Photosynthesis: Light and Dark Reactions, Calvin Cycle

Photosystems

Photosystem I (PSI)

Able to absorb light of λ <700 nm.

Photosystem II (PSII)

Capable of absorbing light of λ <680 nm.

If a photon hits an electron in a photosynthetic pigment molecule, the electron captures the photon’s energy and jumps to a more distant orbit. It can be lost, ionizing the atom. The molecule, now oxidized, seeks electrons, which are supplied by the photolysis of water.

Absorption of light by the reaction center chlorophyll causes it to release an electron, which travels along an electron transport chain to reach NADP⁺. The reaction center, now ionized, takes an electron from H₂O by photolysis.

Electron Transport and Photophosphorylation

Transport and photosynthetic electron flow: Electrons from the reaction center, loaded with photon energy, are transported by proteins in the thylakoid membrane to NADP⁺, reducing it to NADPH.

Photophosphorylation: ATP formation is driven by light. Both NADPH and ATP are needed for the dark phase of photosynthesis.

According to Mitchell’s “chemiosmotic hypothesis,” the energy released by electron transport pumps protons from the stroma into the thylakoid space, creating a gradient. These protons return to the stroma through ATP-ase, which uses the released energy to phosphorylate ADP into ATP.

Dark Phase or Biosynthesis Phase

In the dark phase of photosynthesis or biosynthesis phase, ATP and NADPH from the light phase convert inorganic compounds into organic ones, reducing CO₂ to synthesize carbohydrates.

Calvin Cycle

The Calvin cycle is the metabolic pathway in most photosynthetic organisms that fixes CO₂, obtaining carbon for organic biomolecules. It has three phases:

1. Carboxylic Acid Phase: CO₂ is incorporated into ribulose-1,5-bisphosphate (a 5-carbon molecule) to produce two molecules of 3-phosphoglycerate, catalyzed by ribulose-1,5-bisphosphate carboxylase-oxygenase (RUBISCO), the most abundant enzyme on Earth.
2. Reducing Phase: The carboxyl group of 3-phosphoglycerate (from the carboxylic phase) is reduced, using ATP and NADPH from the light phase.
3. Regeneration Phase: Glyceraldehyde-3-phosphate (from the reducing phase) is converted to glucose-6-phosphate and partly to ribulose-1,5-bisphosphate to close the cycle.

The Calvin cycle balance is: 6 CO₂ + 18 ATP + 12 NADPH + 12 H⁺ + 12 H₂O → glucose + 18 ADP + 18 P_i + 12 NADP⁺

This high ATP consumption reflects that CO₂ is the most oxidized form of carbon, requiring energy to build organic molecule carbon skeletons.