Question

Describe the main chemical changes which occur during photosynthesis in a dark reaction.

Answer 1

The reactions in this phase does not require light energy and occur simultaneously with the light reaction. The time gap between the light and dark reaction is less than one thousandth of a second. In the dark reaction, ATP and NADPH molecules (produced during light reaction) are used to produce glucose (C₆H₁₂O₆) from carbon dioxide. Fixation and reduction of carbon dioxide occurs in the stroma of the chloroplast through a series of reactions. The glucose produced is either immediately used up by the cells or stored in the form of starch.

Answer 2

1. Fixation of CO₂: An unstable 6-carbon compound is produced when the CO₂ in the foliage combines with a five-carbon compound called ribulose diphosphate. This unstable compound dissociates into two molecules of the 3-carbon compound phosphoglyceric acid (PGA). It is the initial stable compound to be produced during photosynthesis.
2. Formation of sugar: ATP and NADPH₂ are required for the conversion of phosphoglyceric acid to phosphoglyceraldehyde (PGAL), which is subsequently converted to glucose.
3. Regeneration of RuDP: Some molecules of phosphoglyceric acid undergo a series of reactions to make ribulose diphosphate (RuDP). The Calvin cycle is a series of reactions that culminate in a dark reaction.