TOPIC 5 EXAM QUESTIONS

Question 1

Describe what happens during photoionisation in the light-dependent
reaction. (2)

Answer 1

1. Chlorophyll absorbs light
   OR
   Light excites/moves electrons in chlorophyll;
   Ignore photosystems.
2. Electron/s are lost
   OR
   (Chlorophyll) becomes positively charged;
   Accept electrons go to electron transport/carrier chain for
   ‘electrons lost’.

Question 2

Heat stress is a condition that often occurs in plants exposed to high
temperatures for a prolonged period of time. Heat stress is a major factor in
limiting the rate of photosynthesis.
(a) Heat stress decreases the light-dependent reaction of photosynthesis.
Explain why this leads to a decrease in the light-independent reaction.
(2)

Answer 2

1. (Less/no) ATP;
2. (Less/no) reduced NADP;

Question 3

A decrease in the activity of the enzyme rubisco would limit the rate of
photosynthesis.
Explain why. (2)

Answer 3

1. (Less/no) carbon dioxide (reacts) with RuBP;
2. (Less/no) GP;

Question 4

In natural ecosystems, most of the light falling on producers is not used in
photosynthesis.
Suggest two reasons why. (2)

Answer 4

1. (Light is) reflected;
   Light is not absorbed on its own is not enough.
2. (Light is) wrong wavelength;
3. (Light) misses chlorophyll/ chloroplasts/photosynthetic tissue;
4. CO2 concentration or temperature is a limiting factor.

Question 5

Atrazine binds to proteins in the electron transfer chain in chloroplasts of
weeds, reducing the transfer of electrons down the chain.
Explain how this reduces the rate of photosynthesis in weeds. (4)

Answer 5

1. Reduced transfer of protons across thylakoid membrane
   OR
   Reduced chemiosomotic gradient / proton gradient across thylakoid
   membrane;
2. (So) less ATP produced;
3. (So) less reduced NADP produced;
   Accept NADPH / NADPH2 / NADPH+
   Reject reduced NAD
4. (So) light-independent reaction slows / stops;
   OR
   Less reduction of GP to triose phosphate.

Question 6

Describe the process of glycolysis (4)

Answer 6

) 1. Phosphorylation of glucose using ATP;
2. Oxidation of triose phosphate to pyruvate;
Accept removal of hydrogen from triose phosphate
for oxidation.
3. Net gain of ATP;
Accept any description that indicates a net gain
e.g., 4 produced, 2 used.
4. NAD reduced;
Accept NADH/NADH2/NADH + H+ produced.

Question 7

Describe the advantage of the Bohr effect during intense exercise. (2)

Answer 7

1. Increases dissociation of oxygen;
   Accept unloading/ release/reduced affinity for
   dissociation
2. For aerobic respiration at the tissues/muscles/cells
   OR
   Anaerobic respiration delayed at the tissues/muscles/cells
   OR
   Less lactate at the tissues/muscles/cells;

Question 8

Describe how acetylcoenzyme A is formed in the link reaction. (2)

Answer 8

1. Oxidation of / hydrogen removed from pyruvate and carbon dioxide released;
2. Addition of coenzyme A.

Question 9

In the Krebs cycle, acetylcoenzyme A combines with four-carbon oxaloacetate to
form six-carbon citrate. This reaction is catalysed by the enzyme citrate synthase.
(i) Oxaloacetate is the first substrate to bind with the enzyme citrate synthase.
Respiration
This induces a change in the enzyme, which enables the acetylcoenzyme A to
bind.
Explain how oxaloacetate enables the acetylcoenzyme A to then bind to the
enzyme. (2)

Answer 9

1. Change (in shape) of active site / active site moulds around the
   substrate;
   Reject: reference to inhibitor
   Accept: change in tertiary structure affecting active site
2. (Substrate / active site) now complementary.

Question 10

Freshwater marsh soils are normally waterlogged. This creates anaerobic
conditions.
Use your knowledge of the nitrogen cycle to suggest why these soils
contain relatively high concentrations of ammonium compounds and low
concentrations of nitrite ions and nitrate ions. (2)

Answer 10

1. Less nitrification
   OR
   Fewer/less active nitrifying bacteria;
   OR
   Nitrification/nitrifying bacteria require oxygen/aerobic conditions;
2. (Less) oxidation/conversion of ammonium (ions) to nitrite
   (ions) and to nitrate (ions);
3. More denitrification
   OR
   More/more active denitrifying bacteria
   OR
   Denitrification/denitrifying bacteria do not require oxygen
   OR
   Denitrification/denitrifying bacteria require anaerobic conditions;
4. (So more) nitrate (ions) reduced/converted to nitrogen (gas);

Question 11

The scientist determined the dry mass of the spinach plants. First, he
heated each sample at 80 °C for 2 hours.
Suggest what the scientist should do to ensure that he has removed all the
water from the sample. (2)

Answer 11

1. Weigh and heat;
   Accept ‘heat until sample remains the same mass’
   for 2 marks.
   Ignore temperature.
2. (Until) mass is constant;

Question 12

Describe the role of saprobionts in the nitrogen cycle. (2)

Answer 12

1. (They use enzymes to) decompose proteins/DNA/RNA/urea;
   Accept any named molecule containing nitrogen eg
   enzymes, NAD, ATP, amino acids
   Accept digest/breakdown/hydrolyse for decompose
   Ignore ‘nitrogen -containing compounds’ unqualified
2. Producing/releasing ammonia/ammonium
   compounds/ammonium ions;

Question 13

Suggest appropriate units for gross productivity. (1)

Answer 13

Unit of energy / mass, per area, per year.

Question 14

.Nitrate from fertiliser applied to crops may enter ponds and lakes. Explain how nitrate may
cause the death of fish in fresh water. (5)

Answer 14

1. Growth of algae / surface plants / algal bloom blocks light;
2. Reduced / no photosynthesis so (submerged) plants die;
3. Saprobiotic (microorganisms / bacteria);
4. Accept: Saprobiont / saprophyte / saprotroph
5. Neutral: decomposer
6. Aerobically respire / use oxygen in respiration;
7. Less oxygen for fish to respire / aerobic organisms die;