Calvin Cycle and Photosynthesis: A Detailed Look

The Calvin Cycle

The Calvin cycle's objective is to fix carbon dioxide (CO2) using ribulose-1,5-bisphosphate. For every triose (3-carbon sugar), two are needed to form a glucose, requiring 3 ribulose and 3 CO2. A hexose (6-carbon sugar) requires 6 ribulose and 6 CO2.

Phases of the Calvin Cycle

1. Carbon Fixation: Ribulose-1,5-bisphosphate (5C) combines with CO2 to form a 6C compound. Enzyme: Ribulose bisphosphate carboxylase. This creates a highly unstable compound. Input: 1 CO2 per ribulose.
2. Reduction: The unstable 6C compound splits into two triose molecules (3C), specifically glyceraldehyde-3-phosphate. Input/Output: None.
3. Phosphorylation: Glyceraldehyde-3-phosphate is phosphorylated using ATP, transforming into 1,3-bisphosphoglycerate. Input: 1 ATP per glyceraldehyde-3-phosphate, output: ADP.
4. Reduction (again): 1,3-bisphosphoglycerate is reduced using NADPH, transforming back into glyceraldehyde-3-phosphate. Input: 1 NADPH, output: NADP+ and inorganic phosphate (Pi).
5. Regeneration: From 6 glyceraldehyde-3-phosphate molecules (from 3 ribulose), 1 is used as a product of the dark phase. The remaining 5 are used to regenerate ribulose-1,5-bisphosphate. This involves complex rearrangements of carbon skeletons. Transformation: Glyceraldehyde-3-phosphate to ribulose phosphate.
6. Phosphorylation (again): Ribulose phosphate is phosphorylated using ATP to regenerate ribulose-1,5-bisphosphate. Input: 1 ATP per ribulose phosphate, output: ADP.
7. Glyceraldehyde-3-phosphate Fate: Glyceraldehyde-3-phosphate can either enter the cytoplasm for the tricarboxylic acid cycle or remain in the chloroplast for other syntheses.

Energy Balance

The overall equation for the Calvin cycle is: 6 CO2 + 18 ATP + 12 NADPH -> 1 Hexose + 18 ADP + 12 NADP+

Electron Transport Chain (Z-Scheme)

The light-dependent reactions occur in the thylakoid membrane, involving two photosystems and electron acceptors.

Key Steps in the Z-Scheme

• Photosystem II (PSII): Located below the water potential. Light excites PSII, causing it to release 2 electrons to pheophytin. To return to its original state, PSII receives 2 electrons from the oxygen-evolving complex.
• Oxygen-Evolving Complex: Performs photolysis of water, releasing 2 H+ (to the lumen), 2 electrons (to PSII), and ½ O2 (to the atmosphere).
• Plastoquinone (PQ): Accepts electrons from pheophytin. Fixed PQ becomes mobile PQ by gaining H+ from the stroma and releasing them into the lumen.
• Cytochrome b6f Complex: Receives electrons from PQ and transfers them to plastocyanin.
• Plastocyanin (PC): Transfers electrons to Photosystem I (PSI).
• Photosystem I (PSI): Transfers electrons to ferredoxin.
• Ferredoxin (Fd): Uses electrons to reduce NADP+ to NADPH. Alternatively, it can transfer electrons back to cytochrome b6f in a cyclic pathway.