Photosynthesis: Electron Transport and Carbon Fixation

Posted on Dec 6, 2024 in Biology

Photosynthesis: Non-Cyclic Electron Transport

Non-Cyclic Electron Transport

In the light phase of photosynthesis, electrons are transported from H2O to NADP+ through the photosynthetic chain. This transport isn’t spontaneous; light energy, captured by pigments in photosystems I and II, is needed. Water molecules break down (photolysis), providing electrons to the chain and releasing O2 as a byproduct.

This transport can be divided into three segments:

Photosystem I (PSI) and NADPH Reduction

A photon excites PSI’s reaction center chlorophyll, which releases an electron to a protein (ferredoxin). Ferredoxin is then oxidized, reducing NADP+ to NADPH. The oxidized chlorophyll needs to regain its electron.

Photosystem II (PSII) Electron Recovery

Light excites PSII, releasing electrons that travel through a chain to PSI’s chlorophyll, replenishing the electron. This leaves a hole in PSII’s chlorophyll.

PSII Electron Recovery and Photolysis

Water photolysis provides electrons to PSII, releasing H+ into the thylakoid lumen (space) and O2 into the atmosphere: H2O -> 2e– + 2H+ + O2

Photophosphorylation

ATP synthesis during the light phase is called photophosphorylation. Electron transport from water to NADP+ releases protons into the thylakoid lumen, creating a proton gradient. Protons flow back into the stroma through ATPase, synthesizing ATP from ADP + Pi (chemiosmotic theory).

Cyclic Electron Transport

An alternative pathway uses only PSI. Excited electrons, instead of going to NADP+, return to PSI. This process, involving the cytochrome b6f complex, transports protons, generating ATP (cyclic photophosphorylation).

Cyclic Electron Transport Characteristics

  • Only uses PSI
  • No NADP+ reduction
  • No water photolysis or O2 release
  • ATP synthesis via H+ translocation

Cyclic transport is important when NADP+ is scarce, as in anoxygenic photosynthetic bacteria (with a single PS).

Dark Phase: Carbon Fixation

Calvin Cycle

The dark phase synthesizes organic molecules from inorganic ones using NADPH and ATP from the light phase. It occurs in the chloroplast stroma and uses CO2, reducing it to simple monosaccharides.

G3P and the Calvin Cycle

The G3P molecules produced are used in various metabolic pathways. Common uses include glucose and fructose synthesis for polysaccharides (starch and cellulose) and sucrose, as well as the synthesis of fatty acids and amino acids.

Calvin Cycle Reactions

  • CO2 Fixation: CO2 is fixed onto ribulose-1,5-bisphosphate (RuBP), forming an unstable six-carbon intermediate that breaks into two 3-carbon molecules of 3-phosphoglycerate (3-PGA).
  • Reduction: 3-PGA is phosphorylated and reduced to glyceraldehyde-3-phosphate (G3P) using NADPH and ATP.
  • Glucose Synthesis and Regeneration: For every six G3P molecules, one is used for glucose synthesis, and five are used to regenerate RuBP.

Chemosynthesis

Chemosynthesis is an anabolic process where organic compounds are synthesized from inorganic ones using chemical energy from the oxidation of inorganic compounds.

Chemosynthesis Steps

  • Phase 1 (equivalent to the light phase): Inorganic compounds (NH3, H2, Fe, H2S) are oxidized, releasing energy and electrons to phosphorylate ADP to ATP and reduce NAD+ to NADH.
  • Phase 2 (equivalent to the dark phase): ATP and NADH are used to reduce inorganic compounds and produce organic ones.

Chemosynthetic Organisms

Chemoautotrophs, mostly aerobic bacteria, play a crucial role in biogeochemical cycles.

  • Nitrifying Bacteria: Nitrosomonas (oxidizes ammonia to nitrites) and Nitrobacter (oxidizes nitrites to nitrates).
  • Sulfur Bacteria: Live in sulfur-rich environments, using sulfur and sulfides as substrates.
  • Iron Bacteria: Oxidize ferrous salts to ferric salts.
  • Hydrogen Bacteria: Use H2 as a substrate.