11.1 - 11.3 Photosynthesis

Question 1

Describe the two stages of photosynthesis

Answer 1

Light-dependent reaction:
- Needs light
- Thylakoid membranes

Light-independent reaction:
- Does not use light directly
- Stroma

Question 2

Describe how ATP is produced during the light dependent reaction

Answer 2

Chemiosmotic theory:
- Energy released as e- are transferred along electron transfer chain
- Used to actively transport protons from stroma to thylakoid lumen
- Protons must then return to stroma due to concentration gradient
- Protons must pass through ATP synthase channel proteins
- As the protons pass through they change the shape of the protein which results in the combination of ADP and Pi to ATP

Question 3

What are the four parts of the light-dependent reaction?

Answer 3

1. Light excited electrons in chlorophyll
2. Photolysis of water
3. Photophosphorylation
4. Reducing NADP

Question 4

Describe photoionisation (1)

Answer 4

• Chlorophyll molecule absorbs light energy
• This excites e- in the chlorophyll
• e- leave chlorophyll and are taken up by election carrier
• Chlorophyll is now positively charged

Question 5

Describe photolysis of water (2)

Answer 5

• Light energy splits water to protons, electrons and oxygen
• e- replace those lost from the chlorophyll molecule

Question 6

Describe photophosphorylation (3)

Answer 6

• High energy e- move along electron transfer chain
• This releases energy
• Used to join ADP+Pi to ATP

Question 7

Describe reducing NADP (4)

Answer 7

• e- from photolysis and protons from the stroma
• Are transferred to NADP
• This forms reduced NADP

Question 8

What is the light-independent reaction also known as?

Answer 8

The Calvin Cycle

Question 9

List the three stages of the Calvin cycle

Answer 9

1. CO2 is combined with RuBP to form 2x GP
2. ATP and reduced NADP are required for the reduction of GP to TP
3. RuBP is regenerated

Question 10

Describe the first stage of the Calvin cycle

Answer 10

• CO2 enters the leaf through the stomata and diffuses into the chloroplast stroma
• Here it is combined with ribulose bisphosphate in a reaction catalysed by rubisco
• This gives two unstable 3-carbon compounds called glycerate 3-phosphate

Question 11

Describe the second stage of the Calvin cycle

Answer 11

• The hydrolysis of ATP provides the energy for the reduction of glycerate 3-phosphate to triose phosphate
• The reaction also requires H+ ions which come from reduced NADP (the reduced NADP is recycled into NADP)
• Some triose phosphate is then converted into useful organic compounds
• The rest returns to the Calvin cycle

Question 12

Describe the third stage of the Calvin cycle

Answer 12

• 5/6 of triose phosphate molecules produced are used to regenerate ribulose bisphosphate
• Regenerating this uses up the rest of the ATP

Question 13

What are the three factors limiting the rate of photosynthesis?

Answer 13

1. Light intensity
2. Carbon dioxide concentration
3. Temperature

Question 14

Explain how light intensity limits the rate of photosynthesis

Answer 14

• Light provides the energy for light-dependent reaction of photosynthesis
• The less light absorbed, the lower the rate of photosynthesis

Question 15

Explain how carbon dioxide concentration limits the rate of photosynthesis

Answer 15

• CO2 required for light-independent reaction
• CO2 conc in atmosphere is only 0.04% so this often the limiting factor

Question 16

Explain how temperature limits the rate of photosynthesis

Answer 16

• Photosynthesis involoves enzymes such as rubisco
• If temp too low, rubisco becomes inactive but above 45 it will denature