lecture 31 - Assimilation of Carbon in Plants

Question 1

What is the first step of carbon assimilation?

Answer 1

CO2 + Ribulose 1,5–bisphosphate –> 2 x
(3- phosphoglycerate)
CO2 fixation….. Rubisco

Question 2

What is the second step of carbon assimilation?

Answer 2

Reduction …….. (cf Gluconeogenesis)
2 (3- Phosphoglycerate) –> Fructose 6-Phosphate

2ATP–>2ADP
2NADPH–>2NADP+

Question 3

What is the third step of carbon assimilation?

Answer 3

Regeneration of Ribulose 1,5–bisphosphate ….
• Analogy with Pentose phosphate pathway:
making a 5C sugar from 6C + 3C sugars.
• Enzymes involved
– Transketolase: transfers a 2C unit from a ketose to an aldose
– Aldolase: condensation between DHAP + an aldehyde
• These reactions are + at equilibrium
• Cycle “driven” by reactions with large –ve ΔG:
– Rubisco;
– Fructose 1,6-bisphosphatase;
– Phosphoribulose kinase

Question 4

Describe the Energetics of CO2 Assimilation to

Fructose 6-phosphate (Fru 6-P)

Answer 4

From 6 CO2 …
1. 1 ATP consumed in phosphorylating each mole of 3-PGA (to 1,3-BPG or 1,3 bisphosphoglycerate)
Since 6 CO2 + 6 Ru 1,5-BP –> 12 PGA.
12 ATP consumed here
2. 1 NADPH consumed in reducing each mole of 1,3-BPG
(to G-3-P or glyceraldehyde-3-phosphate)
12 NADPH consumed here
3. 1 ATP consumed in regenerating each mole of
Ru1,5–BP in the Calvin cycle.
Since 6 Ru 1,5-BP required for fixation of 6 CO2
6 ATP consumed here
i.e. 18 ATP, 12 NADPH consumed/6 CO2 fixed
or 3 ATP & 2 NADPH per CO2 fixed.

Question 5

What is The Fate of Fixed CO2?

Answer 5

Starch synthesis
Starch: a storage polysaccharide, very similar to glycogen
The 2 component polymers:
• α-amylose: unbranched (α 1 4 linked glucose)
• amylopectin: branched (α 1 4 and α 1 6 links)

Question 6

Describe starch synthesis

Answer 6

fru-6P to Glu-6P (PhosphoglucoISOMERASE)
Glu-6P to Glu-1P (PhosphoglucoMUTASE)

The 2 key-reactions in starch synthesis:

Glu 1–P + ATP –> ADP–Glu + PPi
(ADP-glucose pyrophosphorylase)

ADP-Glu + (glucose)n –> (glucose)n + 1 + ADP
(starch synthase)

Synthesis favoured when CO2 fixation rate exceeds the utilization rate of reduced C

Question 7

Describe sucrose synthesis

Answer 7

occurs in the cytosol.
Two stages:
(a) export of reduced CO2 from the stroma
exchanger at inner envelope membrane ensures that P
exported as triose-phosphate is replenished (ultimately for phosphorylation of ADP).

(b) sucrose synthesis from trioses in cytosol

Question 8

What is photorespiration?

Answer 8

This is the oxygenase function of Rubisco
The reaction is “unwanted”: an evolutionary
consequence of increased [O2] in the atmosphere.
O2 competes with CO2 at active site.

Question 9

Describe The C4 and CAM Pathways:

Answer 9

Spatial and temporal CO2 Concentrating
Mechanism to Reduce O2 Competition for Rubisco
The problem: As temperature increases, O2
fixing ability of Rubisco increases more steeply than CO2 fixing ability.
The solution: Devise a mechanism for local [CO2] enrichment at site of CO2 fixation (C4 pathway, CAM metabolism, CO2 pumps).
C4 pathway: Observed in tropical species e.g. sugar cane, maize.