Photosynthesis is a fundamental process for life on Earth, through which plants, algae and some bacteria convert solar energy into chemical energy. This mechanism is divided into two main phases: the light phase and the dark phase. Although its name may suggest otherwise, the dark phase does not occur in the absence of light, but simply does not directly depend on it.
In this phase, carbon dioxide is fixed into organic compounds through a series of chemical reactions regulated by enzymes, which leads to the synthesis of molecules essential for plant life. One of the most important processes within this stage is the famous Calvin cycle, where carbohydrates which will be used by the plant as a source of Energy and for the construction of cellular structures.
What is the dark phase of photosynthesis?
Also known as the biosynthetic phase or Calvin-Benson cycle, the dark phase is the stage in which CO is incorporated2 atmospheric into organic compounds. It takes place in the stroma of the chloroplasts of plant cells and uses the products generated in the light phase: ATP (adenosine triphosphate) and the NADPH extension (reduced nicotinamide adenine dinucleotide phosphate).
Although it does not directly require light, its operation is conditioned by it, since many of the enzymes involved in this process depend on light regulation and factors such as temperature and CO concentration.2.
The Calvin cycle: the key to the process
The Calvin cycle is a set of chemical reactions that allow the fixation of carbon in the form of organic compounds that can be used by the plant. It is divided into three main phases:
- Carbon fixation: the CO2 binds to a ribulose-1,5-bisphosphate (RuBP) molecule thanks to the action of the enzyme RuBisCO (ribulose-1,5-bisphosphate carboxylase-oxygenase). This results in an unstable six-carbon molecule, which is split into two molecules of 3-phosphoglyceric acid (3-PGA).
- Reduction: 3-PGA is transformed into glyceraldehyde-3-phosphate (G3P) by using ATP y NADPH extensionSome of these molecules are used to generate glucose and other organic compounds.
- Regeneration of RuBP: Some G3P molecules are recycled to regenerate ribulose-1,5-bisphosphate, allowing the cycle to continue.
Types of carbon fixation
There are different ways in which plants carry out carbon fixation in the dark stage of photosynthesis, depending on their adaptation to the environment:
- C plants3: Most plants use this method, in which CO2 It is incorporated directly into the Calvin cycle.
- C plants4: In warm environments, some plants capture CO2 in cells other than those in the Calvin cycle, avoiding photorespiration and improving photosynthetic efficiency.
- CAM plants: Typical of arid areas, these plants capture CO2 overnight and store it in the form of malate for use during the day.
Importance of RuBisCO in the dark phase
La RuBisCO It is the enzyme responsible for fixing CO2 in the Calvin cycle and is considered the most abundant in the biosphere. However, it has a defect: it can also fix oxygen, giving rise to photorespiration, a process that decreases photosynthetic efficiency. For this reason, plants have developed strategies to minimize this effect.
Final products and their usefulness
The products generated in the dark phase are essential for the growth and development of the plant:
- Glucose: used as an energy source or stored as starch.
- Saccharose: transported to other parts of the plant for immediate use.
- Cellulose: used in the construction of cell walls.
Factors influencing the dark phase
Environmental conditions play a crucial role in the efficiency of the dark phase:
- Temperature: The enzymatic activity of the Calvin cycle is temperature dependent, being optimal in moderate ranges.
- CO concentration2: an increase in CO2 promotes carbon fixation and glucose production.
- Availability of ATP and NADPH: essential for the process, come from the light phase.
The study of photosynthesis and in particular the dark phase continues to arouse great interest in the scientific community, since its optimization could have a positive impact on agricultural production and on the reduction of CO2 atmospheric. Understanding the Calvin cycle and its implications allows us to assess the importance of this process in the ecological balance of the planet.