested food through their finger-like shape which increases surface area.

Practice questions

1 a) The following diagram shows the effect of a protease enzyme on film.

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i) What is a protease enzyme?

(1 mark)

ii) Suggest the chemicals produced when the film coating is fully digested.

(1 mark)

b) The graph in Figure 4.10 plots the effect of pH on the action of the protease enzyme on film.

i) Explain the shape of the graph.

(3 marks)

ii) Describe and explain what would happen to the film if a lipase enzyme was used.

(2 marks)

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2 Soft-centred mint chocolates are made using enzymes. The mint centre is hard when first made so that it will not melt when covered with hot chocolate. After the chocolate has solidified the sweet is kept at 18 °C for 14 days. During this time an enzyme called invertase breaks down the complex sugar in the mint centre making it softer and sweeter. One manufacturer wanted to find a new enzyme to use in this process which would reduce the time needed to soften the mints. Their scientists carried out experiments using four new enzymes: A, B, C and D. Each experiment used the same mass of solid mint centre and the same concentration of enzyme. Table 4.4 shows the time taken by each of the new enzymes to make the mint go soft.

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a) Suggest which of the new enzymes, A, B, C or D, the manufacturer would use. Give a reason for your answer.

(2 marks)

Another scientist checked the method and results of the experiment and concluded that some of the factors had not been controlled.

b) Choose two factors from the list below which should have been controlled in the manufacturer’s experiment.

(2 marks)

 ✵ Temperature

 ✵ Humidity

 ✵ Light intensity

 ✵ Oxygen concentration

 ✵ pH

 ✵ Softness of the mint at the end

c) What term describes the role of the other scientist checking the work of the manufacturer’s scientists?

(1 mark)

3 Figure 4.11 shows the absorption of glucose into the blood.

Use the information from Figure 4.12 to help explain why starch needs to be digested.

(2 marks)

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4 Figure 4.12 shows a structure from the lining of the ileum.

a) Name this structure.

(1 mark)

The structure is adapted to maximise the absorption of digested food molecules.

b) i) Explain how the overall shape of the structure helps this function.

(1 mark)

ii) Parts A and B also help this function. Identify each feature and explain how it helps to maximise absorption.

(2 marks)

iii) Name part C and describe its function.

(1 mark)

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