Photosynthesis and Chemosynthesis Explained

Photosynthesis

Photosynthesis is the process by which some cells can capture light energy from the sun and transform it into chemical energy. This process is carried out by plants, green or red algae, and some bacteria. Its purpose is to synthesize organic matter from inorganic matter (CO₂, H₂O, mineral salts) using light energy and releasing molecular oxygen into the atmosphere during the process.

In eukaryotic cells, it occurs in chloroplasts in two phases:

• Light Phase

  This occurs in the thylakoid membrane of chloroplasts, driven by chlorophyll. It captures light energy from the sun and converts it into chemical energy in the form of energy-rich ATP and a molecule with great reducing power, NADPH. The photolysis of water liberates O₂.

• Dark Phase or Calvin Cycle

  It takes place in the chloroplast stroma. The dark phase of photosynthesis does not require light, although it typically occurs during the day. It involves the use of the chemical energy obtained in the light phase reactions to reduce CO₂, i.e., to synthesize organic matter.

  The purpose of the Calvin cycle is to use the reducing power (NADPH + H⁺) and energy (ATP) produced in the light phase to reduce CO₂ and incorporate it into organic matter.

  The overall equation for the dark phase is:

  6 CO₂ + 18 ATP + 12 NADPH + 12 H⁺ → C₆H₁₂O₆ + 18 P_i + 12 NADP⁺

Chemosynthesis

Chemosynthesis consists of ATP synthesis from the energy released by the oxidation of inorganic substances. The organisms carrying out these processes are called chemoautotrophs. All are bacteria.

These microorganisms are key players in biogeochemical cycles, enabling life on the planet by returning oxidized substances from the decomposition of dead organisms back to the substrate. Thus, the remains of living beings are transformed into mineral salts of nitrogen or sulfur that can be absorbed by plants again.

Photosynthesis is an anabolic process because it consumes energy (CO₂, H₂O, photons from sunlight, mineral salts) to synthesize more complex molecules (carbohydrates). It is also endergonic and endothermic because it consumes energy and does not release heat into the external environment.

While there is atmospheric release of O₂ during the light phase, O₂ is not used by the chloroplast for the biological degradation of carbohydrates. Only mitochondria carry out this process (cellular respiration).