How Is Glucose Made in Plants?

Unlike animals, plants can make their own food.

Plant cells fix atmospheric carbon using energy from sunlight in a process called photosynthesis. Photosynthesis is divided into two stages, light reactions and light-independent reactions. Glucose is manufactured during the second stage.

Features

The main feature of light-independent reactions is a cyclic series of reactions called the Calvin cycle. To power these reactions, the Calvin cycle uses sunlight energy stored in chemical bonds in the form of a molecule called ATP. The light reactions synthesize ATP for use in the Calvin cycle.

Function

An enzyme called rubisco catalyzes addition of a molecule of carbon dioxide to a five-carbon sugar called ribulose bisphosphate, which is so unstable, it breaks up into two 3-carbon sugars called 3-phosphoglycerate. These are then phosphorylated (a phosphate group is added) and reduced to form glyceraldehyde 3-phosphate or PGAL. One molecule of PGAL leaves the cycle while the other is regenerated to form ribulose bisphosphate.

Effects

Two molecules of PGAL from the Calvin cycle are combined to form a single molecule of glucose, a six-carbon sugar. The PGAL from the Calvin cycle is also the starting point for the biosynthesis of various other molecules the cell needs. The net effect of photosynthesis is to convert 6 molecules of CO2 and 6 molecules of water into 1 molecule of glucose and 6 molecules of oxygen.

References

• "Biology"; Neil A. Campbell, Jane B. Reece, Lisa A. Urry, Michael L. Cain, Peter V. Minorsky, Steven A. Wasserman, Robert B. Jackson; 2008
• Wayne's Word: Photosynthesis and Cellular Respiration