Topic 4 Photosynthesis: Alternatives to C3 Photosynthesis

Question 1

• Process involving fixation of oxygen, instead of CO2, by rubisco
• Produces no ATP or glucose
• Rubisco: not efficient or fast because fixes CO2 and oxygen at same time if both present
• Peroxisomes: break down products of process

Answer 1

1. Photorespiration

Question 2

• Process evolving from normal photosynthesis, or C3 photosynthesis
• Type of spatial separation
• CO2 enters leaf and absorbed by mesophyll cells then moved to bundle sheath cells
• CO2 combines with phosphoenolpyruvate forming oxaloacetate by phosphoenolpyruvate carboxylase in mesophyll
  a. Oxaloacetate has 4C
  b. Oxaloacetate converted to malate and transferred through plasmodesmata into bundle sheath cell, which requires conversion of 1 ATP to 1 AMP
  c. Malate converted to pyruvate and CO2. CO2 used in Calvin and pyruvate shuttled back to mesophyll and converted back into phosphoenolpyruvate

Answer 2

2. C4 Photosynthesis

Question 3

Overall Purposes:

1. Move CO2 from mesophyll to bundle sheath cells. This process is Hatch-Slack pathway, explains that little oxygen reduces competition while rubisco is deciding to fix carbon dioxide or oxygen
2. Minimize photorespiration and water loss from stomata, or lead pores
   - C4 Photosynthesis is found in hot, dry climates
   - Occurs in plants like corn or sugarcane
3. Required one additional ATP molecule that becomes AMP

Answer 3

Overall Purposes of C4 Photosynthesis

Question 4

• Crassulacean acid metabolism
• Another process similar to C3 photosynthesis, and almost identical to C4 photosynthesis
  a. PEP carboxylase fixes CO2 + PEP to OAA making malic acid
  b. It is then shuttled into the vacuole
  c. At night, stomata are open, PEP carboxylase is active, and malic acid accumulates
  d. During day, stomata is closed. Malic acids out of vacuole and converted to OAA which requires 1 ATP, releases CO2 which moves to Calvin

Answer 4

3. CAM Photosynthesis

Question 5

Overall Advantages

- Photosynthesis can proceed during day reducing water loss

Answer 5

Advantages to CAM Photosynthesis

Question 6

• As leaves age, chlorophyll breaks down to extract valuable components like Mg2+, and carotenoids become visible
• Splitting of water in the light reactions provides 2 electrons for non-cyclic photophosphorylation and are incorporated into NADPH and Calvin
• Calvin is light-independent but requires ATP and NADPH produced from light-depended reactions

Answer 6

Photosynthesis