Photosynthesis

Question 1

Non-cyclic Photophosphorylation

Answer 1

1. A photon of light strikes photosynthetic pigments in PS2 and PS1
2. Energy from the photon of light is passed to neighbouring pigment molecules via resonance energy until it reaches special chlorophyll a, P680 and P700 respectively in the reaction centres
3. Electrons in special chlorophyll a is excited and boosted to higher energy levels, and is then accepted by the primary electron acceptor
4. Primary electron acceptors passes electrons down the ETC, comprising of a series of electron carriers of progressively lower energy levels from PS2 to PS1
5. Energy released during the transfer is used to pump H+ via active transport from the stroma, across the thylakoid membrane into the thylakoid lumen
6. In PS2, photolysis of water is catalysed by the water splitting enzyme, splitting water into 2H+, 2 electrons and half oxygen
7. Accumulation of H+ in the thylakoid lumen sets up an electrochemical and proton gradient between the stroma and thylakoid lumen
8. H+ diffuses down the proton gradient from the thylakoid lumen back into the stroma through ATP synthase, releasing electrical potential energy, driving phosphorylation of ADP to ATP, catalysed by ATP synthase
9. Electrons released from special cholrophyll a of PS2 replaces electrons lost by special chlorophyll a of PS1. Electrons released from photolysis of water replaces electrons lost by special chlorophyll a of PS2
10. Electrons released from special chlorophyll a of PS1 combines with H+ to form hydrogen atoms, which are then used to reduce NADP to NADPH, catalysed by NADP+ reductase

Question 2

Cyclic photophosphorylation

Answer 2

Excited electrons from special chlorophyll a of PS1 are passed to the ETC between PS2 and PS1, before returning to the same special chlorophyll a of PS1

Question 3

Explain how non-cyclic photophosphorylation differs from cyclic
photophosphorylation.

Answer 3

1. Photolysis of water occurs during non-cyclic photophosphorylation only
2. PS2 and PS1 involved in non-cyclic photophosphorylation but only PS1 involved during cyclic photophosphorylation
3. 2 ETC involved during non-cyclic photophosphorylation vs only 1 ETC during cyclic photophosphorylation

Question 4

Calvin Cycle

Answer 4

1. During CO2 fixation, 6 molecules of CO2 is fixed by combining with 6 molecules of ribulose bisphosphate (RuBP), forming 6 molecules of 6C unstable intermediate, catalysed by rubisco. The intermediates are immediately broken down into 12 molecules of glycerate phosphate (GP)
2. During PGA reduction, 12 molecules of GP is phosphorylated by 12 ATP to form 12 molecules of glycerate bisphosphate (GBP). 12 molecuels of glycerate bisphosphate is then reduced by NADPH, forming 12 triose phosphate (TP)
3. During RuBP regeneration, 10 molecules of TP and 6 molecules of ATP are used to regenerate 6 molecules of RuBP, for subsequent Calvin Cycle.
4. Rest of the TP exits the Calvin cycle to produce hexose sugar (a-glucose/b-glucose)

Question 5

Importance of ATP and NADPH to photosynthesis

Answer 5

1. ATP serves as energy source during light independent reactions
2. ATP is needed to regenerate RuBP
3. ATP is needed to phosphorylate glycerate phosphate to glycerate bisphosphate
4. NADPH is needed to reduce glycerate bisphosphate to triose phosphate, for regeneration of RuBP and synthesis of sugars

Question 6

Role of NADP

Answer 6

1. Acts as a hydrogen atom carrier and coenzyme for dehydrogenase
2. Hydrogen atom transferred to NADP, reducing NADP to NADPH, catalysed by NADP+ reductase
3. NADPH carries hydrogen atom from light dependent reactions to the Calvin cycle for reduction of glycerate bisphosphate to triose phosphate.