Photosynthesis

Question 1

What are the key stages of the light dependent reaction?

Answer 1

1. Absorption of light by pigments
2. Release of electrons from pigments
3. Electron transfer along the ETC
4. Photolysis of water to provide protons (H+) and electrons (to replace those lost)
5. Chemiosmosis - movement of protons through the thylakoid membrane
6. Production of ATP and reduced NADP

Question 2

Outline the process of non-cyclic photophosphorylation

Answer 2

1. Light absorption excites electrons at the reaction centres of PSI and PSII
2. The excited electrons are emitted from reaction centres and passed along chains of electron carriers
3. Energy is released, allowing protons to be pumped into the thylakoid space
4. The electrons lost from PSII replace those lost from PSI
5. Photolysis of water generates electrons (to replace those lost from PSII) and more protons in the thylakoid space
6. Protons diffuse through ATP synthase into the stroma, and ATP is synthesised via chemiosmosis
7. The electrons lost from PSI and protons pumped through ATP synthase are used to reduce NADP

Question 3

What are the three key steps in the light independent reaction?

Answer 3

1. Fixation - carbon dioxide is fixed to form glycerate-3-phosphate (GP)
2. Reduction - GP is reduced to triose phosphate (TP) using protons and electrons from reduced NADP and energy from ATP
3. Regeneration - ribulose bisphosphate (RuBP) is regenerated from TP and energy from ATP

Question 4

Explain the light independent reaction in full detail

Answer 4

1. Reaction of carbon with RuBP (5C) to form an unstable 6C intermediate
2. The 6C compound breaks down, forming two GP molecules (3C)
3. Energy from ATP, and protons and electrons from reduced NADP are used to reduce GP to a phosphorylated 3C sugar - TP
4. Most TP is used to regenerate RuBP, using ATP from light independent reaction, and the rest is used to produce organic molecules needed by the plant