Photosynthesis - Light Independent

Question 1

Light independent (overview)

Answer 1

ATP & NADPH from dependent: energy for light independent rxn

Calvin Cycle (Stroma-chloroplast): 
Carbon fixation: conversion of inorganic carbon (CO2) to organic carbon (carbohydrate, glucose)

Question 2

Light dependent (3 steps)

Answer 2

1) Carbon fixation (fixation of CO2 to RuBP, catalyzed by Rubisco enzyme)

2) Reduction (Reduction of CO2-RuBP complex to G3P, with E from ATP &
NADPH - light dependent);
Most G3P out of chloroplast: gives glucose (reverse rxn of glycolysis)

3) Regeneration of ribulose 1,5-biphosphate (RuBP) with rest of G3P, Calvin cycle can continue

6 CO2 + 18 ATP + 12 NADPH + 12 H2O —> C6H12O6 + 18 ADP + 18 Pi +
12 NADP+

Yield: 6 turns (fixation of 6 CO2) =
1 glucose

Question 3

Photorespiration (Rubisco)

Answer 3

Rubisco: large multi-enzyme complex
Catalyses fixation of CO2 to RuBP

but also O2 to RuBP (photorespiration - competitive inhibition for same active site)

Photorespiration: produces CO2, counterproductive to - photosynthesis Calvin cycle - fixation of CO2

Question 4

Photorespiration (evolution)

Answer 4

Evolution: photorespiration —> protection from photo-oxidative damage (too much oxygen or light - harmful)

Before; less oxygen in atmosphere, so less competition for active site (rubisco evolution)

Question 5

Normal conditions

Answer 5

Rate of Carbon fixation 4 times rate of photorespiration (20% CO2 misused in photorespiration)

Change based on T, water levels, O2/CO2 levels change

Question 6

High temp

Answer 6

1) Stomata close to prevent water loss
2) Accumulation of O2 in plant, Decrease CO2
3) Increase photorespiration
4) Decrease efficiency of Calvin cycle

Question 7

3 mechanisms to combat effects of photorespiration

Answer 7

C3 plants: No mechanism to regulate photorespiration (energy efficient, but water loss)

C4 plants: Requires ATP to regenerate PEP - corn, sugar cane, grass
CO2 fixation to PEP (mesophyll cell) & Calvin cycle (bundle sheath cell) - (2 diff cells)

C4 & CAM: water loss minimized, but requires more energy

1) Fix CO2 to PEP (4-C molecule) - energy needed
2) PEP modified + decarboxylated (release CO2) at site of Rubisco —> high C of CO2 close to enzyme (less competition for active site for CO2)

CAM plants: cacti, pineapples

1) CO2 Fixation to PEP & Calvin cycle (same mesophyll cell, but diff times of day)
2) C4 rxn - night (cool temp, stomata open)
3) close - day (CO2 removed, Calvin cycle)