Photosynthesis

Every living thing can synthesize AMP, but only green plants and a few microorganisms have the capacity to make it from energy-poor materials. Through the process of photosynthesis, these organisms can convert light energy to chemical bond energy and reduce carbon dioxide to carbohydrate.

When light strikes a photosynthetic organism, energy is absorbed by the green pigment chlorophyll. This energy is used to convert ADP to ATP and to reduce NADP by the addition of hydrogen ions donated by water.

2 ADP + 2 pi + 2 NADP⁺ + 2 H₂ O + light energy = 2 ATP + 2 NADPH + °2

Because light is essential for the production of ATP and the reduction of NADP, these events are known as the light reactions of photosynthesis.

The light reactions in photosynthetic bacteria differ somewhat from the green plants. Bacteria do not use water as a source of hydrogen ions and oxygen is not formed. Some use organic molecules, others use hydrogen sulfide (H₂S) and give off sulfur.

The remaining reactions can take place whenever ATP, NADPH, and carbon dioxide are present; they are, therefore, called the dark reactions. During the dark reactions, carbon dioxide combines with a five-carbon sugar which immediately splits, forming two molecules of the three-carbon compound, phosphoglyceric acid (PGA). Five-sixths of the PGA is lIsed to synthesize more of the five-carbon sugar; the remainder is reduced by NADPH to phosphoglyceraldehyde (PGAL). The dark reactions of photosynthesis are: