Essence and conditions
Unlike the first stage, the light-independent reactions do not require the direct participation of light quanta. The process is localized in the chloroplast stroma—the fluid internal environment of the organelle, where energy-rich ATP molecules and the NADPH₂ reducing agent are delivered.
The main task of this stage is the fixation of carbon derived from atmospheric $CO_2$ and its transformation into complex organic molecules. Unlike the light-dependent reactions, the evolution of free oxygen does not occur here.
Process mechanism: Calvin cycle
The main pathway of carbon dioxide biochemical transformations is a closed loop of reactions.
- Carboxylation ($CO_2$ fixation). The stroma contains a special five-carbon sugar — ribulose bisphosphate (RuBP) — which acts as an acceptor. With the participation of the enzyme ribulose-1,5-bisphosphate carboxylase, the RuBP molecule attaches carbon dioxide. An extremely unstable six-carbon compound is formed.
- Cleavage. The unstable molecule instantly breaks down into two three-carbon molecules: 3-phosphoglyceric acid (PGA).
- Reduction. Using the energy of ATP and the reducing power of NADPH₂, PGA molecules are converted into glyceraldehyde 3-phosphate (G3P).
- Regeneration and synthesis. The fate of the formed G3P is twofold. One portion of the molecules goes toward the regeneration of the initial acceptor (RuBP) so that the cycle is not interrupted. The second portion, after passing through the fructose formation stage, is used to construct glucose.
The overall chemical equation for glucose biosynthesis is as follows: $6CO_2 + 12NADPH_2 + 18ATP \rightarrow C_6H_{12}O_6 + 12NADP^+ + 18ADP + 18P_i + 6H_2O$
Factors regulating process intensity
The rate of light-independent reactions is closely linked to environmental conditions and the general state of the plant.
- Carbon dioxide concentration. Normally, ambient air contains about 0.03–0.04% $CO_2$. Artificially increasing this indicator to 0.1% significantly accelerates carbon fixation.
- Water balance. Despite the fact that water is directly split in the light-dependent phase, its deficiency is fatal to the dark phase as well. When moisture is lacking, the plant closes its stomata, blocking the diffusion of $CO_2$ into the tissues.
- Temperature regime. The enzymes of the Calvin cycle have their own temperature optimum. For flora of temperate latitudes, maximum productivity is observed at +25°C.
Alternative pathway: Hatch-Slack pathway
Some plant species (maize, sugarcane, millet) have evolutionarily established the $C_4$ photosynthetic pathway. Its main difference lies in the nature of the primary fixation product: instead of a three-carbon acid, the four-carbon oxaloacetate is synthesized.
This metabolic maneuver provides colossal adaptive advantages. Plants with the $C_4$ pathway use carbon dioxide extremely sparingly by reusing it from weak photorespiration processes. This allows them to survive under conditions of severe drought, high temperatures, and still accumulate significant biomass.
To easily remember the sequence of carbon transformations in the Calvin cycle, use the three-letter rule: Ribulose bisphosphate (R) fixes the gas $\rightarrow$ breaks down into Acid (PGA) $\rightarrow$ is reduced to Aldehyde (G3P).
Class and exam questions
Do the dark reactions of photosynthesis occur at night?
Yes, the dark reactions can proceed both day and night, as light is not directly involved. The main requirement is the presence of accumulated ATP and NADPH₂ molecules in the chloroplasts.
What else is produced in the dark phase besides glucose?
In addition to sugars, the biochemical pathways of the dark phase provide monomers for the synthesis of other vital macromolecules: amino acids, nucleotides, glycerol, and fatty acids.
Why does the dark phase stop during drought?
During moisture deficiency, the plant closes the stomata on its leaves to minimize water evaporation. Because of this, carbon dioxide ceases to enter the tissues, and the Calvin cycle is deprived of its main substrate.
What else to study on this topic
- Features of anoxygenic photosynthesis in purple and green sulfur bacteria.
- The cosmic role of plants in climate regulation and the formation of the ozone layer (according to K.A. Timiryazev).
- The relationship between mineral nutrition and the efficiency of chlorophyll synthesis.
- The effect of light radiation intensity on the destruction of photosynthetic pigments.
- Quantitative indicators of photosynthetic productivity and methods for their optimization.
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