3.5.1 Photosynthesis

Question 1

Where do the light-dependent and light-independent reactions occur in plants?

Answer 1

Light-dependent: in the thylakoids of chloroplasts
Light-independent: stroma of chloroplasts

Question 2

Explain the role of light in photoionisation.

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 light-dependent reaction.

Answer 3

1. Electron transfer chain
2. Chemiosmosis

Question 4

What happens in the electron transfer chain (ETC)?

Answer 4

Electrons released from chlorophyll move down a series of carrier proteins embedded in the thylakoid membrane and undergo a series of redox reaction 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 light-dependent stage?

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 and 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 light-dependent reaction?

Answer 9

• NADP + 2H⁺ + 2e⁻ -> reduced NADP
• 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 and 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 to 2 x triose phosphate (TP)
• Requires 2 x reduced NADP and 2 x ATP
• Forms 2 x NADP and 2 x ADP

Question 14

How does the light-independent reaction 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 x ATP
• Forms 1x ADP