Photosynthesis is a mechanism for using light energy to synthesize glucose from carbon dioxide and water. It is not the only way in which a cell can synthesize glucose, but it is crucial because it opens the way to the use of a virtually unlimited source of free energy, the sun.

The overall reaction is the reverse of glucose oxidation:

A photosynthesizing plant needs a source of carbon atoms (from C0₂) and a source of reducing hydrogen atoms (from H₂0). The reaction above is the one followed by all photosynthetic eucaryotes and blue-green algae. Some photosynthetic bacteria use CO₂ as their carbon source, but obtain reducing hydrogens from H₂S, H₂, or organic molecules. Other bacteria can use organic matter as both C and H sources. No bacteria use water and release 0₂ in the way that blue-green algae and higher plants do. Bacterial photosynthesis will be discussed later, but for the moment we shall focus on the O₂-releasing process of photosynthesis.

The photosynthetic machinery can be divided into two stages, which are connected by ATP and NADPH (not NADH) but otherwise seem to operate quite independently of one another. NADPH, or reduced nicotinamide adenine dinucleotide phosphate, is a carrier molecule identical to NADH except for another phosphate group esterified with the 2' hydroxyl of the adenosine ribose ring (next page). The extra phosphate group may function as a label, to say, in effect, "This reduced nucleotide belongs to photosynthesis. Do not use for respiration."

The first of the two stages of photosynthesis, the "dark reactions," involve the synthesis of glucose from C0₂ and a reducing agent, or the "fixation" of C0₂. These reactions can take place perfectly well in the absence of light, as long as supplies of NADPH as the reducing agent and ATP for driving energy are available. ATP and the reducing agent are produced by the "light reactions," which involve the trapping of light energy by chlorophyll molecules, and which can operate only in the presence of light. Although the light reactions are what we ordinarily think of as photosynthesis, they appear to be a later addition to the older synthetic machinery of the dark reactions.