13. Photosynthesis

Question 1

Limiting factor

Answer 1

One of several factors that is in its shortest supply and that therefore affects rate of reaction

Question 2

Explain why the illumination of chloroplasts leads to optimum pH conditions (pH8) for rubisco. [3m]

Answer 2

• upon illumination, excited electrons are emitted from chlorophyll a…
• …and are passed along the ETC
• photolysis of water occurs and gives out protons…
• …that are pumped into the thylakoid space
• pH in stroma increases due to the leaving of protons
• rubisco is in stroma

Question 3

State and explain the expected relationship between light intensity and time taken to decolourise DCPIP. [4m]

Answer 3

As light intensity increases, time taken for decolourisation decreases

• more LIGHT ENERGY absorbed
• more photolysis
• more electrons excited
• more protons
• more REDUCTION of DCPIP

Question 4

To produce 1 molecule of glucose, the C3 pathway uses 18 molecules of ATP and the C4 pathway uses 30 molecules of ATP.
Suggest why C4 plants can afford this high cost of ATP. [2m]

Answer 4

• C4 plants are adapted for high light intensities/high temperature
• so high rate of photophosphorylation / light dependent reaction
• so much ATP produced

Question 5

Describe the effects on a plant if its environmental temperature rises well above the usual temperature range. [5m]

Answer 5

• decrease in rate of photosynthesis
• rubisco/enzyme denatured
• less CO2 fixed to RuBP
• photorespiration occurs
• increase in transpiration
• eventually stomata close

Question 6

Explain how sorghum is able to prevent photorespiration at high temperatures. [4m]

Answer 6

• bundle sheath cells are where light independent stage takes place
• RuBP and rubisco are kept away from oxygen
• by mesophyll cells
• maintains higher concentration of CO2 in bundle sheath cells
• PEP carboxylase has high optimum temperature, approximately 45°C
• PEP carboxylase is not denatured

Question 7

Outline how the biochemistry of C4 plants differs from that of C3 plants. [2m]

Answer 7

• the 1st product of photosynthesis is 4 carbon compound oxaloacetate
• 1st CO2 acceptor is PEP
• CO2 released from 4C compound to enter Calvin cycle

Question 8

5 differences between cyclic and non-cyclic photophosphorylation

Answer 8

1. cyclic: electron emitted returns to PSI; 
non-cyclic: electron emitted from PSII absorbed by PSI
Non-cyclic:
2. reduced NADP produced
3. oxygen produced
4. photolysis occurs
- photolysis only involves PSII
5. only PSI vs both PSI and PSII

Question 9

Describe the role of rubisco in the Calvin cycle. [3m]

Answer 9

• fixation of CO2
• catalyses the reaction between RuBP and CO2
• to give 2 GP
• via an unstable intermediate compound