Reduction Phase
A series of reactions utilizing ATP for phosphorylation and NADPH for reduction to convert 3-PGA into glyceraldehyde-3-phosphate (triose phosphate).
What is Reduction Phase?
A 3-carbon compound, the first stable product of carbon fixation in the C3 pathway, which is phosphorylated and reduced in the Calvin Cycle.
Key formula / rule: Phosphorylation of 3-PGA
Key points
- Describe the sequence of reactions in the reduction phase of the Calvin Cycle.
- Identify the intermediate compounds formed during the reduction phase.
- State the role of ATP and NADPH in the reduction phase.
- Name the key enzymes involved in the reduction phase.
Common exam trap
Confusing the roles of ATP and NADPH: ATP provides phosphate/energy, NADPH provides electrons/reducing power.
Definitions
- Term
3-Phosphoglycerate (3-PGA)
- Meaning
A 3-carbon compound, the first stable product of carbon fixation in the C3 pathway, which is phosphorylated and reduced in the Calvin Cycle.
- Term
1,3-Bisphosphoglycerate
- Meaning
An intermediate 3-carbon compound formed from 3-PGA during the reduction phase, prior to its reduction to G3P.
- Term
Glyceraldehyde-3-phosphate (G3P)
- Meaning
Also known as triose phosphate, it is a 3-carbon sugar phosphate, the direct product of the reduction phase, used for both regeneration of RuBP and synthesis of glucose.
- Term
Phosphoglycerate kinase
- Meaning
The enzyme that catalyzes the phosphorylation of 3-PGA to 1,3-bisphosphoglycerate using ATP.
- Term
Glyceraldehyde-3-phosphate dehydrogenase
- Meaning
The enzyme that catalyzes the reduction of 1,3-bisphosphoglycerate to glyceraldehyde-3-phosphate using NADPH.
Learning objectives
Describe the sequence of reactions in the reduction phase of the Calvin Cycle.
Identify the intermediate compounds formed during the reduction phase.
State the role of ATP and NADPH in the reduction phase.
Name the key enzymes involved in the reduction phase.
Explain the fate of glyceraldehyde-3-phosphate (G3P) produced.
Formulae
- Name
Phosphorylation of 3-PGA
- Note
Catalyzed by phosphoglycerate kinase
- Expression
3-Phosphoglycerate + ATP → 1,3-Bisphosphoglycerate + ADP
- Name
Reduction of 1,3-Bisphosphoglycerate
- Note
Catalyzed by glyceraldehyde-3-phosphate dehydrogenase
- Expression
1,3-Bisphosphoglycerate + NADPH + H⁺ → Glyceraldehyde-3-phosphate + NADP⁺ + Π
- Name
Overall for 1 CO2 equivalent (2 molecules of 3-PGA)
- Note
Represents the energy cost for the reduction of carbon fixed from one CO2 molecule
- Expression
2 (3-Phosphoglycerate) + 2 ATP + 2 NADPH → 2 (Glyceraldehyde-3-phosphate) + 2 ADP + 2 Π + 2 NADP⁺
Prerequisites
Basic understanding of photosynthesis and its two main phases (light-dependent and light-independent reactions).
Knowledge of ATP and NADPH as energy carriers and reducing agents.
Familiarity with the first stage of the Calvin Cycle: Carbon Fixation (RuBisCO and formation of 3-PGA).
Common mistakes
Confusing the roles of ATP and NADPH: ATP provides phosphate/energy, NADPH provides electrons/reducing power.
Incorrectly stating the number of ATP/NADPH molecules used per CO2 fixed (it's 2 ATP and 2 NADPH in this phase for 2 molecules of 3-PGA derived from 1 CO2).
Forgetting the names of the intermediate compounds (1,3-bisphosphoglycerate) or the enzymes.
Not understanding that G3P is the actual sugar product that can be used to synthesize glucose.
Keywords
Calvin Cycle
C3 Pathway
Reduction Phase
3-PGA
Glyceraldehyde-3-phosphate
G3P
Triose phosphate
ATP
NADPH
Phosphorylation
Reduction
Phosphoglycerate kinase
Glyceraldehyde-3-phosphate dehydrogenase
Photosynthesis
Practice preview
What is the net output of one turn of the Calvin cycle in terms of triose phosphate molecules that can be used for sugar synthesis?…
medium
Which molecule is directly produced from 3-phosphoglycerate during the reduction phase of the Calvin cycle?…
easy
The reduction of 1,3-bisphosphoglycerate to glyceraldehyde-3-phosphate in the Calvin cycle requires which of the following?…
easy