Anabolism and Photosynthesis: Processes of Building Life

Posted on Oct 28, 2024 in Biology

Anabolism

The part of metabolism dedicated to constructing organic molecules from smaller, oxidized molecules.

Light-Dependent Reactions

These reactions capture light energy and convert it into chemical energy by synthesizing ATP and reduced pyridine nucleotides.

Chemiosmotic Photophosphorylation

ATP synthesis in chloroplasts, much like in mitochondria, is explained by Mitchell’s chemiosmotic hypothesis. Electron transport creates a proton gradient across the thylakoid membrane. Protons flowing back into the stroma through ATP synthase drive ATP synthesis from ADP and phosphate. Electrons reduce NADP+ to NADPH. ATP and NADPH are then used to reduce simple molecules like CO2 to form carbohydrates.

Dark Phase (Calvin Cycle)

Occurring in the chloroplast stroma, this phase uses the chemical energy from the light phase to reduce CO2, nitrates, and sulfates, assimilating bioelements C, H, and S to synthesize carbohydrates, amino acids, and other molecules. CO2 enters through leaf stomata.

  1. Carboxylation: CO2 binds to ribulose 1,5-bisphosphate (5C), forming an unstable 6C compound that splits into two 3-phosphoglycerate (PGA) molecules.
  2. Reduction: PGA is reduced to glyceraldehyde 3-phosphate (PGAL) using ATP and NADPH.
  3. Regeneration/Synthesis: Of six PGAL molecules, five regenerate ribulose 1,5-bisphosphate to continue the cycle, while one synthesizes glucose, fatty acids, amino acids, and other necessary molecules.

The enzyme Rubisco (ribulose 1,5-bisphosphate carboxylase/oxygenase) plays a key role. At low CO2 concentrations, it acts as an oxidase, leading to photorespiration, which oxidizes carbohydrates and reduces photosynthetic efficiency. At higher CO2 concentrations, it acts as a carboxylase, promoting CO2 fixation.

Glycogenesis

The synthesis of glucose and glycogen.

  • Initial substrates: pyruvate, lactate, some amino acids, Krebs cycle intermediates, glycerol.
  • Final substrate: monosaccharides and polysaccharides.

This process occurs in the cytosol, except for the pyruvate conversion, which occurs in the mitochondrial membrane.

Types of Organisms

  • Photosynthetic: Build their structures from inorganic materials and produce complex organic molecules using light energy.
  • Chemosynthetic: Obtain energy from oxidizing inorganic molecules.
  • Anaerobes: Use other acceptor molecules in metabolic reactions, resulting in partial or incomplete oxidation.
  • Aerobes: Use molecular oxygen as the final electron acceptor, resulting in complete oxidation.

Chemosynthesis

ATP synthesis using energy from oxidizing inorganic substances. Chemoautotrophic and chemolithotrophic bacteria carry out these processes, using reduced compounds like NH3 (from decomposing organic matter) and oxidizing them into minerals that plants can absorb, closing biogeochemical cycles.

Chemosynthesis occurs in two phases:

  1. ATP and a reduced coenzyme (NADH in bacteria) are produced.
  2. ATP and NADH are used to synthesize organic compounds from inorganic substances.

In the first phase, inorganic substance oxidation provides energy for ADP phosphorylation in the respiratory chain (oxidative phosphorylation). Some ATP drives reverse electron transport to produce NADH. The second phase follows similar pathways to the dark phase of photosynthesis.

Types of Chemosynthetic Bacteria

  • Colorless sulfur bacteria: Oxidize sulfur or sulfur compounds (obligate aerobes).
  • Nitrogen bacteria: Oxidize reduced nitrogen compounds. Nitrifying bacteria convert ammonia to nitrite, and nitrifying bacteria convert nitrite to nitrate.
  • Iron bacteria: Oxidize ferrous compounds to ferric compounds.
  • Hydrogen bacteria: Use molecular hydrogen.