Photosynthesis and Chemosynthesis: Energy Conversion in Organisms

Posted on Mar 12, 2025 in Biology

Light Phase (Photochemical)

Photosynthetic pigments are associated with membrane proteins, constituting photosystems. By absorbing a photon, the pigment is ionized (oxidized). The pigment acts as an electron donor to a molecule called an electron acceptor. Then, a series of electron acceptors are reduced and successively oxidized, forming a transport chain. During this process, energy is released, which is harnessed by ATP synthases to produce ATP via the chemiosmotic hypothesis (proton accumulation inside the thylakoid). This process is called photophosphorylation. There are two transport modes:

  • Noncyclic: Involves both photosystems.
  • Cyclic: Involves only photosystem I.

To replenish electrons to chlorophyll, water photolysis occurs.

  • Noncyclic photophosphorylation: Uses the two photosystems in series. It produces ATP and NADPH.
  • Cyclic photophosphorylation: Involves only photosystem I. It produces ATP without generating NADPH.

Dark Phase (Biosynthetic)

The dark phase uses the energy (ATP) and reduced coenzyme (NADPH) from the light phase to synthesize organic material. It uses CO2 as a carbon source. It takes place in the stroma of chloroplasts and is known as the Calvin cycle. There are three stages:

  1. Fixation of CO2: Ribulose-1,5-bisphosphate (RuBP) is carboxylated by the enzyme RuBisCO, forming two molecules of 3-phosphoglycerate (PGA). Because PGA has 3 carbon atoms, plants that follow this metabolic pathway are called C3 plants.
  2. Reduction: 3-phosphoglycerate is reduced to glyceraldehyde 3-phosphate (G3P) using ATP and NADPH. This glyceraldehyde can follow two pathways: regenerate ribulose-1,5-bisphosphate or be exported to the cytoplasm to produce glucose and starch.
  3. Regeneration (complex process).

Photorespiration and C4 Plants

In hot, dry environments, plants are forced to close their stomata to avoid water loss, leading to low CO2 concentration in the leaf. If CO2 is low, RuBisCO oxidizes ribulose-1,5-bisphosphate (5C), yielding 3-phosphoglycerate (3C) and glycolic acid (2C). This process, called photorespiration, reduces photosynthetic capacity by 50%.

Chemosynthesis

Chemosynthesis involves the synthesis of ATP from energy released by the oxidation of certain inorganic substances. All organisms capable of chemosynthesis are bacteria. Similar to photosynthesis, it occurs in two steps:

  1. Obtaining ATP and reduced coenzyme.
  2. Using ATP and NADH to synthesize organic compounds from inorganic substances.

Examples: sulfur bacteria, nitrogen-fixing bacteria.

Heterotrophic Anabolism

Heterotrophic anabolism is the metabolic process of forming complex organic molecules. We distinguish between the biosynthesis of monomers from their precursors and the biosynthesis of polymers. It is a reduction process, and the energy needed is supplied by ATP. Most of these reactions occur in the cytosol, except for protein synthesis, phospholipid and cholesterol synthesis, and protein glycosylation, which occur in the endoplasmic reticulum (ER).

Anabolism of Carbohydrates

Gluconeogenesis is the process of obtaining glucose from non-carbohydrate substances, such as pyruvic acid. Pyruvic acid can come from glycolysis, amino acid catabolism, and lactic acid produced in muscles. Gluconeogenesis from pyruvate requires 6 ATP molecules to synthesize 1 glucose molecule; therefore, it is an expensive process.

Glycogenogenesis and Amylogenesis: Glycogen synthesis starts from glucose. First, UTP activates glucose, which then joins a glycogen chain that acts as a primer. Subsequently, branching occurs via 1-6 linkages. Amylogenesis is the synthesis of starch. It takes place in amyloplasts. The process is similar to glycogen synthesis, but the activator is ATP.

Anabolism of Lipids

Fat biosynthesis involves obtaining fatty acids and glycerol, and the synthesis of triglycerides.

  • Fatty Acid Synthesis: Occurs in the cytoplasm and is made from acetyl-CoA of mitochondrial origin. Acetyl-CoA is transformed into malonyl-CoA (3C) by the enzyme acetyl-CoA carboxylase, costing one ATP. Subsequently, malonyl-CoA (3C) joins an acetyl-CoA (2C), yielding a 4C molecule with the detachment of CO2. The enzyme complex fatty acid synthase, using NADPH, catalyzes the successive unions of acetyl-CoA to complete the fatty acid chain.
  • Glycerol Synthesis: Glycerol is formed from products of glycolysis.