In order for photosynthesis to take place, photosynthetic organisms need a supply of carbon dioxide, which they get from the air.

What exactly is air?

Air is primarily nitrogen and oxygen with a small amount of carbon dioxide. Its composition is 78% nitrogen, 21% oxygen, and 0.03% carbon dioxide.

Carbon dioxide and oxygen enter and exit a plant through pores in the leaves called stomata. A single pore is a stoma. When carbon dioxide enters the leaf, it diffuses into the stroma, the fluid matrix of the chloroplast that surrounds the thylakoids.

In the stroma, carbon dioxide reacts with the five-carbon acceptor molecule, ribulose-1,5-bisphosphate.

The enzyme rubisco catalyzes the covalent attachment of carbon dioxide to this five-carbon sugar. In the presence of water, the resulting six-carbon sugar spontaneously decomposes into two molecules of the three-carbon sugar 3-phosphoglycerate.

In reactions that consume ATP and NADPH, the three-carbon sugar, 3-phosphoglycerate, formed in stage 1 is converted into glyceraldehyde-3-phosphate. This is the second stage of the Calvin-Benson cycle. This stage includes an oxidation-reduction reaction, because NADPH is oxidized to NADP+, and 3-phosphoglycerate is reduced to glyceraldehyde-3-phosphate.

After stage 2, some of the glyceraldehyde 3-phosphate exits the cycle, where it is used to form sugar and starch or as an immediate energy source.

For the continuous flow of carbon dioxide through the Calvin-Benson cycle, the starting acceptor molecule, ribulose-1,5-bisphosphate, must be constantly regenerated. This is accomplished through a series of reactions that involves the rearrangement of covalent bonds. Another molecule of ATP is consumed in these reactions. Most plants assimilate carbon dioxide through the Calvin-Benson cycle and are called C3 plants.

For every three-carbon glyceraldehyde-3-phosphate molecule that leaves the Calvin-Benson cycle, three molecules of carbon dioxide must enter. This consumes six molecules of NADPH and nine molecules of ATP. Taking carbon in its most oxidized form and reducing it to a sugar phosphate is an energy-expensive process!

Carbohydrate metabolism in plant cells is more complex than in animal cells, because the processes of glycolysis and gluconeogenesis must be coordinated with the Calvin-Benson cycle and associated pathways.

In addition, plants must have a light-sensitive control mechanism. During the day, plants are able to satisfy their energy needs through photosynthesis. At night, they obtain energy through glycolysis like other organisms do. To prevent the consumption of the ATP and NADPH generated through glycolysis and other catabolic processes from being used in the Calvin-Benson cycle, the activity of the enzyme rubisco is regulated by light-dependent factors, such as pH.

Based on what you know about photosynthesis, does high pH activate or inhibit rubisco? Recall that pH increases as the concentration of protons decreases. In the presence of light, protons are pumped out the stroma, which increases the stroma pH from about 7 to 8. The rubisco reaction occurs in the stroma, and rubisco is most active at pH 8, so the increase in the stroma’s pH when light is available activates rubisco.

In plants, light stimulates the Calvin-Benson cycle and inhibits glycolysis, whereas darkness has the opposite effect.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education