The reactions in this phase do not require light energy, and occur simultaneously with the light reaction.
Energy and electrons provided by ATP and NADPH, respectively, are used to incorporate CO2 into carbohydrate via a cyclic pathway called the Calvin–Benson cycle.
Light independent phase (Calvin cycle): The reactions in this phase do not require light energy, and occur simultaneously with the light reaction (time gap between the two being less than even one–thousandth of a second).
It utilizes the chemical energy temporarily stored in the products of light reaction. It is a series of reactions taking place in a cyclic manner. Energy and electrons provided by ATP and NADPH (energy carries) , respectively, are used to incorporate CO2 into carbohydrate via a cyclic pathway called the Calvin–Benson cycle.
Illustration of Photosynthesis
The incorporation of carbon dioxide into organic compounds is known as carbon fixation. This synthesis occurs in a number of steps using certain special intermediary (enzymatic) compounds and chiefly RuBP (ribulose biphosphate). Carbon dioxide is captured by the chemical ribulose bisphosphate (RuBP). RuBP is a 5–C chemical. Six molecules of carbon dioxide enter the Calvin Cycle, eventually producing one molecule of glucose (carbohydrates).
The dark reaction (biosynthetic phase) may be summarized in an overall chemical equation as follows :
6RuBP + 6CO2 + 18ATP + 12NADPH + 12H+
6RuBP + C6H12O6 + 18ADP + 18Pi + 12NADP + 6H2O
The carbon skeletons produced by photosynthesis are then variously used to form other organic compounds, such as the building material cellulose, as precursors for lipid and amino acid biosynthesis, or as a fuel in cellular respiration.
The glucose moleculeC6H12O6 is a simple sugar readily soluble in water. Most green plants convert glucose into starch as soon as it is formed during photosynthesis. Several glucose molecules are transformed to produce one molecule of starch; this process is called polymerization. Some other plants change glucose to sucrose (cane sugar, which chemically is called disaccharide or double sugar), or some into oils, etc.