Contents

How to use this book 4

How Science Works 5

The AS examination 6

BY1 Basic Biochemistry and Cell Structure Overview 8

Biological molecules 9

Cell structure and organisation 25

Cell membranes and transport 35

Enzymes 45

Nucleic acids and cell division 57

BY2 Biodiversity and Physiology of Body Systems Overview 69

Biodiversity, evolution and classi! cation 70

Adaptations for gaseous exchange 83

Transport in animals and plants 98

Reproductive strategies 124

Adaptions for nutrition 130

Answers Exam practice questions 143

Knowledge checks 152

Glossary 153

Index 156

3

Contents

How to use this bookThe contents of the book closely match the speci! cation for WJEC AS Level Biology and provide you with information and plenty of practice examination questions in order to prepare successfully for the BY1 and BY2 examinations.

This book covers all three of the Assessment Objectives required for your WJEC AS level Biology course. The main text covers AO1 Knowledge and Understanding, which consists of the main factual content of the speci! cation and AO2 Application of Knowledge and Understanding. The other Assessment Objective, AO3 How Science Works, which covers around 5% of the assessment weighting of the speci! cation, is covered in the form of a margin feature ‘How Science Works’. You will not be asked to recall the information given under this heading for the purpose of the examination.

The book content is clearly divided into two Assessment Units, BY1 and BY2.

! BY1 covers Basic Biochemistry and Cell Structure.

! BY2 covers Biodiversity and Physiology of Body Systems.

At the start of each unit there is an overview page. Each unit is divided into a number of topics. Topic openers give a summary of the content to be covered together with a list of learning objectives.

At the end of the book there are answers to the Exam practice and Knowledge check questions, a glossary of key terms used in the WJEC speci! cation and a detailed index to help you navigate through the book.

The text is supplemented with a number of features.

Study pointAs you progress through your studies, advice is provided to help you understand and use the knowledge content. This may provide some extra information not included in the main text or simply point out that the information is relevant but may not be tested in the examination.

Examiner tip! e examiner may provide general or specifi c advice to help you with your studies and to prepare you for the exam.

Links are highlighted in the margin near the relevant text. They are accompanied by a reference to any areas where sections relate to one another. It may be suggested that you recap a topic before beginning to study the current topic.

Link

Key TermsAlthough there are terms that you need to defi ne and understand included in the body of the text, where terms are not explained within the same topic these are highlighted in bold type within the text and are highlighted in blue as margin features. Terms can also be looked up in the glossary section which appears at the back of the book. ! e use of key terms is an important feature since examination papers contain a number of terms that need to be defi ned.

These are short questions to check your understanding of the subject, allowing you to apply the knowledge that you have acquired. These questions are of two types: Filling in blanks in a passage, or matching terms with phrases speci! c to the topic under study. Answers are supplied at the back of the book.

How Science WorksThis feature helps you understand something about science itself, how scienti! c knowledge has been obtained, how reliable it therefore is and what its limitations are. It may also help you to have a deeper awareness of how science is used to improve our quality of life. You will not be examined on the information provided by this feature.

YOU SHOULD KNOW ›››

››› ! e learning objectives provided are more specifi c to the sub-topic being studied than the more general learning objectives listed at the start of the topic.

AS Biology

4

BY1Enzymes

Topic contents46 Enzyme structure

47 Properties of enzymes

49 Factors affecting enzyme action

51 Enzyme inhibition

52 Medical and industrial applications of enzymes

In cells, metabolic reactions take place quickly and thousands of reactions are taking place simultaneously. Order and control is essential if reactions are not to interfere with each other. These features of metabolism are made possible by the action of enzymes.

By the end of this topic you should be able to:

! Describe the structure of enzymes and how their properties are related to their structure.

! Explain the mechanisms of action of enzyme molecules with reference to speci! city, active site, enzyme–substrate complex.

! Explain how enzymes are affected by factors such as temperature, pH, and the concentration of the reactants.

! Explain how enzymes are affected by inhibitors.

! Explain the principle of immobilised enzymes and their advantages over ‘free’ enzymes.

! Describe the use of a biosensor to test for blood glucose.

45

Key TermsCatalyst = a molecule which speeds up a chemical reaction, but remains unchanged at the end of the reaction.

Metabolism = all the chemical processes that take place in living organisms.

Revisit protein structure on page 16.

Link

Enzyme structureEnzymes are globular proteins that act as biological catalysts.

They speed up the rate of metabolic reactions. Some enzymes catalyse reactions where larger molecules are broken down into smaller ones. Others catalyse reactions where small molecules are built up into larger, more complex molecules.

Enzymes are tertiary globular proteins where the protein chain is folded back on itself into a spherical or globular shape with hydrophilic R groups on the outside of the molecule, making enzymes soluble. Each enzyme has its own sequence of amino acids and is held in its tertiary form by hydrogen bonds, disulphide bridges and ionic bonds. This complex and precise 3-D shape gives the enzyme many of its properties. Although the enzyme molecule is large, only a small region, called the active site, is functional.

" Enzyme structure

Enzymes react with another molecule called a substrate. Each enzyme has its own special shape, with an area, the active site, onto which the substrate molecules bind. Only three to twelve amino acids make up the active site but its shape is an exact fi t for the substrate. The combination of enzyme and substrate is called the enzyme–substrate complex, temporary bonds holding the substrate in place. When the reaction is complete, more products are released, leaving the enzyme unchanged and ready to receive another substrate molecule.

" Enzyme–substrate complex

Fig 41

Substrate moleculesituated within the enzyme’s active site

bonds maintainenzyme’s shape

Fig 42

enzyme enzymeenzyme–substratecomplex

substrate products

active site

YOU SHOULD KNOW ›››

››› how the structure of enzymes is related to their function

››› that enzymes lower the activation energy

››› the formation of an enzyme–substrate complex

46

AS Biology: BY1 Basic Biochemistry and Cell Structure

9

Properties of enzymes ! Each enzyme will act on only one type of substrate molecule. This is because the shape

of the active site allows only one shape of molecule to ! t with it. The particular enzyme is said to be speci! c for that particular substrate.

! Enzymes are very fast acting and are said to have a high turnover number. This means that they can convert many molecules of substrate per unit time. For example, catalase, which breaks down the waste product hydrogen peroxide in the body, has a turnover number of several million!

! Reactions need energy to start them off otherwise the reactants will not be converted to products. This energy is called activation energy and is the energy needed to break existing chemical bonds inside molecules. One way of making chemicals react together is to increase the energy in the form of heat. Living organisms cannot do this as temperatures above 40°C will cause irreversible damage to the structural and functional proteins in their bodies.

" Enzymes and activation energy

For a reaction to occur the reactants must collide with suffi cient energy to alter the arrangement of the atoms in the molecules. In other words, an initial boost of energy is needed, that is, an activation energy level which must be overcome before the reaction can proceed. In chemical reactions this may be carried out by heating. Enzymes work by lowering the activation energy. When a substrate enters the active site of an enzyme, the shape of the molecule is altered slightly. This enables metabolic processes to take place at temperatures relatively lower than chemical reactions not catalysed by enzymes.

Fig 43

Progress of reaction

Without enzyme With enzyme

Ener

gy activation energy

substrate

Progress of reaction

Ener

gy

activation energy

substrateproduct product

Examiner tipIt is incorrect to state that the substrate has the ‘same shape’ as the active site. It has a complementary shape to the active site.

Study pointsEnzymes are inactive at 0°C and if the temperature is raised they become active again. Enzymes are denatured at temperatures above 40°C.

By lowering the activation energy, enzymes reduce the input of energy required to allow reactions to take place at the lower temperatures found in cells.

Identify the missing word or words.

Enzymes are tertiary globular proteins that act as •••• catalysts. They are held in this shape by hydrogen, disulphide and ••••

bonds. During an enzyme-controlled reaction the substrate ! ts into a region on the enzyme called the •••• ••••. The combination of enzyme and substrate is called the enzyme–substrate ••••.

# Reactions need to overcome an energy barrier before they can get started. Compare this to a man pushing a boulder over the top of a hill. First, he has to exert energy to get the boulder to the top of the hill and over before it can roll down the other side.

BY1: Enzymes

47

Models of enzyme actionScientists often use a scientifi c model as a representation of how something works. The lock and key theory proposes that enzymes work in the same way as a key operates a lock. Each key has a specifi c shape that fi ts and operates a particular lock. In a similar way, a substrate will only fi t the active site of one particular enzyme.

However, observations that the enzyme’s shape was being altered by the binding molecule, suggest that the structure was fl exible and not rigid as originally thought. Scientists then proposed an alternative model, called the induced fi t model. This theory suggests that the enzyme has a certain general shape which alters slightly to accommodate the substrate. As the enzyme changes its shape it places a strain on the substrate molecule and distorts a particular bond. This lowers the activation energy needed to break the bond.

The course of an enzyme-controlled reactionThe progress of an enzyme-catalysed reaction for a fi xed concentration of substrate can be followed by measuring either the formation of product or the disappearance of substrate.

" Graph showing the measurement of formation of a product

The shape of the graph may be explained as follows:

! When the enzyme and substrate are ! rst mixed together, there are a large number of substrate molecules.

! Both types of molecules are in constant motion.

! Substrate molecules come in contact with the empty active sites of the enzyme molecules.

! All active sites become ! lled with substrate molecules, which are rapidly broken down to product.

! As the reaction proceeds there is a decreasing amount of substrate and more and more product.

! In due course the graph " attens out because all the substrate has been used up and so no further product can be formed.

How Science WorksObservation of enzyme molecules led scientists to propose the induced ! t model of enzyme action.

Study pointsHow could you carry out an experiment to determine the effect of temperature on the rate of breakdown of hydrogen peroxide by the enzyme, catalase?

If substrate becomes limiting, the addition of extra enzyme will not increase the rate of reaction.

Examiner tip! e curve is steepest at the start of the reaction. ! is is called the initial rate of reaction.

How Science Works

Fig 44

0123456789

10

1 2Time/min

Volu

me

of p

rodu

ct/c

m3

YOU SHOULD KNOW ›››

››› the lock and key and induced fi t models of enzyme action

››› how factors such as temperature, pH and substrate concentration a" ect the rate of an enzyme-catalysed reaction

48

AS Biology: BY1 Basic Biochemistry and Cell Structure

Factors affecting enzyme actionEnzymes are made inside living cells but may act inside the cell (intracellular) or outside (intercellular, extracellular) such as the digestive enzymes of the alimentary canal. Environmental conditions, such as temperature and pH, change the three-dimensional structure of enzyme molecules. Bonds are broken and hence the configuration of the active site is altered.

The effect of temperature on rate of enzyme actionA rise in temperature increases the kinetic energy of molecules and they move around more quickly and collide with each other more often. In an enzyme-catalysed reaction, the enzyme and substrate molecules collide more often in a given time, increasing the rate of reaction. As a general rule, the rate of reaction doubles for each 10°C rise in temperature until an optimum temperature is reached. For most enzymes this is 40°C. Above this temperature the increasing vibration of the molecules causes the hydrogen bonds to break and cause a change in the tertiary structure of the enzyme. This alters the shape of the active site and the substrate will not fit into the active site. The enzyme is then said to be denatured. This is a permanent change in the structure. If enzymes are subjected to low temperatures, such as freezing, the enzyme is inactivated as the molecules have no kinetic energy. However, the enzyme can work again if the temperature is raised.

The effect of pH on rate of enzyme actionMost enzymes have an optimum pH at which the rate of reaction is at its maximum. Small changes in pH outside the optimum can cause small reversible changes in enzyme structure and result in inactivation. Extremes of pH can denature an enzyme.

The charges on the amino acid side-chains of the enzyme’s active site are affected by free hydrogen ions or hydroxyl ions. In the formation of an enzyme substrate complex the charge on the active site must match those on the substrate. If the active site has too many H+ ions (say) the active site and the substrate may both have the same charge and the enzyme will repel the substrate.

Fig 45

0 10 20 30 40 50 60 70

Temperature (°C)

Rate

of r

eact

ion

Fig 46

0 1 3 5 7 9 11 13

pH

Rate

of r

eact

ion

Study point

Enzymes, rather than inorganic catalysts, are used widely in industry because they are more ef!cient. They have a higher turnover number and are very speci!c. They are also more economical as they work at lower temperatures.

Fig 47

+–

–+

++

++

––

––

enzyme

substrate

optimum pH

enzyme

substrate

low pH

enzyme

substrate

high pH

charges on active site match those of substrate so an enzyme–substrate complex forms

charges on active site repel substrate

charges on active site repel substrate

$ How change in pH affects enzyme action

% Effect of temperature on rate of reaction

% Effect of pH on rate of reaction

BY1: Enzymes

49

10

At extremes of pH the hydrogen bonding is affected and the three-dimensional shape of the enzyme is altered and so is the shape of the active site.

Enzymes are also affected by the concentration of the substrate and the concentration of the enzyme itself.

Substrate concentrationThe rate of an enzyme-catalysed reaction will vary with changes in substrate concentration. If the amount of enzyme is constant, the rate of reaction will increase as the substrate increases. At low substrate concentrations the enzyme molecules have only a limited number of substrate molecules to collide with. In other words, the active sites are not working to full capacity. As more substrate is added there must come a point when all the enzyme’s active sites are fi lled. In other words, the rate of reaction is at a maximum.

% Effect of increasing substrate concentration

$ Effect of low and high substrate concentration

Enzyme concentration Once a product leaves the active site of an enzyme, that enzyme molecule can be reused. Therefore only a small concentration of enzyme is needed to catalyse a large number of substrate molecules. The number of substrate molecules that one molecule of enzyme can turn into products in one minute is called the turnover number. The enzyme, catalase, has a turnover number of several million.

The rate of an enzyme-catalysed reaction will vary with changes in enzyme concentration. Increasing the enzyme concentration will increase the number of available active sites and therefore the rate of reaction.

Examiner tipIn enzyme experiments it is essential that bu" ers and controls are used.Bu" ers maintain a constant pH. When a bu" er is used in an experiment the pH changes little when a small quantity of acid or alkali is added. It may be said that a bu" er ‘soaks up hydrogen ions’.Controls are duplicate experiments, identical in every respect to the actual experiment, except for keeping the variable being investigated constant.

Study pointInclude all experimental work in your revision. There are often questions involving practical work in theory exams, particularly involving enzymes.

Examiner tipProvided temperature and pH are favourable and there is an excess of substrate, then the rate of reaction will be directly proportional to the enzyme concentration.

Fig 48

Substrate concentration

Rate

of r

eact

ion

Fig 49

substratemolecules

emptyactive sites

excess substratemolecules unable to !nd

any free active sites

Low substrateconcentration

High substrateconcentration

only a limitednumber of substrate molecules to !llactive sites

the rate ofreactionis at amaximum

Identify the missing word or words.

A rise in temperature increases the •••• energy of molecules. At 0°C enzymes are ••••. In an enzyme-catalysed reaction as the temperature increases, the rate of reaction increases until an •••• temperature of approximately 40°C is reached. Above this temperature the increasing vibration of the molecules causes the •••• bonds to break and cause a change in the •••• structure of the enzyme. This alters the shape of the active site and the •••• will no longer ! t into the active site. The enzyme is then said to be ••••.

50

AS Biology: BY1 Basic Biochemistry and Cell Structure

Exam practice questions

Enzymes

1 The graph represents the effect of an increase in temperature on the rate of an enzyme-controlled reaction:

(a) Explain the shape of the curve between A and B and between C and D. (4)

(b) Suggest why the temperature of a mammal is maintained just below the optimum temperature for the enzymes in that mammal. (2)

2 A student investigated the action of the enzyme catalase. This enzyme catalyses the breakdown of hydrogen peroxide into oxygen and water. The student collected the oxygen given off in a measuring cylinder. The volume of gas was recorded every 20 seconds as shown by the line labelled Y in the graph below left.

(a) The rate of reaction can be calculated using the formula:Volume of oxygen collected Time taken to collect

Use the formula to calculate the rate in cm3 min–1 for the !rst 30 seconds. (2)

(b) The initial rate is the rate of reaction at the start and is the maximum rate. It is shown by line X. The initial rate is 19 cm3 min–1 Explain why the initial rate is greater than the rate calculated in (a). (2)

(c) The graph above right shows the effect of temperature on the activity of an amylase enzyme found in bacteria that live in hot water in volcanic regions:

(i) Using the graph, describe and explain the effect of temperature on the rate of activity of the amylase. (6)

(ii) State the difference between bacterial amylase and an amylase found in humans. (2)

Fig ST15

0 10 20 30 40 50 60 70 80Temperature (°C)

Rate

of r

eact

ion

A

B

C

D

Fig ST16

0 20 40 60 80 100 120 140Time/s

Volu

me

of o

xyge

n/cm

3

0

1

2

3

4

5

6

7

8

9

10

X

Y

Fig ST17

20 40 60 80 100 120 140Temperature/°C

Rate

of r

eact

ion/

arbi

trar

y un

its

0

50

10

54

AS Biology: BY1 Basic Biochemistry and Cell Structure

Exam practice questions

3 The graph shows the mass of product formed when a fixed concentration of enzyme is added to a fixed concentration of substrate and then placed in water baths at three different temperatures.

(a) (i) Explain why the mass of product formed at 60˚C is greater during the !rst !ve minutes than the masses formed at 25˚C and 37˚C. (3)

(ii) Explain why there is less overall product formed at 60˚C than at 37˚C. (3)

(b) Explain why the mass of product formed at 37˚C levels off after approximately 20 minutes. (1)

(c) Explain why the curve at 25˚C has not levelled off after 60 minutes. (3)

4 (a) Apart from the presence of substrate concentration and inhibitors, state three factors that affect the rate of an enzyme-controlled reaction. (3)

(b) The graph shows how the rate of an enzyme-catalysed reaction varies with substrate concentration when affected by a competitive and a non-competitive inhibitor.

(i) Which line shows the competitive inhibitor? (1)

(ii) Give a reason for your answer to (b) (i). (1)

(iii) Explain how a competitive inhibitor works. (3)

Fig ST18

0 10 20 30 40 50 60Time/min

Mas

s of p

rodu

ct fo

rmed

/g

25°C

37°C

60°C

Fig ST19

Substrate concentration/arbitrary units

Rate

of r

eact

ion/

arbi

trar

y un

its

no inhibitor

A

B

BY1: Enzymes

55