What is Dark Reaction in Photosynthesis?

dark reaction in photosynthesisBefore we elaborate on dark reaction in photosynthesis, let us understand as to what exactly is the process of photosynthesis. It is basically a process employed by plants and other organisms for the conversion of light energy into chemical energy. They use sunlight for synthesizing nutrients from carbon dioxide and water. In plants photosynthesis mainly involves the green pigment chlorophyll. It exudes oxygen as a by-product. The energy generated is later released to fuel the activities of the organisms.

Dark Reaction in Photosynthesis

Dark reaction is the second stage in the process of photosynthesis with the first one being the light reaction. The concept of dark reaction in photosynthesis was initially explained by Blackman. It is thus also known by the name Blackman’s reaction. This is basically the stage wherein the chemical process of photosynthesis takes place without the use of sunlight. The reaction occurs in the stroma of the chloroplast. The process is comparatively slower than the light reaction and is exclusively enzymatic.

Sugar is synthesised from CO2 in this reaction. During the process, the energy rich carbon dioxide is fixed to energy rich carbohydrates. This happens via the use of energy rich compound ATP, the light reaction’s assimilatory power NADPH2.

There are mainly two types of cyclic reactions that take place in dark reactions. These are:

  • Calvin Cycle or C3 Cycle: This is basically a series of biochemical redox reactions that occur in the stroma of chloroplast. These reactions are also referred to as carbon fixation. The main enzyme of the cycle is known as RuBisCo.
  • Hatch Slack Cycle or C4 Cycle: This is an alternative to the Calvin cycle that occurs during dark reaction in photosynthesis. The first stable compound in C4 cycle is 4-carbon compound.

The products of dark reaction of photosynthesis are sugar molecules and organic molecules that are essential for functioning of the cell and metabolism.

Difference between Dark Reaction and Light Reaction in Photosynthesis

While the dark reaction in photosynthesis is light independent reaction, light reaction, on the other hand, is dependent on light. Dark reaction of photosynthesis takes place in the stroma of living chloroplast. Light reactions take place in chloroplasts’ thylakoid membranes in a leaf. In dark reaction in photosynthesis, C3 pathway is noticed in green plants and C4 pathway is observed in desert plants. In light reaction in photosynthesis, the cyclic and non-cyclic photophosphorylation takes place in the cells.

The major product in dark reaction of photosynthesis is sugar where as in light reaction oxygen is released.

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