Photosynthesis

Question 1

Where do the LDR and LIDR occur in plants?

Answer 1

• LDR = thylakoid membranes.
• LIDR = stroma.

Question 2

Explain the role of light in phosphoionisation.

Answer 2

Chlorophyll molecules absorb energy from photons of light.
This ‘excites’ 2 electrons (raises them to a higher energy level), causing them to be released from the chlorophyll.

Question 3

Name the 2 main stages involved in ATP production in the LDR.

Answer 3

1. electron transfer chain
2. chemiosmosis

Question 4

What happens in the electron transfer chain?

Answer 4

Electrons released from the chlorophyll move down a series of carrier proteins embedded in the thylakoid membrane & undergo a series of redox reactions, which releases energy.

Question 5

How is a proton concentration gradient established during chemiosmosis?

Answer 5

Some energy released from the ETC is coupled to the active transport of H+ ions (protons) from the stroma into the thylakoid space.

Question 6

How does chemiosmosis produce ATP in the LDR.

Answer 6

H+ ions (protons) move down their concentration gradient from the thylakoid space into the stroma via the channel protein ATP synthase.
ATP synthase catalyses ADP + Pi –> ATP.

Question 7

Explain the role of light in photolysis.

Answer 7

Light energy splits molecules of water.
2H2O –> 4H+ + 4e- + O2

Question 8

What happens to the products of the photolysis of water?

Answer 8

• H+ ions: move out of thylakoid space via ATP synthase & are used to reduce the coenzyme NADP.
• e-: replace electrons lost from chlorophyll.
• O2: used for respiration or diffuses out of leaf as waste gas.

Question 9

How and where is reduced NADP produced in the LDR?

Answer 9

• NADP + 2H+ + 2e- –> NADPH.
• Catalysed by dehydrogenase enzymes.
• Stroma of chloroplasts.

Question 10

Where do the H+ ions and electrons used to reduce NADP come from?

Answer 10

• H+ ions: photolysis of water.
• Electrons: NADP acts as the final electron acceptor of the electron transfer chain.

Question 11

Name the 3 main stages in the Calvin cycle.

Answer 11

1. Carbon fixation.
2. Reduction.
3. Regeneration.

Question 12

What happens during carbon fixation?

Answer 12

• Reaction between CO2 & ribulose bisphosphate (RuBP) catalysed by rubisco.
• Forms unstable 6C intermediate that breaks down into 2x glycerate 3-phosphate (GP).

Question 13

What happens during reduction (in the Calvin cycle)?

Answer 13

• 2 x GP are reduced into 2 x triose phosphate (TP).
• Requires 2 x reduced NADP & 2 x ATP.
• Forms 2 x NADP & 2 x ADP.

Question 14

How does the LIDR result in the production of useful organic substances?

Answer 14

1C leaves the cycle (i.e., some of the TP is converted into useful organic molecules).

Question 15

What happens during regeneration (in the Calvin cycle?)

Answer 15

• After 1C leaves the cycle, the 5C compound RuP forms.
• RuBP is regenerated from RuP using 1 ATP.
• Forms 1 ADP.