pooled

spent

To provide energy and reducing power for the rest of the cell, the ATP and NADPH are instead used within the chloroplast stroma to produce a simple three-carbon sugar that can be exported to the cytosol by specific carrier proteins in the chloroplast inner membrane. This production of sugar from CO 2 and water, which occurs during stage 2 of photosynthesis, is called carbon fixation.

In the central reaction of photosynthetic carbon fixation, CO 2 from the atmosphere is attached to a five-carbon sugar derivative, ribulose 1,5-bisphosphate, to yield two molecules of the three-carbon compound 3-phosphoglycerate. This carbon-fixing reaction is catalyzed in the chloroplast stroma by a large enzyme called ribulose bisphosphate carboxylase or Rubisco.

Rubisco works much more slowly than most other enzymes: it processes about three molecules of substrate per second― compared with 1,000 molecules per second for a typical enzyme. The enzyme generally represents more than 50% of the total chloroplast protein, and it is widely claimed to be the most abundant protein on Earth. The energy and reducing power needed to regenerate ribulose 1,5-bisphosphate come from the ATP and NADPH produced by the photosynthetic light reactions.

The elaborate series of reactions in which CO 2 combines with ribulose 1,5-bisphosphate to produce a simple three-carbon sugar—a portion of which is used to regenerate the ribulose 1,5-bisphosphate that’s consumed—forms a cycle, called the carbon-fixation cycle, or the Calvin cycle. For every three molecules of CO 2 that enter the cycle, one molecule of glyceraldehyde 3-phosphate is ultimately produced, at the expense of nine molecules of ATP and six molecules of NADPH, which are consumed in the process. Glyceraldehyde 3-phosphate, the three-carbon sugar that is the final product of the cycle, provides the starting material for the synthesis of the many other sugars and other organic molecules that the plant needs.

弯曲

约束；限制

以销子联接的

固定措施

联接；接合

实际上的

适当的

正确

错误

正确

错误

正确

错误