Question

How is one molecule of glucose formed during photosynthesis?

Answer 1

The second stage of photosynthesis, which takes place in the stroma of the chloroplast, can occur without the presence of sunlight. In this stage, known as the Calvin Cycle, carbon molecules from CO2 are fixed into glucose (C6H12O2). The reactions of the Calvin Cycle are
1. A five-carbon sugar molecule called ribulose bisphosphate, or RuBP, is the acceptor that binds CO2 dissolved in the stroma. This process, called CO2 fixation, is catalyzed by the enzyme RuBP carboxylase, forming an unstable six-carbon molecule. This molecule quickly breaks down to give two molecules of the three-carbon 3-phosphoglycerate (3PG), also called phosphoglyceric acid (PGA).

2. The two 3PG molecules are converted into glyceraldehyde 3-phosphate (G3P, a.k.a. phosphoglyceraldehyde, PGAL) molecules, a three-carbon sugar phosphate, by adding a high-energy phosphate group from ATP, then breaking the phosphate bond and adding hydrogen from NADPH + H+.

3. Three turns of the cycle, using three molecules of CO2, produces six molecules of G3P. However, only one of the six molecules exits the cycle as an output, while the remaining five enter a complex process that regenerates more RuBP to continue the cycle. Two molecules of G3P, produced by a total of six turns of the cycle, combine to form one molecule of glucose.

Answer 2

carbon dioxide and ATP are turned into glucose in what is called the dark reaction (because it does not directly require light). Three ATP molecules react with three molecules of carbon dioxide via a series of chemical reactions (called the Calvin cycle) to form an intermediate called glyceraldehyde 3-phosphate. Two of the glyceraldehyde 3-phosphate molecules then are combined to form a glucose molecule. Thus, six molecules of carbon dioxide and light energy are needed to make one molecule of glucose for photosynthesis.