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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.
Philip Allan, an imprint of Hodder Education, an Hachette UK company, Market Place, Deddington,
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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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Contents
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
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-andchallenge 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.
Unit BY4: Metabolism, Microbiology and Homeostasis
5
Respiration
the hydrolysis of ATP releases small quantities of energy (30.6 kJ mol−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
ATP
Energy from
respiration or
photons of light
Energy for
cellular work
ADP + P i
Figure 2 The interconversion of ATP, ADP and P i
ATP is reformed from ADP and P i 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).
After studying this topic you should be able to:
l฀ understand the importance of chemical energy in
biological processes
l฀
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.
recognise the structure of ATP and describe its role
as an energy carrier and its use in the liberation of
energy for cellular activity
Summary
l฀
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 + 6H 2O
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−)
Unit BY4: Metabolism, Microbiology and Homeostasis
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.
7
Respiration
Glycolysis
The link reaction
Krebs cycle
(formation of
acetyl coenzyme A)
Electron transport
chain
Glucose
Acetyl
coenzyme
A
Krebs
cycle
Electron
transport and
chemiosmosis
2 ATP
34 ATP
Pyruvate
2 ATP
Figure 5 Outline of the stages of aerobic respiration
Glycolysis (splitting of glucose)
Glycolysis occurs in the cytoplasm — this is where the enzymes for glycolysis are
located. The main stages in the pathway are shown in Figure 6.
Glucose
6C
2 ATP
2 ADP
Hexose bisphosphate
6C
2 × triose phosphate
3C
2 NAD
4 ADP + Pi
2 NADH2
4 ATP
2 × pyruvate
3C
Figure 6 The steps involved in glycolysis
l฀
Step 1: Two molecules of ATP are required for the phosphorylation of glucose to
produce hexose bisphosphate. The energy from the hydrolysis of ATP activates
glucose and makes the molecule more reactive.
l฀
Step 2: Hexose bisphosphate is split (lysis) producing two molecules of triose
phosphate.
Unit BY4: Metabolism, Microbiology and Homeostasis
9
Respiration
Acetyl CoA
2C
Examiner tip
Figure 8 shows the
names of some of the
intermediate molecules
involved in the Krebs cycle.
They may appear in the
exam, but you are not
expected to know them.
What is important is that
you understand what is
produced when these
molecules are recycled.
Coenzyme A
(Oxaloacetate)
4C
(Citrate)
6C
FADH2
NAD
FAD
CO2
CO2
NADH2
2 NADH2
2 NAD
(Ketoglutarate)
5C
ATP
Knowledge check 3
ADP + Pi
Explain why carbon
dioxide is not produced
when glucose is added to
a preparation of isolated
mitochondria.
Figure 8 The steps involved in the Krebs cycle
l฀
the reduction of the coenzymes NAD and FAD to NADH 2 and FADH 2
l฀
the production of ATP from ADP and P i by substrate-level phosphorylation
The electron transport chain (ETC)
The electron transport chain involves a chain of electron carriers located on the inner
mitochondrial membrane (cristae). The cristae have a large surface area so there
are more electron carriers, which increases ATP synthesis. The reduced coenzymes,
NADH 2 and FADH 2, produced during glycolysis, the link reaction and the Krebs cycle
act as a source of electrons and protons.
Figure 9 shows the electron carriers at progressively lower energy levels. As electrons
pass along the chain of carriers in a series of redox reactions, they release energy. This
energy is used to synthesise ATP by oxidative phosphorylation. Oxygen is the
terminal electron acceptor. It combines with protons (H+) and electrons (e−) and is
reduced to water.
Oxidative
phosphorylation
involves the synthesis of
ATP using energy released
from redox reactions.
ADP + Pi
Potential energy level
NADH2
Carrier 2
ADP + Pi
Reduced
carrier 3
NAD
ADP + Pi
Reduced
carrier 2
Carrier 4
H2O
Carrier 3
ATP
Reduced
carrier 4
ATP
O2
ATP
Figure 9 The electron transport chain
Unit BY4: Metabolism, Microbiology and Homeostasis
11
Respiration
Glucose (6C)
ATP
Glycogen
Starch
(plants)
ADP
Glucose phosphate (6C)
ATP
ADP
Hexose bisphosphate (6C)
Triose phosphate (3C)
Triose phosphate (3C)
2ADP
NAD
Reduced
NAD
2ATP
Glucose
pyruvate = glycolysis
Pyruvate
NAD
Pyruvate
Reduced
NAD
acetyl coA = link reaction
Coenzyme A
CO 2
Acetyl coenzyme A
Reduced
NAD
Coenzyme A
Oxaloacetate (4C)
Citrate (6C)
NAD
NAD
Reduced
NAD
Krebs cycle
CO 2
Reduced
FAD
FAD
NAD
ATP ADP + Pi
CO 2
Reduced
NAD
Reduced hydrogen carriers
(reduced NAD and FAD are oxidised)
Oxygen
Water
Oxidative phosphorylation
ATP
ADP + Pi
Figure 11 Summary of aerobic respiration
Unit BY4: Metabolism, Microbiology and Homeostasis
Examiner tip
On an exam paper, the
reduced forms of NAD
and FAD are likely to
be written as reduced
NAD and reduced FAD
respectively (as in Figure
11). When answering
questions it is perfectly
acceptable to use NADH2
or NADH + H+ and
FADH2 or FADH + H+.
13
Respiration
2 × Triose phosphate (3C)
4ADP
4ATP
2×
NAD
2H
2 × Reduced
NAD
2 × Pyruvate (3C)
2 × Lactate (3C)
Figure 12 Anaerobic respiration in animals
2 × Triose phosphate (3C)
Knowledge check 7
4ADP
State the name of the
molecule that acts as the
terminal electron acceptor
in the following processes:
2×
NAD
2H
4ATP
2 × Reduced
NAD
(a) aerobic respiration
2 × Pyruvate (3C)
2 × Ethanal
(2C)
2 × Ethanol
(2C)
2CO 2
Figure 13 Anaerobic respiration in plants and fungi
(b) anaerobic respiration in
a muscle cell
(c) anaerobic respiration in
yeast
Comparison of energy yields
Not all the energy of the glucose molecule is transferred to ATP. There is a loss of
energy as heat energy.
Aerobic respiration involves the complete breakdown of glucose to carbon dioxide
and water. It produces 38 molecules of ATP per molecule of glucose. It is about
40% efficient.
Anaerobic respiration involves the incomplete breakdown of glucose and produces
two molecules of ATP per molecule of glucose. It is about 2% efficient. Energy still
remains locked up in lactate/ethanol.
Alternative respiratory substrates
Under certain circumstances fats and proteins may be used as respiratory substrates.
Individuals are able to survive for long periods without food because they can use
their reserves of carbohydrate, fat and protein:
l฀
Glycerol is converted into triose phosphate and enters glycolysis. Long-chain fatty
acid molecules are split into 2C fragments, which enter the pathways as acetyl
coenzyme A.
l฀
When a person is starving, tissue protein used as a source of energy. It is hydrolysed
into its constituent amino acids, which are deaminated (NH 2 groups are removed).
This leaves an organic acid that can enter the Krebs cycle.
Unit BY4: Metabolism, Microbiology and Homeostasis
Examiner tip
All organic molecules can
be respired, but glucose
is the main respiratory
substrate. Most questions
on respiration use glucose
as the starting point.
15
Photosynthesis
Outer membrane
Inner membrane
Chloroplast envelope
Lipid droplet
Intergranal lamella
Thylakoid
Stroma
Granum
Examiner tip
Synoptic link to BY1:
Starch grains are found
in chloroplasts. Within
the stroma molecules
of glucose undergo
condensation reactions to
form starch. Therefore,
there must be an enzyme
in the stroma that catalyses
these reactions.
Starch grain
DNA
70S ribosomes
Cytosol
Figure 14 The structure of a chloroplast
l฀ The light-independent stage involves a series of enzyme-catalysed reactions in
which carbon dioxide is reduced to form a carbohydrate. This requires NADPH 2
and energy released from the hydrolysis of ATP.
l฀ Figure 15 shows an overview of the two main stages in photosynthesis occurring
within a chloroplast.
Light energy
H2O
Light-dependent stage
ADP + Pi
CO2
ATP
O2
NADP NADPH2
Light-independent stage
CHO
(carbohydrate)
Figure 15 Outline of the stages of photosynthesis
Photosynthetic pigments
There are several photosynthetic pigments found in plants. They can be divided
into two main groups, the chlorophylls and the carotenoids. The function of the
pigments is to absorb light energy, thereby converting it into chemical energy.
Chromatography can be used to separate out the leaf pigments so that they can be
identified.
Unit BY4: Metabolism, Microbiology and Homeostasis
17
Photosynthesis
(a)
Knowledge check 8
Outer
membrane
Intermembrane
space
Inner
membrane
Explain the advantage to a
plant of having chloroplasts
that contain several
different light-absorbing
pigments.
Stroma
(b)
Thylakoid lumen
Granum
(stack of
thylakoids)
Thylakoid
Lamellae
Knowledge check 9
Explain why plants appear
green.
(c)
Cluster of pigment molecules
embedded in membrane (see Figure 18)
Thylakoid membrane
Stroma
Examiner tip
Synoptic link to BY1: In
BY1, you learnt about the
structure of chloroplasts
and mitochondria. You
may be given electron
micrographs or drawings
of these organelles and
asked to identify structures
or to indicate where the
stages of photosynthesis or
respiration occur.
Figure 17 (a) The structure of a chloroplast; (b) A section through
a single thylakoid; (c) Pigments in a thylakoid membrane
Knowledge check 10
Light energy
Transfer of energy
State exactly where
in the chloroplast you
would expect to find
photosystems.
Knowledge check 11
Antenna complex
Accessory
pigments
Reaction centre
(chlorophyll a)
Figure 18 A photosystem
Unit BY4: Metabolism, Microbiology and Homeostasis
Photosystems contain
several pigments. State
the location of the
following pigments within a
photosystem:
(a) chlorophyll a
(b) chlorophyll b
19
Photosynthesis
into the thylakoid space. The protons accumulate so that steep concentration and
electrochemical gradients are established between the thylakoid space and the
stroma. These gradients are also maintained by:
l฀
the photolysis of water, which occurs in the thylakoid space and increases the H+
concentration
l฀
the reduction of NADP, which occurs in the stroma and decreases the H+ concentration
The protons (H+) diffuse back into the stroma through the chemiosmotic protein
channels where the enzyme ATP synthase is located. The flow of protons through
ATP synthase provides the energy required to produce ATP from ADP and P i.
H+
H
H+
+
H+
H
H+
1/ O
2 2
H+
+
Thylakoid space
+
H
2H+
H+
Proton pump
H+
H+
Electron carrier
H2O
e−
e−
Thylakoid
membrane
−
−
e
e
PS II
+
H
Light energy
P680
Light energy
P700
e−
PS I
NADPH2
NADP + 2H+
Stroma
ATP synthase
ATP
ADP + Pi
Figure 20 Chemiosmosis and non-cyclic photophosphorylation
Electrons from the chlorophyll a molecules in photosystem I are used to reduce NADP
and are replaced indirectly by electrons from the photolysis of water. This is known
as non-cyclic phosphorylation and is represented by stages 1 to 5 in Figure 19.
You can see from Figure 19 that the electron acceptor at stage 4 is at the highest energy
state. It is possible for some of these excited electrons to return to the chlorophyll a
molecule in photosystem I via the electron transport chain. This is known as cyclic
phosphorylation and is represented by stages 4, 6 and 3 in Figure 19.
Cyclic and non-cyclic photophosphorylation are compared in Table 3.
Table 3 Cyclic and non-cyclic photophosphorylation
Feature
Cyclic
photophosphorylation
Non-cyclic
photophosphorylation
Photosystems involved
I only
I and II
Photolysis of water
No
Yes
Electron donor
Chlorophyll a in
photosystem I
Chlorophyll a in
photosystem I
Terminal electron acceptor
Chlorophyll a in
photosystem I
NADP
Products
ATP
ATP, NAPH2 and oxygen
Unit BY4: Metabolism, Microbiology and Homeostasis
21
Photosynthesis
Investigating photosynthesis: Calvin’s lollipop
Melvin Calvin was an American biochemist who investigated the pathway by which
carbon dioxide is converted into organic compounds during photosynthesis. A
suspension of the unicellular alga Chlorella was placed in a flattened glass vessel that
was called the ‘lollipop’ (see Figure 22a). The suspension of Chlorella was supplied
with radioactive carbon dioxide. The lollipop was illuminated and the algae allowed
to photosynthesise. As the Chlorella photosynthesised, the radioactive carbon
dioxide was ‘fixed’ and incorporated into organic molecules (the intermediate
compounds), which became radioactive. At specific time intervals samples of the
Chlorella were released into boiling alcohol. This denatured enzymes, killed the
Chlorella and stopped the light-independent reactions at a particular point in time.
Compounds that the radioactive carbon had reached at a particular moment were
determined by chromatography and autoradiography (see Figure 22b). The order
in which each compound is produced was found by identifying the molecules and
analysing the results. From these results Calvin discovered the metabolic pathway
that is now known as the light-independent stage of photosynthesis.
(a)
Suspension of
algal cells
Light
Syringe for
injecting
radioactive
carbon
dioxide
Rapid
action
tap
Hot
alcohol
(b)
Autoradiogram produced after
5 seconds of photosynthesis
Autoradiogram produced after
15 seconds of photosynthesis
Aspartic
acid
Sugar
phosphates
Glycerate-3phosphate
Triose
phosphate
Sugar
diphosphates
Original position
of extract
Original position
of extract
Figure 22 (a) Calvin’s lollipop; (b) autoradiograms showing the
different molecules synthesised during Calvin’s experiments
Unit BY4: Metabolism, Microbiology and Homeostasis
23