Photosynthesis and Chemosynthesis: Processes Explained

Posted on Nov 1, 2024 in Biology

Photosynthesis

Light-Dependent Reactions

Acyclic Phase

  1. Photon Absorption and Water Photolysis: Photons arrive at photosystem II (PSII), exciting electrons in chlorophyll P680. These electrons are passed to an electron acceptor. To replace the lost electrons, water molecules are split (photolysis) within the thylakoid lumen, releasing oxygen and protons.
  2. Photophosphorylation: Electrons move through the electron transport chain, from PSII to plastoquinone (PQ), then to the cytochrome complex, and finally to photosystem I (PSI). As electrons move through the cytochrome complex, protons are pumped into the thylakoid lumen, creating a proton gradient. This gradient drives ATP synthesis through ATP synthase.
  3. NADP+ Reduction: Photons excite electrons in chlorophyll P700 in PSI. These electrons are passed to ferredoxin and then to NADP+ reductase. This enzyme uses the electrons and protons from the stroma to reduce NADP+ to NADPH.

Cyclic Phase

This phase only involves PSI and generates ATP. Electrons from PSI are passed to ferredoxin, then to cytochrome, and back to PSI. This cyclic flow creates a proton gradient, leading to ATP synthesis. No water is split, and no NADPH is produced.

Factors Influencing Photosynthesis

  • Temperature: Each species has an optimal temperature range for photosynthesis.
  • CO2 Concentration: Photosynthesis increases with CO2 concentration up to a saturation point.
  • O2 Concentration: High O2 levels can inhibit photosynthesis.
  • Light Intensity: Photosynthesis increases with light intensity up to a point where it can damage pigments.
  • Water Availability: Water shortage reduces photosynthesis as plants close stomata to conserve water, limiting CO2 intake.

Chemosynthesis

Chemosynthesis is the process of producing ATP from the energy released by oxidizing inorganic substances. Chemoautotrophic bacteria perform this process. They oxidize reduced compounds (e.g., NH3) and use the released energy to synthesize ATP and NADH. These are then used to synthesize organic compounds from inorganic substances, similar to the dark phase of photosynthesis (Calvin cycle).

Calvin Cycle

  1. Carboxylation: CO2 is fixed to ribulose-1,5-bisphosphate (RuBP), forming an unstable 6-carbon compound that splits into two molecules of 3-phosphoglycerate (3-PGA).
  2. Reduction: 3-PGA is reduced to glyceraldehyde-3-phosphate (G3P) using ATP and NADPH.
  3. Regeneration: Five out of six G3P molecules are used to regenerate RuBP, while one is used to synthesize glucose, fatty acids, amino acids, and other molecules.

Photorespiration and the Hatch-Slack Pathway

Photorespiration: Occurs in hot and dry conditions when stomata close to prevent water loss. High O2 levels cause rubisco to act as an oxidase, destroying RuBP and reducing photosynthetic efficiency.

Hatch-Slack Pathway (C4 Plants): C4 plants have evolved a mechanism to minimize photorespiration. CO2 is initially fixed to phosphoenolpyruvate (PEP) in mesophyll cells, forming oxaloacetate (a 4-carbon compound). Oxaloacetate is then converted to malate, which is transported to bundle sheath cells. In these cells, CO2 is released and enters the Calvin cycle. This spatial separation of CO2 fixation and the Calvin cycle reduces photorespiration and increases efficiency in hot and dry environments.