Photosynthesis: Light-Independent Reactions
The light-independent reactions (also known as the Calvin cycle) are the second stage of photosynthesis. They take place in the stroma of the chloroplast and do not require light directly. However, they depend on the products of the light-dependent reactions — ATP and NADPH — so they cannot continue indefinitely in the dark.
Key Definition: The light-independent reactions (Calvin cycle) use ATP and NADPH from the light-dependent reactions to fix CO₂ into organic molecules in the stroma of the chloroplast.
The Calvin Cycle: Overview
The Calvin cycle was elucidated by Melvin Calvin and his colleagues in the 1950s using the "lollipop experiment" with the green alga Chlorella. They used radioactive carbon-14 (¹⁴C) in CO₂ to trace the path of carbon through the cycle, identifying the intermediates using two-dimensional paper chromatography and autoradiography.
The cycle has three main stages:
- Carbon fixation
- Reduction
- Regeneration of RuBP
Stage 1: Carbon Fixation
- Carbon dioxide (CO₂) from the atmosphere diffuses into the chloroplast stroma.
- CO₂ combines with a 5-carbon (5C) acceptor molecule called ribulose bisphosphate (RuBP).
- This reaction is catalysed by the enzyme RuBisCO (ribulose bisphosphate carboxylase-oxygenase).
- RuBisCO is the most abundant enzyme on Earth, reflecting its critical importance in global carbon fixation.
- The result is an unstable 6C intermediate that immediately breaks down into two molecules of glycerate-3-phosphate (GP), each a 3C compound.
CO₂ + RuBP (5C) → 2 × GP (3C)
Key Definition: Carbon fixation is the incorporation of inorganic CO₂ into an organic molecule (GP). This is the first step in the Calvin cycle and is catalysed by RuBisCO.
Stage 2: Reduction of GP to GALP
- Each molecule of GP (3C) is reduced to glyceraldehyde-3-phosphate (GALP), also known as G3P or triose phosphate (TP). GALP is a 3C sugar phosphate.
- This reduction requires:
- ATP (from the light-dependent reactions) — provides the energy (phosphorylation).
- NADPH (from the light-dependent reactions) — provides the hydrogen atoms (reducing power).
- ATP is hydrolysed to ADP + Pi, and NADPH is oxidised to NADP⁺. Both ADP and NADP⁺ are recycled back to the light-dependent reactions on the thylakoid membranes.
GP (3C) + ATP + NADPH → GALP (3C) + ADP + Pi + NADP⁺
Exam Tip: The reduction of GP to GALP is the step that requires both ATP and NADPH. If either runs out (e.g., in the dark), GP accumulates and GALP decreases.
Stage 3: Regeneration of RuBP
- For every 3 molecules of CO₂ fixed, 6 molecules of GALP (3C) are produced.
- 5 out of 6 GALP molecules are used to regenerate 3 molecules of RuBP (5C), ensuring the cycle can continue.
- This regeneration requires ATP (but not NADPH).
- The rearrangement of five 3C molecules into three 5C molecules involves several enzyme-catalysed steps.
5 × GALP (3C) + 3 ATP → 3 × RuBP (5C) + 3 ADP + 3 Pi
The Fate of GALP
The remaining 1 out of every 6 GALP molecules (i.e., the net product of the cycle) is exported from the chloroplast and used as a building block for larger organic molecules:
Glucose and Other Carbohydrates
- Two GALP (3C) molecules can be combined to form glucose (6C) — a hexose sugar.
- Glucose can then be used to make:
- Sucrose — the main transport sugar in plants (glucose + fructose).
- Starch — the main storage polysaccharide in plants (polymer of alpha-glucose).
- Cellulose — a structural polysaccharide forming plant cell walls (polymer of beta-glucose).
- Fructose — another hexose sugar found in fruits and nectar.