Second stage of photosynthesis
The second stage of photosynthesis is known as the dark reaction or the Calvin cycle
The second stage of photosynthesis is known as the dark reaction or the Calvin cycle. It occurs in the stroma of the chloroplasts and does not require light directly, although it is dependent on the products of the light reaction.
The key goal of the dark reaction is to convert the energy stored in the ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate) molecules produced during the light reaction into carbohydrates. These carbohydrates, such as glucose, are essential for the survival and growth of plants.
The dark reaction consists of three main steps: carbon fixation, reduction, and regeneration.
1. Carbon Fixation: The dark reaction starts with the enzyme RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) catalyzing the reaction between carbon dioxide (CO2) and a 5-carbon compound called RuBP (ribulose 1,5-bisphosphate). This reaction leads to the formation of two molecules of 3-phosphoglycerate (3-PGA).
2. Reduction: In this step, ATP and NADPH from the light reaction are used to convert the 3-PGA molecules into glyceraldehyde-3-phosphate (G3P). For every three molecules of CO2 fixed, six molecules of G3P are produced. One of the six molecules is further utilized by the plant, while the remaining five molecules undergo the next step for regeneration of RuBP.
3. Regeneration: The five molecules of G3P are rearranged and converted back into three molecules of RuBP, which can again be used in the carbon fixation step. This regeneration process requires additional ATP molecules.
Overall, the dark reaction of photosynthesis is a complex series of biochemical reactions that enables plants to convert carbon dioxide from the atmosphere into energy-rich carbohydrates. It plays a crucial role in sustaining plant growth, as well as providing oxygen to the atmosphere as a byproduct of the light reaction.
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