WJEC A2 Biology Student Unit Guide: Unit BY4: Metabolism, Microbiology and Homeostasis

STUDENT UNIT GUIDE

WJEC A2 Biology Unit BY4

Metabolism, Microbiology and

Homeostasis

Andy Clarke

I would like to thank Alex Cook and Phil Evans for their help and advice in writing this book.

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© Andy Clarke 2013

ISBN 978-1-4441-8297-2

First printed 2013

Impression number 5 4 3 2 1

Year 2015 2014 2013

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P2197

responsibility for the content remains with the publisher.

Getting the most from this book ...........................................................................................................4

About this book .............................................................................................................................. 5

Content Guidance

Energy and living things ...............................................................................................................................6

Respiration .............................................................................................................................. 7

Photosynthesis .............................................................................................................................. 16

Microbiology .............................................................................................................................. 24

Populations .............................................................................................................................. 33

Excretion .............................................................................................................................. 41

The nervous system .............................................................................................................................. 51

Responses in plants ....................................................................................................................................65

Questions & Answers

Q1 ATP and respiration .............................................................................................................................. 71

Q2 Respiration .............................................................................................................................. 73

Q3 Photosynthesis .............................................................................................................................. 75

Q4 Microbiology .............................................................................................................................. 78

Q5 Populations .............................................................................................................................. 81

Q6 The kidney............................................................................................................................... 83

Q7 The nervous system .............................................................................................................................. 86

Q8 The nitrogen cycle .............................................................................................................................. 89

Knowledge check answers .................................................................................................................... 91

Index .............................................................................................................................. 93

Contents

4 WJEC A2 Biology

Getting the most from this book

Examiner tips

Advice from the examiner on key points in the

text to help you learn and recall unit content,

avoid pitfalls, and polish your exam technique in

order to boost your grade.

Knowledge check answers

1 Turn to the back of the book for the Knowledge

check answers.

Summaries

l Each core topic is rounded off by a

bullet-list summary for quick-check

reference of what you need to know.

Knowledge check

Rapid-fire questions throughout the Content

Guidance section to check your understanding.

The nitrogen cycle Question 8

Unit BY4: Metabolism, Microbiology and Homeostasis 89

Questions & Answers

Exam-style questions

Examiner comments

on the questions

Tips on what you

need to do to gain full

marks, indicated by the

icon .

Sample student

answers

Practise the questions,

then look at the student

answers that follow

each set of questions.

Examiner commentary

on sample student

answers

Find out how many marks

each answer would be

awarded in the exam and

then read the examiner

comments (preceded by

the icon ) following

each student answer.

Question 8 The nitrogen cycle

Describe the nitrogen cycle. Include the form that nitrogen takes in each part and the role of bacteria

(giving names wherever possible). Any diagrams included in your answer must be fully annotated.

Total: 10 marks

The last question on every WJEC exam paper is an essay-style question worth 10 marks.

There are always two alternatives and you are required to answer one. Although the subject

matter of the questions will differ, on the whole these questions are testing recall with

understanding (AO1). Therefore, if you have revised the whole of the unit and are well prepared

you should gain most of the marks on this type of question. You may include diagrams within

your answer and you are strongly advised to do this. Biology is a visual subject and your notes

will probably contain many diagrams to help you understand the biology. You will not gain any

credit for just drawing a diagram. However, if you annotate it then the annotations will gain

credit. Drawing diagrams will also help you to construct a coherent answer.

Summary

l Nitrogen is an important element and is found in proteins and nucleic acids in

plants and animals. a

l When plants and animals die decomposers b release ammonia into the soil. c

l The ammonia is then converted into nitrates by bacteria. d

l The plants can then absorb the nitrates to make more proteins. e

l Other bacteria change nitrates into nitrogen gas f which enters the atmosphere. g

l Some plants, such as peas, can absorb the nitrogen directly. h

Student A

3/10 marks awarded The diagram of the nitrogen cycle is incomplete and there are

six statements about the cycle, demonstrating a lack of preparation. a, c g These points gain

marks for correctly identifying the locations of different forms of nitrogenous compound. b, d,

e, f There is no reference made to any of the processes involved (putrefaction, nitrification,

denitrification and nitrogen fixation) or to the names of the bacteria involved. h This statement is

incorrect and shows a lack of understanding.

Plan:

Plants

NO3

–

NO2

–

N2

NH4

+ Decomposers

Animals

Essay:

Plants absorb nitrates from the soil a to provide a source of nitrogen for the synthesis

of amino acids and proteins. b Animals consume the plants, digest the proteins into

amino acids, which they then use to make proteins.

Student B

Unit BY4: Metabolism, Microbiology and Homeostasis 5

About this book

This guide will help you to prepare for BY4, the examination for WJEC A2 Biology

Unit  4: Metabolism, Microbiology and Homeostasis. Your understanding of

many of the principles in Unit 1 may be re-examined here as well.

Content Guidance

The Content Guidance section covers all the concepts you need to understand

and  facts you need to know for the BY4 exam. It also includes examiner tips and

knowledge checks to help you prepare for BY4.

The order in which topics appear in the guide follows the order of the specification

with the exception of the detail of chemiosmosis, which is included in respiration and

photosynthesis, rather than with ATP.

The concepts in each topic are presented first followed by details of the processes

and  adaptations of the various structures involved. You are advised to familiarise

yourself with the key ideas before attempting to learn the associated facts.

The A2 biology course is more demanding than AS and includes stretch-and￾challenge and synoptic aspects.

Stretch and challenge: At A2 you have to develop a greater understanding of biological

concepts and demonstrate a greater ability to apply your knowledge and understanding

(AO2). The Content Guidance section contains boxes detailing investigations carried

out on particular aspects of biology. The specification does not require you to know the

details of these investigations, but they will give you an idea of the sort of information

you could be provided with to assess AO2.

Synoptic element: You need to start piecing together the topics you have studied so

far and try to see the links between them; this is the synoptic element. In Unit BY1

you learnt the ‘core concepts’ in biology — the fundamentals of biochemistry and cell

biology. This knowledge underpins all aspects of A2 biology. To ensure you have a

good understanding of Unit BY4 it is essential that you revisit these concepts. Synoptic

links are highlighted throughout the Content Guidance section.

Questions and Answers

This section will help you to:

l familiarise yourself with the question styles you can expect in the unit test

l understand what the examiners mean by terms such as ‘describe’ and ‘explain’

l interpret the question material — especially any data that the examiners give you

l write concise answers to the questions that the examiners set

It would be impossible to give examples of every kind of question in one book, but

these should give you a flavour of what to expect. Two students, Student A and Student

B, attempt each question in this section. Their answers, along with the examiner

comments, should help you to see what you need to do to score a good mark — and

how you can easily not score a mark even if you understand the biology.

6 WJEC A2 Biology

Content Guidance

Energy and living things

Key concepts you must understand

l Most energy available to living organisms is derived directly or indirectly from

the sun.

l Autotrophic organisms (e.g. plants) convert light energy into chemical energy

during photosynthesis. This chemical energy is locked up within organic molecules.

l All organisms, both autotrophic and heterotrophic, break down these organic

molecules during respiration to produce adenosine triphosphate (ATP).

l ATP is the only source of immediate energy within the cell for processes such

as active transport, muscle contraction and the synthesis of organic molecules,

such as proteins.

l ATP is often referred to as the ‘universal energy currency’ because it transfers

energy for biochemical reactions in the cells of all living organisms.

Adenosine triphosphate (ATP)

The structure of ATP is shown in Figure 1. It is a free RNA nucleotide consisting of a ribose

sugar, the base adenine and three phosphate groups (adenosine = ribose + adenine).

Examiner tip

Synoptic link to BY1: In

Unit BY1 you studied the

structure of nucleotides

and the functions of nucleic

acids. Revisiting these

topics will help you with

aspects of energy transfer.

Examiner tip

Make sure that you get

the name right. You

could be given a diagram

similar to Figure 1 and

asked to name it or label

it. Many students get this

wrong by labelling the

base adenosine, or calling

the molecule adenine

triphosphate or adenosine

triosephosphate.

C

C

H C C

Adenine

N

N N

H

C H

N

NH2

H2

C

Ribose

O O O

O

O

O

O

O

O

P O P P

OH OH

H Phosphate groups

H H

O

Figure 1 A molecule of ATP

As energy is released when ATP is hydrolysed to adenosine diphosphate (ADP) and

inorganic phosphate (Pi), it is an exergonic reaction (see Figure 2). This reaction is

catalysed by the enzyme ATPase and involves the removal of the terminal phosphate

group. The reaction is always coupled with an energy-requiring reaction (endergonic

reaction) so that energy is transferred.

ATP acts as an energy carrier and is suited to its function because:

l the molecule is soluble and can be transported within the cell (but cannot leave the

cell), transferring chemical energy to energy-requiring processes

Respiration

Unit BY4: Metabolism, Microbiology and Homeostasis 7

l the hydrolysis of ATP releases small quantities of energy (30.6kJmol−1) that are

matched closely to the energy required in the coupled reaction

l the energy is transferred quickly as the hydrolysis of ATP requires only one enzyme

Summary

After studying this topic you should be able to:

l understand the importance of chemical energy in

biological processes

l recognise the structure of ATP and describe its role

as an energy carrier and its use in the liberation of

energy for cellular activity

Respiration

Key concepts you must understand

l Respiration is a process that occurs within the cells of all living organisms.

It can be represented by the following simple chemical equation:

C6H12O6 + 6O2 6CO2 + 6H2O

l Respiration releases chemical energy from the oxidation of organic molecules,

such as glucose, to synthesise ATP.

l There are three ways in which molecules can be oxidised or reduced:

Oxidation Reduction

1 Gaining oxygen Losing oxygen

2 Losing hydrogen Gaining hydrogen

3 Losing electrons (e−) Gaining electrons (e−)

Examiner tip

In biology it is more helpful

to think about oxidation in

terms of loss of hydrogen

and loss of electrons and

reduction in terms of gain

of hydrogen and electrons.

Examiner tip

The ‘law of conservation of

energy’ states that energy

can neither be created

nor destroyed. However,

energy can be converted

from one form to

another. When answering

questions relating to the

hydrolysis of ATP, you

must refer to energy being

released. You will not gain

credit for stating that

energy is produced.

Knowledge check 1

(a) Give three examples

of cellular activities that

require ATP.

(b) Describe three

advantages of ATP

for its function as the

universal source of

energy.

ATP

Energy for

cellular work

Energy from

respiration or

photons of light

ADP + P i

Figure 2 The interconversion of ATP, ADP and Pi

ATP is reformed from ADP and Pi by a condensation reaction. This requires the input of

energy, i.e. it is an endergonic reaction. The energy required can come from cellular

respiration or from the transduction of light energy during photosynthesis. This

reaction is catalysed by the enzyme ATP synthase (also known as ATP synthetase).

Content Guidance

8 WJEC A2 Biology

Examiner tip

Synoptic link to BY1:

Respiration is a series

of enzyme-catalysed

reactions. Therefore

factors that affect

enzymes affect the

rate of respiration. The

most important factor

influencing the rate of

respiration is temperature.

Aerobic respiration

Mitochondria are present in all eukaryotic cells and they are involved in synthesis

of ATP during aerobic respiration. Figure 4 shows the structure of a mitochondrion.

The organelle is composed of a double membrane enclosing a fluid-filled matrix. The

inner membrane is highly folded to form cristae. This increases the surface area for

the synthesis of ATP.

Figure 4 Structure of a mitochondrion

Figure 5 shows the location of the four stages of respiration. ATP is synthesised

mainly in the mitochondria.

l Oxidation and reduction reactions always take place together because as one

molecule is oxidised another molecule is reduced. These chemical reactions are

called redox reactions.

l Figure 3 represents a typical step in the respiratory pathway. Note that the

coenzyme (NAD) is reduced as the organic molecule is oxidised.

NAD

Organic molecule

Reduced

NAD

Oxidised

organic molecule

Figure 3 Oxidation of an organic molecule coupled with the reduction of a coenzyme

l The oxidation reactions involved in respiration are exergonic. The energy

released from the organic molecules is used to reduce the coenzymes NAD and

FAD, as these reactions are coupled. During each oxidation reaction a small

quantity of energy is, in effect, transferred to these coenzymes.

l During glycolysis, the link reaction and Krebs cycle, organic molecules are

repeatedly oxidised and therefore most of the energy contained within glucose is

transferred to the reduced coenzymes NADH2 and FADH2. When the coenzymes

are re-oxidised the stored energy is used to synthesise ATP via oxidative

phosphorylation.

l The series of oxidation reactions in respiration brings about the gradual release

of chemical energy from organic molecules in a series of small steps (as opposed

to combustion which is the uncontrolled release of energy in a single step).

l Aerobic respiration occurs in the presence of oxygen. Respiration that takes

place in the absence of oxygen is called anaerobic respiration.

Matrix

Cristae

Outer

membrane

Inner

membrane