To grow, all organisms require both carbon and energy. As we saw in Chapter 8, photosynthetic organisms use energy from the sun to form ATP and NADPH, which reduce CO₂ to carbohydrates. As photosynthesis pulls CO₂ out of the atmosphere (or water, for aquatic organisms), the carbon is transferred to carbohydrates, and oxygen is given off as a by-product. The overall chemical reaction is shown here:

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

It is estimated that photosynthetic organisms remove about 210 billion metric tons of carbon (in nearly 770 billion metric tons of CO₂) from the atmosphere each year, about 60% of it by land plants and the rest by phytoplankton and seaweeds in the oceans. This annual photosynthetic removal of CO₂ equals about 25% of the total CO₂ in the atmosphere. In the absence of processes that restore CO₂ to the atmosphere, high rates of photosynthesis would use up the atmospheric reservoir of CO₂ in just a few years. But the Keeling curve tells us that atmospheric CO₂ is not decreasing rapidly—it is rising. Moreover, the geologic record indicates that photosynthesis has fueled ecosystems for billions of years without causing CO₂ depletion.

What processes return CO₂ to the atmosphere, balancing photosynthetic removal? In Chapter 7, we discussed aerobic respiration. Humans and many other organisms gain both the energy and carbon needed for growth from organic molecules (that is, by consuming food). As explained in Chapter 7, aerobic respiration uses oxygen to oxidize organic molecules to CO₂, converting chemical energy in the organic compounds to ATP for use in cellular processes. Aerobic respiration can be summarized by this chemical equation:

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

Plants respire, too, converting the chemical energy in carbohydrates into useful work. On the present-day Earth, the amount of CO₂ returned annually to the atmosphere by aerobic respiration and related processes is about equal to the amount that plants and algae remove by photosynthesis. That is why photosynthesis does not deplete the CO₂ in air.

As we discussed in Chapter 8, the equations for oxygenic photosynthesis and aerobic respiration mirror each other. Photosynthesis continually converts CO₂ to organic molecules, using energy from the sun, and respiration harvests the chemical energy stored in organic molecules, releasing CO₂ and water in the process. Photosynthesis and aerobic respiration are complementary metabolic pathways: Each uses the products of the other and generates a new supply of substrates for its complement. The result is a cycle (Fig. 25.2).