3.5.1 Photosynthesis

Question 1

What happens when chlorophyll absorbs light energy?

Answer 1

Chlorophyll absorbs light energy leading to photoionisation where electrons are excited and released.

Question 2

What is the fate of electrons released during photoionisation?

Answer 2

Electrons are transferred down the electron transfer chain and their energy is conserved in ATP and reduced NADP.

Question 3

How is ATP produced in the light-dependent reaction?

Answer 3

ATP is produced by electron transfer down the electron transfer chain which drives proton movement across chloroplast membranes via ATP synthase (chemiosmosis).

Question 4

What is photolysis and what are its products?

Answer 4

Photolysis is the splitting of water using light energy producing protons electrons and oxygen.

Question 5

How is reduced NADP formed in the light-dependent reaction?

Answer 5

Electrons from the electron transfer chain and protons from photolysis reduce NADP to form reduced NADP.

Question 6

What is the role of ATP and reduced NADP in the light-independent reaction?

Answer 6

ATP provides energy and reduced NADP provides hydrogen to reduce glycerate 3-phosphate (GP) to triose phosphate (TP).

Question 7

What enzyme catalyses the fixation of carbon dioxide in the Calvin cycle?

Answer 7

Rubisco catalyses the reaction of carbon dioxide with ribulose bisphosphate (RuBP) to form two molecules of glycerate 3-phosphate (GP).

Question 8

What happens to triose phosphate in the Calvin cycle?

Answer 8

Some triose phosphate regenerates RuBP and some is converted into useful organic substances.

Question 9

What are the limiting factors of photosynthesis?

Answer 9

Light intensity carbon dioxide concentration and temperature.

Question 10

How do farmers overcome limiting factors of photosynthesis?

Answer 10

Use of artificial lighting carbon dioxide enrichment and temperature control in greenhouses.

Question 11

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

Answer 11

1. Chlorophyll absorbs light
   OR
   Light excites/moves electrons in chlorophyll;
   Ignore photosystems.
2. Electron/s are lost
   OR
   (Chlorophyll) becomes positively charged;

Question 12

Light Dependent Reaction –
Describe the process (6)

Answer 12

1. Excites electrons / electrons removed from chlorophyll;
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;.
4. Photolysis of water produces protons, electrons and oxygen;
5. NADP reduced by electrons and protons / hydrogen

Question 13

Light Dependent Reaction. Describe

the electron transport chain (5)

Answer 13

1. Excites electrons (from chlorophyll);
2. Electrons move along carriers/electron transfer chain releasing energy;
3. Energy used to join ADP and Pi to form ATP;
4. Photolysis of water produces protons, electrons and oxygen;
5. NADP reduced by electrons;

Question 14

Light Independent Reaction.

Describe the Calvin Cycle (6)

Answer 14

1. Carbon dioxide combines with RuBP;
2. Produces two GP;
3. GP reduced to Triose Phosphate;
4. Using reduced NADP;
5. Using energy from ATP;
6. TP converted to glucose / hexose / RuBP / ribulose bisphosphate / named organic substance;

Question 15

Why weedkillers cause plants to
give off heat? (2)

Answer 15

1. Energy is released from high energy/excited electron/s (that were lost from chlorophyll
2. This energy which should be used to form ATP, cannot do this as the weedkiller blocks the ETC, (so the energy is lost as heat)

Question 16

Why does increased light intensity
stimulate plant growth? (5)

Answer 16

1. Have faster production of ATP and reduced NADP;
2. (So) have faster/more light-independent reaction;
3. (So) produce more sugars that can be used in respiration;
4. (So) have more energy for growth;
5. Have faster/more synthesis of new organic materials;

Question 17

Explain why the student marked the origin of a chromatogram using a pencil rather than using
ink.

Answer 17

Ink and (leaf) pigments would mix

Question 18

Describe the method the student used to separate the pigments after the
solution of pigments had been applied to the origin.

Answer 18

1. Level of solvent below origin/line;

Reject water or any named aqueous solution.
Accept named organic solvent.

2. Remove/stop before (solvent) reaches top/end;

Question 19

The pigments in leaves are different colours. Suggest and explain the
advantage of having different coloured pigments in leaves.

Answer 19

(Absorb) different/more wavelengths (of light) for photosynthesis;

Question 20

Heat stress decreases the light-dependent reaction of photosynthesis.

Explain why this leads to a decrease in the light-independent reaction.

Answer 20

1. (Less/no) ATP;
2. (Less/no) reduced NADP;
   Accept NADPH, NADPH + H, NADPH2 NADPH+
   H+
   Reject reduced NAD, NADH etc,

Question 21

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

Explain why

Answer 21

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

Question 22

Where precisely is rubisco found in a cell?

Answer 22

Stroma

Question 23

Name the two products of the light-dependent reaction that are required for
the light-independent reaction.

Answer 23

1. ATP;
2. Reduced NADP;

Question 24

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

Answer 24

1. (Light is) reflected;
   Light is not absorbed on its own is not enough.
2. (Light is) wrong wavelength;