Photosynthesis

Question 1

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.

Answer 1

1. (Less/no) ATP;

2. (Less/no) reduced NADP;

Question 2

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

Answer 2

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

2. (Less/no) GP;

Question 3

Where precisely is rubisco found in a cell?

Answer 3

1. Stroma (of/in chloroplast);

Question 4

The solution that the student used to produce the chloroplast suspension had the same water potential as the chloroplasts.
Explain why it was important that these water potentials were the same.

Answer 4

1. Osmosis does not occur;

2. Chloroplast / organelle does not burst / lyse / shrivel / shrink;

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.

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

When treated with Atrazine, weeds have been shown to give off small amounts of heat.
Suggest an explanation for this observation.

Answer 6

Idea that energy is released from high energy / excited electron/s (that were lost from chlorophyll)

Question 7

Where precisely in a cell does the Calvin cycle take place?

Answer 7

Stroma (of chloroplasts);

Question 8

Some bacteria use hydrogen sulfide, H2S, to produce organic compounds.
The hydrogen sulfide has a similar role to that of water in photosynthesis.
A simple equation for this process in bacteria is shown below:
hydrogen sulfide + carbon dioxide → glucose + sulfur + water
Suggest what the hydrogen sulfide is used for in these bacteria.

Answer 8

1. (Provides) hydrogen / protons/H+ and electrons/e-;
   Ignore: if water is used as source of hydrogen.
2. For reduction;
   Reject: reduction of NAD.
   Reject: reduction by H+ or protons on their own.
3. Source of electrons for chlorophyll/electron transfer chain;

Question 9

Crops use light energy to produce photosynthetic products.

Describe how crop plants use light energy during the light-dependent reaction.

Answer 9

1. Excites electrons / electrons removed (from chlorophyll);
   Accept: higher energy level as ‘excites’.
2. Electrons move along carriers/electron transfer chain releasing energy;
   Accept: movement of H+/protons across membrane releases energy.
   Reject: ‘produces energy’ for either mark but not for both.
3. Energy used to join ADP and Pi to form ATP;
   Reject: ‘produces energy’ for either mark but not for both.
   Accept: energy used for phosphorylation of ADP to ATP
   Do not accept P as Pi but accept phosphate.
4. Photolysis of water produces protons, electrons and oxygen;
5. NADP reduced by electrons / electrons and protons / hydrogen;

Question 10

Scientists studied the rate of carbon dioxide uptake by grape plant leaves. Grape leaves have stomata on the lower surface but no stomata on the upper surface.
The scientists recorded the carbon dioxide uptake by grape leaves with three different treatments:
Treatment 1 − No air-sealing grease was applied to either surface of the leaf.
Treatment 2 − The lower surface of the leaf was covered in air-sealing grease that prevents gas exchange.
Treatment 3 − Both the lower surface and the upper surface of the leaf were covered in air–sealing grease that prevents gas exchange.
Suggest the purpose of each of the three leaf treatments.

Answer 10

1.      (No grease)
            means stomata are open
            OR
            allows normal CO2 uptake;
Allow ‘gas exchange’ for CO2 uptake.
‘As a control’ is insufficient on its own.
2.      (Grease on lower surface)
seals stomata
OR
stops CO2 uptake through
stomata
OR
to find CO2 uptake through
stomata
OR
shows CO2 uptake through cuticle / upper surface;
3.      (Grease on both surfaces) shows sealing is effective
OR
stops all CO2 uptake.

Question 11

The stomata close when the light is turned off.

Explain the advantage of this to the plant.

Answer 11

1. (Because) water is lost through stomata;
2. (Closure) prevents / reduces water loss;
3. Maintain water content of cells.

Question 12

What measurements should the student have taken to determine the rate of photosynthesis?

Answer 12

Oxygen production / concentration and time.

Question 13

A student investigated the effect of different wavelengths of light on the rate of photosynthesis. She used the apparatus shown in Figure 1.

(b) Other than temperature and pH, give two factors which should be kept constant during this investigation.

Answer 13

1. Intensity of light;
   Accept: distance from light
2. Amount / number / mass / species of algae / photosynthesising cells;
3. Carbon dioxide (concentration / partial pressure);
4. Time.

Question 14

The student did not use a buffer to maintain the pH of the solution.
Explain what would happen to the pH of the solution during this investigation

Answer 14

1. (pH) increases;
   Neutral: becomes more alkaline / less acidic
2. As (more) carbon dioxide removed (for photosynthesis).

Question 15

Suggest and explain why the rate of photosynthesis was low between 525 nm and 575 nm wavelengths of light.

Answer 15

1. Less absorption / (more) reflection (of these wavelengths of light);
   Reject: no absorption or cannot absorb unless in context of green light.
   Note: no green light absorbed or green light reflected = 2 marks.
2. (Light required) for light dependent (reaction) / photolysis
   Accept: for excitation / removal of electrons (from chlorophyll)
3. (Represents) green light / colour of chlorophyll.

Question 16

During the light-independent reaction of photosynthesis, carbon dioxide is converted into organic substances. Describe how.

Answer 16

1. Carbon dioxide combines with ribulose bisphosphate / RuBP;
2. Produces two glycerate (3-)phosphate / GP;
   Accept: any answer which indicates that 2 x as much GP produced from one RuBP.
3. GP reduced to triose phosphate / TP;
   Must have idea of reduction. This may be conveyed by stating m.p. 4.
4. Using reduced NADP;
   Reject: Any reference to reduced NAD for m.p.4 but allow reference to reduction for m.p. 3.
5. Using energy from ATP;
   Must be in context of GP to TP.
6. Triose phosphate converted to glucose / hexose / RuBP / ribulose bisphosphate / named organic substance;

Question 17

Explain the relationship between stomatal opening and photosynthesis.

Answer 17

1. Stomata allow uptake of carbon dioxide;

2. Carbon dioxide used in / required for photosynthesis;