[3.5] Energy Transfers In & Between Organisms

Question 1

What are the stages of photosynthesis and where do they happen?

Answer 1

1. Light dependant reaction.
   
   • Thylakoid membrane of chloroplast.
2. Light independent reaction.
   
   • Stroma of chloroplast.

Question 2

Describe photoionisation in the light-dependant reaction (LDR).

Answer 2

1. Chlorophyll absorbs light energy which excites its electrons (higher energy level).
2. So electrons are released from chlorophyll (chlorophyll becomes positively charged).

Question 3

Describe what happens after photoionisation in the LDR.

Answer 3

• Some energy from electrons released in photoionisation is conserved in the production of ATP / reduce NADP (chemiosmotic theory).

1. Electrons move along electron transfer chain (electron carriers), releasing energy.
2. This energy is used to actively pump protons from stroma into thylakoid.
3. Protons move by facilitated diffusion down electrochemical gradient intro stroma via ATP synthase.
4. Energy used to join ADP and Pi to form ATP (photophosphorylation).
5. NADP accepts a proton and an electron to become reduced NADP.

Question 4

Describe the photolysis of water in the LDR.

Answer 4

• Water splits to produce protons, electrons and oxygen (H₂O -> 1/2 O₂ + 2e⁻ + 2H⁺).
  
  • Electrons replace those lost from chlorophyll.

Question 5

Describe the light-independent reaction of photosynthesis (Calvin cycle).

Answer 5

1. CO₂ reacts with ribulose bisphosphate (RuBP) which is catalysed by the enzyme rubisco.
2. Forming 2 glycerate 3-phosphate (TP).
3. GP reduced to triose phosphate (TP) using produced from light-dependant reaction (reduced NADP & energy from ATP)
4. Some TP converted to useful organic substances (e.g. glucose).
5. Some TP used to **regenerate RuBP in the Calvin cycle (using energy from ATP).