Understanding the Dark Phase of Photosynthesis: Carbon Fixation

Posted on Mar 1, 2025 in Biology

The Dark Phase of Photosynthesis: Carbon Fixation

Plant cells contain the enzymes necessary to reduce and assimilate nutrients, transforming them into biomolecules within living matter. The dark phase involves the transformation of carbon, nitrogen, and sulfur compounds that can be incorporated into anabolic pathways.

Carbon Fixation

The photosynthetic fixation of CO2 occurs in the stroma of the chloroplasts via the Calvin-Benson cycle. This cycle leads to the incorporation of CO2 to form organic compounds (CH2O)n.

Plants that utilize this process are called C3 plants because the first stable organic compound formed (3-phosphoglycerate) contains three carbon atoms.

The Calvin-Benson cycle consists of three main stages:

  1. Carboxylation: Ribulose-1,5-bisphosphate is carboxylated, giving rise to two molecules of 3-phosphoglycerate. This reaction is catalyzed by the enzyme ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco).
  2. Reduction: 3-phosphoglycerate is reduced to glyceraldehyde-3-phosphate through two reactions that consume ATP and NADPH. Part of the glyceraldehyde-3-phosphate is used in cellular metabolism and the synthesis of sugars.
  3. Regeneration: Ribulose-1,5-bisphosphate is regenerated in a series of reactions.

The glyceraldehyde-3-phosphate can be converted into starch, fatty acids, and amino acids through a series of reactions in the stroma of chloroplasts. It can also be transported to the cytoplasm to form sucrose.

Rubisco, the primary enzyme involved in carbon fixation, is considered the most abundant protein on Earth, accounting for up to 50% of the soluble proteins found in green leaves. Photosynthetic efficiency largely depends on its activity. It is a bifunctional enzyme that can react with either CO2 (carboxylase activity) or O2 (oxygenase activity), depending on the concentrations of both gases.

Photorespiration

Photorespiration is a metabolic cycle that occurs simultaneously with photosynthesis in the presence of light. It involves oxygen consumption and the release of CO2.

Photorespiration occurs in three cellular compartments: chloroplasts, peroxisomes, and mitochondria.

Photorespiration counteracts carbon fixation, decreasing photosynthetic efficiency by 30% to 50%. It is a consequence of the oxygenase activity of Rubisco.

When plants are in dry and warm environments, they close their stomata to prevent water loss. However, this creates a problem: the O2 evolved by photosynthesis accumulates within the leaf, while atmospheric CO2 cannot enter, leading to increased photorespiration and decreased photosynthesis.

In greenhouse cultivation, enriching the atmosphere with CO2 increases the photosynthetic activity of some plants grown in temperate climates.

Tropical plants adapted to warmer environments have developed a special mechanism to maintain high CO2 levels around ribulose-bisphosphate carboxylase, even when their stomata are closed to prevent moisture loss. In these plants, called C4 plants, CO2 is concentrated in specific areas of the plant. The pumping of CO2 is performed through the Hatch-Slack cycle, which begins with the fixation of CO2 into a four-carbon compound. This compound is transported to the bundle sheath cells surrounding the vascular bundles, maintaining an elevated CO2 concentration. This transport requires energy but is worthwhile in dry environments, preventing the loss of photosynthetic efficiency due to photorespiration.