A2 revision
starters & activities
Question 1
The light-dependent stage of photosynthesis takes place on the thylakoid
membranes in chloroplasts. These membranes surround the thylakoid space (lumen)
and are arranged into stacks known as grana.
Figure 1a shows the arrangement of photosystems in the thylakoid membrane as
well as summarising the processes that occur there.
Figure 1a
Figure 1a
(a) Using the information in Figure 1a explain:
(i) why the pH of the thylakoid space (lumen) is lower than that of the
stroma
(ii) what significance this has for ATP production.
[4]
(b) Herbicides (weedkillers) interfere with electron transport by accepting
electrons.
Explain how this causes plants to die.
[3] [Total: 7]
Sample answer
a)
i.
ii.
b)
Hydrogen ions are pumped into the thylakoid space as electrons are passed
along the electron carriers (G in Figure 1a). This creates a higher
concentration of hydrogen ions inside the thylakoid space, which lowers the
pH in the lumen.
Because of this, the hydrogen ions diffuse down the concentration gradient
across the thylakoid membrane from the lumen to stroma through ATP
synthetase channels, which in turn generates ATP. So a low pH results in ATP
formation.
By accepting electrons, weedkillers could prevent production of ATP from
chemiosmosis. Also, NADP will not be reduced so no NADPH would be
produced. This means that the light-independent stage (Calvin cycle) will
not happen, so there will be no fixation of carbon dioxide into sugars or
other organic molecules such as fatty acids or amino acids. As a result
the plant will die.
Examiner’s comments
a)
This is a Stretch and Challenge question because it requires you to use
information from a diagram to bring together related concepts – pH and ATP
production.
i.
Here you should discuss hydrogen ions not H or simply hydrogen. Notice
that some of the main terms used in the answer – e.g. thylakoid lumen,
stroma, ATP synthetase – are present in Figure 3, so remember to use
them to ensure you get all the available marks! Referring to the figure also
demonstrates that you are using it as requested in the question.
ii.
This answer is a good one for Stretch and Challenge as it brings
everything together – it relates back to what is being asked for in the
question. Remember to always answer the question!
You could also gain marks in part (a) for correctly referring to chemiosmosis
or to an electrochemical gradient or proton motive force.
Examiner’s comments
.

Here you are not expected to know exactly how weedkillers work – the
command word suggest tells you this. However, especially for a Stretch and
Challenge question, you should use the information given to put forward a
possible reason – but don’t take a wild guess!
Notice that the answer to part (b) builds upon your previous answer to part
(a). This is a common occurrence in Stretch and Challenge questions and
can give you a clue as to where to begin your answer.
You can also gain marks in part (b) for mentioning that autotrophic nutrition
stops or that no respiratory substrate will be produced – however don’t
mention food as in ‘the plant does not produce food’.
Note: In the diagram can you identify the pigments P680, P700 and the type
of molecule represented by G? These are not Stretch and Challenge
elements but core knowledge for revision.
Question 2
Figure 4 shows the relationship between various metabolic processes in yeast.
Figure 2a
In an investigation, yeast cells were homogenised (broken up) and the resulting
homogenate centrifuged. Portions containing only nuclei, ribosomes,
mitochondria or cytosol (residual cytoplasm) were isolated. Samples of each
portion and of the complete homogenate were incubated in four different ways:
1. with glucose.
2. with pyruvate
3. with glucose and cyanide
4. with pyruvate and cyanide.
Cyanide inhibits carriers in the electron transport chain such as cytochromes.
After incubation, the presence or absence of carbon dioxide and lactate in each
sample was determined.
The results are summarised in Table 2b below.
= absent  = present  = a little
Samples of homogenate
Complete
Nuclei only
Ribosomes only
Mitochondria only
Cytosol
Carbon
dioxide
Ethanol
Carbon
dioxide
Ethanol
Carbon
dioxide
Ethanol
Carbon
dioxide
Ethanol
1. Glucose










2. Pyruvate










3. Glucose
and cyanide










4. Pyruvate
and cyanide










a)
Carbon
dioxide
Ethanol
Suggest why more carbon dioxide is produced when the complete
homogenate is incubated with just glucose or pyruvate present than
when cyanide is also present
[3]
The results are summarised in Table 2b below.
= absent  = present  = a little
Samples of homogenate
Complete
Nuclei only
Ribosomes only
Mitochondria only
Cytosol
Carbon
dioxide
Ethanol
Carbon
dioxide
Ethanol
Carbon
dioxide
Ethanol
Carbon
dioxide
Ethanol
1. Glucose










2. Pyruvate










3. Glucose
and cyanide










4. Pyruvate
and cyanide










b)
Carbon
dioxide
Suggest why carbon dioxide is produced when mitochondria are
incubated with pyruvate but not when incubated with glucose.
Ethanol
[3]
The results are summarised in Table 2b below.
= absent  = present  = a little
Samples of homogenate
Complete
Nuclei only
Ribosomes only
Mitochondria only
Cytosol
Carbon
dioxide
Ethanol
Carbon
dioxide
Ethanol
Carbon
dioxide
Ethanol
Carbon
dioxide
Ethanol
1. Glucose










2. Pyruvate










3. Glucose
and cyanide










4. Pyruvate
and cyanide










a)
Carbon
dioxide
Ethanol
Suggest why ethanol production can still occur in the presence of cyanide.
[3]
[Total: 9]
Sample answer
(a)
Without cyanide the complete homogenate can break down the
glucose/pyruvate fully to produce carbon dioxide. However, when
cyanide is present pyruvate does not enter the mitochondria.
Cyanide inhibits an enzyme involved in oxidative phosphorylation.
It reduces ATP production. If oxidative phosphorylation stops, so
does Krebs cycle and link reaction. Some carbon dioxide is
produced when pyruvate is converted to ethanal.
You could also say that breakdown of the glucose/pyruvate is
incomplete. A correct reference to anaerobic respiration will also
gain a mark.
Sample answer
b)
Pyruvate is the end product of glycolysis and it can enter
mitochondria where carbon dioxide is produced (in the Krebs
cycle and the link reaction). Glucose cannot enter the
mitochondria, and the enzymes for breaking down glucose are
only found in the cytoplasm not in mitochondria.
You can also gain a mark for saying that carbon dioxide is
produced by decarboxylation or a decarboxylase enzyme.
Furthermore, glucose cannot enter mitochondria because there
are no carriers for glucose in mitochondrial membrane. Also,
glycolytic enzymes are not found in mitochondria but they are
found in cytoplasm (cytosol).
Sample answer
c)
Pyruvate is converted to ethanal in cytoplasm and the ethanal is
converted to ethanol. The enzymes for these conversions are not
inhibited by cyanide.
You can gain a mark by using the information in the question that
tells you that conversion of pyruvate to ethanol does not involve
electron carriers in the electron transport chain, so cyanide will not
affect it.
Examiner’s comments
This question requires some background knowledge of cellular
respiration: ribosomes and nuclei are not involved; glycolysis
occurs only in the cytoplasm not in mitochondria, etc. However, it
is a Stretch and Challenge question since the answers have to be
deduced from the table and the information given. For instance,
you can deduce from the table that mitochondria can only operate
with pyruvate and are affected by cyanide whereas cytosol is not.



Note: Some parts of this exam question have been omitted as we
are only concentrating on the Stretch and Challenge aspects.
The omitted questions were on core concepts in cellular
respiration and asked about processes A, B and C. Can you
identify and discuss these processes?
Stretch and Challenge elements are likely to be included in this
way as an extension to sections on recall and core understanding.



Note: Some parts of this exam question have been omitted as we
are only concentrating on the Stretch and Challenge aspects.
The omitted questions were on core concepts in cellular
respiration and asked about processes A, B and C. Can you
identify and discuss these processes?
Stretch and Challenge elements are likely to be included in this
way as an extension to sections on recall and core understanding.