Module 5- Photosynthesis

Question 1

Explain LDR

Answer 1

1. Light excites an electron- chlorophyll has been photoionised (chlorophyll absorbs light energy)
2. Electrons have been raised to a higher energy level
3. The electrons are gained by an electron carrier molecule (reduction of electron carrier molecule, oxidation of chlorphyll)
4. ETC- series of redox reactions- electrons lose energy
5. Protons (H+) are pumped through the proton pumps from stoma–> lumen
6. High concentration of H+ in the lumen of thylakoid
7. H+ pass through ATP synthase back to stroma (chemiosmotic theory)
8. Energy is used to join ADP+Pi–> ATP
9. e- excited at PSI NADP + e- +H+–> NADPH (NADP is reduced by electrons)
10. Photolysis provides the e- that becomes excited

Question 2

6 marks

Describe the light- independent reaction of photosynthesis

Answer 2

1. Carbon dioxide reacts with RuBP
2. Produces two GP using Rubisco
3. GP reduced to TP
4. Using reduced NADP
5. Using energy from ATP
6. TP converted to useful organic substances like glucose
7. RuBP is regenerated

Question 3

6 marks

Desribe the role of electron trasnport chains in the light- dependent reactions of photosynthesis

Answer 3

1 electron transport chain accepts excited electrons
2 from chlorophyll
3 electrons lose energy along chain
4 ATP produced
5 from ADP and Pi
6 reduced NADP formed
7 when electrons and H+ combine with NADP
8 H+ from photolysis

Question 4

6 marks

Absorption of light energy and electron transfer chain

Answer 4

• Chlorophyll molecule absorbs light energy
• Pair of electrons become excited and move to higher energy level
• Electrons leave, chlorophyll molecule = oxidised
• Taken up by electron carrier molecule = reduced
• Move down chain in series of oxidation-reduction reactions
• Losing energy at each stage - used to combine ADP+Pi = ATP

Question 5

3 marks

What structural features of the thylakoids increase rate of photosynthesis

Answer 5

1. Contain photosynthetic pigments to absorb light energy
2. Large SA as discs stacked into grana to increase light absorption
3. Contain ATP synthase enzymes required to make ATP in LIR

Question 6

5 marks

Describe how light energy absorbed by chlorophyll molecules is used to synthesise ATP

Answer 6

Excitation of electrons raised to higher energy level
Electrons emimted from chlorophyll molecule
Electrons to electron transport chain
Loss of energy by electrons along electron transport chain
Energy lost by electrons is used to synthesise ATP; From ADP + Pi

Question 7

5 marks

The concentrations of carbon dioxide in the air at different heights above ground in a forest
changes over a period of 24 hours. Use your knowledge of photosynthesis to describe
these changes and explain why they occur.

Answer 7

1. High concentration of carbon dioxide at night
2. No photosynthesis in the dark as light is required for LDR
3. In dark plants respire
4. In light, plants use more carbon dioxide than they produce / rate of photosynthesis greater than rate of respiration
5. Decrease in carbon dioxide concentration with height
6. At ground level fewer leaves/ less photosynthesising