[3.5.1] Photosynthesis

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 2x glycerate 3-phosphate (GP).
3. GP reduced to triose phosphate (TP) using products 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).

Question 6

Describe and explain how temperature affects rate of photosynthesis.

Answer 6

1. As temperature increases, rate increases.
   
   • Enzymes e.g. rubisco gain kinetic energy.
   • So more enzyme-substrate (E-S) complexes form.
2. Above an optimum temperature, rate decreases.
   
   • Enzymes denature as H bonds in tertiary structure break.
   • So fewer enzyme-substrate (E-S) complexes form.

Question 7

Describe and explain how light intensity affects rate of photosynthesis.

Answer 7

1. As light intensity increases, rate increases.
   
   • Light-dependant reaction increases (e.g. more photoionisation of chlorophyll) so more ATP and reduced NADP produced.
   • So light-independent reaction increases as more GP reduced to TP and more TP regenerates RuBP.
2. Above a certain light intensity, rate stops increasing.
   
   • Another factor is limiting e.g. temperature / CO₂ concentration.

Question 8

Describe and explain how CO₂ concentration affects rate of photosynthesis.

Answer 8

1. As CO₂ concentration increases, rate increases.
   
   • Light-independent reaction increases.
   • As more CO₂ combined with RuBP to form GP.
   • So more GP reduced to TP.
   • So more TP converted to organic substances and more RuBP regenerated.
2. Above a certain CO₂ concentration, rate stops increasing.
   
   • Another factor is limiting e.g. temperature / light intensity.

Question 9

Explain the key consideration when evaluating data relating to agricultural practices used to overcome the effect of limiting factors.

Answer 9

• Agricultural practice should increase rate of photosynthesis, leading to increased yield.
  
  • As more glucose produced for faster respiration.
  • So more ATP to release energy for growth e.g. cell division, protein synthesis.
• But profit from extra yield should be greater than costs (money & environmental costs).