All animals and most microorganisms rely on the continual uptake of large amounts of organic compounds from their environment. These compounds are used to provide both the carbon skeletons for biosynthesis and the metabolic energy that drives cellular processes. It is believed that the first organisms on the primitive Earth had access to an abundance of the organic compounds produced by geochemical processes, but that most of these original compounds were used up billions of years ago. Since that time, the vast majority of the organic materials required by living cells have been produced by photosynthetic organisms, including many types of photosynthetic bacteria.

The most advanced photosynthetic bacteria are the cyanobacteria, which have minimal nutrient requirements. They use electrons from water and the energy of sunlight when they convert atmospheric CO₂ into organic compounds—a process called carbon fixation. In the course of splitting water [in the overall reaction nH₂O + nCO₂

In plants and algae, which developed much later, photosynthesis occurs in a specialized intracellular organelle—the chloroplast. Chloroplasts perform photosynthesis during the daylight hours. The immediate products of photosynthesis, NADPH and ATP, are used by the photosynthetic cells to produce many organic molecules. In plants, the products include a low-molecular-weight sugar (usually sucrose) that is exported to meet the metabolic needs of the many nonphotosynthetic cells of the organism.

Biochemical and genetic evidence strongly suggest that chloroplasts are descendants of oxygen-producing photosynthetic bacteria that were endocytosed and lived in symbiosis with primitive eucaryotic cells. Mitochondria are also generally believed to be descended from an endocytosed bacterium. The many differences between chloroplasts and mitochondria are thought to reflect their different bacterial ancestors, as well as their subsequent evolutionary divergence. Nevertheless, the fundamental mechanisms involved in light-driven ATP synthesis in chloroplasts are very similar to those that we have already discussed for respiration-driven ATP synthesis in mitochondria.

The actual reaction in which CO₂ is fixed is energetically favorable because of the reactivity of the energy-rich compound ribulose 1,5-bisphosphate, to which each molecule of CO₂ is added (see Figure 14-38). The elaborate metabolic pathway that produces ribulose 1,5-bisphosphate requires both NADPH and ATP; it was worked out in one of the first successful applications of radioisotopes as tracers in biochemistry. This carbon-fixation cycle (also called the Calvin Cycle) is outlined in Figure 14-39. It starts when 3 molecules of CO₂ are fixed by ribulose bisphosphate carboxylase to produce 6 molecules of 3-phosphoglycerate (containing 6 × 3 = 18 carbon atoms in all: 3 from the CO₂ and 15 from ribulose 1,5-bisphosphate). The 18 carbon atoms then undergo a cycle of reactions that regenerates the 3 molecules of ribulose 1,5-bisphosphate used in the initial carbon-fixation step (containing 3 × 5 = 15 carbon atoms). This leaves 1 molecule of glyceraldehyde 3-phosphate (3 carbon atoms) as the net gain.

A total of 3 molecules of ATP and 2 molecules of NADPH are consumed for each CO₂ molecule converted into carbohydrate. The net equation is:

Thus, both phosphate-bond energy (as ATP) and reducing power (as NADPH) are required for the formation of organic molecules from CO₂ and H₂O.