DARK PHASE OF PHOTOSYNTHESIS: CHARACTERISTICS, MECHANISM, PRODUCTS - BIOLOGY - 2025

The dark phase of photosynthesis is the biochemical process by which organic substances (based on carbon) are obtained from inorganic substances. It is also known as the carbon fixation phase or the Calvin-Benson cycle. This process occurs in the stroma of the chloroplast.

In the dark phase, chemical energy is supplied by products generated in the light phase. These products are the energetic molecules ATP (adenosine triphosphate) and NADPH (a reduced electron carrier).

Light phase and dark phase. Maulucioni, from Wikimedia Commons

The fundamental raw material for the process in the dark phase is carbon, which is obtained from carbon dioxide. The end product is carbohydrates or simple sugars. These carbon compounds obtained are the fundamental basis of the organic structures of living beings.

General characteristics

Dark phase of photosynthesis. pixabay.com

This phase of photosynthesis is called dark due to the fact that it does not require the direct participation of sunlight for its development. This cycle occurs during the day.

The dark phase develops primarily in the chloroplast stroma in most photosynthetic organisms. The stroma is the matrix that fills the internal cavity of the chloroplast around the thylakoid system (where the light phase takes place).

In the stroma are the enzymes necessary for the dark phase to occur. The most important of these enzymes is rubisco (ribulose bisphosphate carboxylase / oxygenase), the most abundant protein, representing between 20 to 40% of all existing soluble proteins.

Mechanisms

The carbon required for the process is in the form of CO ₂ (carbon dioxide) in the environment. In the case of algae and cyanobacteria, CO ₂ is dissolved in the water that surrounds them. In the case of plants, CO ₂ reaches the photosynthetic cells through the stomata (epidermal cells).

-Calvin-Benson cycle

This cycle has several reactions:

Initial reaction

CO ₂ binds to a five-carbon acceptor compound (ribulose 1,5-bisphosphate or RuBP). This process is catalyzed by the enzyme rubisco. The resulting compound is a six-carbon molecule. It breaks down quickly and forms two compounds of three carbons each (3-phosphoglycerate or 3PG).

Calvin cycle. Calvin-cycle4.svg: Mike Jonesderivative work: Aibdescalzo, via Wikimedia Commons

Second process

In these reactions, the energy provided by ATP from the light phase is used. An energy-driven phosphorylation of ATP and a process of reduction mediated by NADPH occurs. Thus, 3-phosphoglycerate is reduced to glyceraldehyde 3-phosphate (G3P).

G3P is a phosphated three-carbon sugar, also called triose phosphate. Only one sixth of glyceraldehyde 3-phosphate (G3P) is transformed into sugars as a product of the cycle.

This photosynthetic metabolism is called C3, because the basic product obtained is a three-carbon sugar.

Final process

The parts of the G3P that are not transformed into sugars are processed to form ribulose monophosphate (RuMP). RuMP is an intermediate that is converted to ribulose 1,5-bisphosphate (RuBP). In this way, the CO ₂ acceptor is recovered and the Kelvin-Benson cycle is closed.

Of the total RuBP produced in the cycle in a typical leaf, only a third is converted to starch. This polysaccharide is stored in the chloroplast as a source of glucose.

Another part is converted into sucrose (a disaccharide) and transported to other organs of the plant. Subsequently, sucrose is hydrolyzed to form monosaccharides (glucose and fructose).

-Other photosynthetic metabolisms

In particular environmental conditions, the photosynthetic process of plants has evolved and has become more efficient. This has led to the appearance of different metabolic pathways for obtaining sugars.

C4 metabolism

In warm environments the stomata of the leaf are closed during the day to avoid the loss of water vapor. Therefore the concentration of CO ₂ in the leaf decreases in relation to that of oxygen (O ₂). The rubisco enzyme has a double substrate affinity: CO ₂ and O ₂.

At low CO ₂ and high O ₂ concentrations, rubisco catalyzes the condensation of O ₂. This process is called photorespiration and it decreases photosynthetic efficiency. To counteract photorespiration, some plants in tropical environments have developed a particular photosynthetic anatomy and physiology.

During C4 metabolism, carbon is fixed in the mesophyll cells and the Calvin-Benson cycle occurs in the chlorophyll sheath cells. The fixation of CO ₂ occurs during the night. It does not occur in the stroma of the chloroplast, but in the cytosol of the mesophyll cells.

The fixation of CO ₂ occurs by a carboxylation reaction. The enzyme that catalyzes the reaction is phosphoenolpyruvate carboxylase (PEP-carboxylase), which is not sensitive to low concentrations of CO ₂ in the cell.

The CO ₂ acceptor molecule is phosphoenolpyruvic acid (PEPA). The intermediate product obtained is oxaloacetic acid or oxaloacetate. Oxaloacetate is reduced to malate in some plant species or to aspartate (an amino acid) in others.

Subsequently, the malate moves into the cells of the vascular photosynthetic sheath. Here it is decarboxylated and pyruvate and CO ₂ are produced.

CO ₂ enters the Calvin-Benson cycle and reacts with Rubisco to form PGA. For its part, pyruvate returns to mesophyll cells where it reacts with ATP to regenerate the carbon dioxide acceptor.

CAM metabolism

Crassulaceae acid metabolism (CAM) is another strategy for CO ₂ fixation. This mechanism has evolved independently in various groups of succulent plants.

CAM plants use both the C3 and C4 pathways, just as they do in C4 plants. But the separation of both metabolisms is temporary.

CO ₂ is fixed at night by the activity of PEP-carboxylase in the cytosol and oxaloacetate is formed. Oxaloacetate is reduced to malate, which is stored in the vacuole as malic acid.

Later, in the presence of light, the malic acid is recovered from the vacuole. It is decarboxylated and CO ₂ is transferred to the RuBP of the Calvin-Benson cycle within the same cell.

CAM plants have photosynthetic cells with large vacuoles where malic acid is stored, and chloroplasts where the CO ₂ obtained from malic acid is transformed into carbohydrates.

Final products

At the end of the dark phase of photosynthesis, different sugars are produced. Sucrose is an intermediate product that is rapidly mobilized from the leaves to other parts of the plant. It can be used directly to obtain glucose.

Starch is used as a reserve substance. It can accumulate on the leaf or be transported to other organs such as stems and roots. There it is kept until it is required in different parts of the plant. It is stored in special plastids, called amyloplasts.

The products obtained from this biochemical cycle are vital for the plant. The glucose produced is used as a carbon source to constitute compounds such as amino acids, lipids and nucleic acids.

On the other hand, the sugars produced from the dark phase represent the base of the food chain. These compounds represent packages of solar energy transformed into chemical energy that are used by all living organisms.

References

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