Carbohydrates are synthesized from CO₂ molecules during photosynthesis, yet they have more energy stored in their chemical bonds than is contained in the bonds of CO₂ molecules. Therefore, to build carbohydrates using CO₂ requires an input of energy. This energy comes from sunlight.

How does energy from sunlight become incorporated into chemical bonds? The answer to this question arises from the fact that the synthesis of carbohydrates from CO₂ and water is a reduction–oxidation, or redox, reaction. In Chapter 7, we saw that reduction reactions are reactions in which a molecule acquires electrons and gains energy, whereas oxidation reactions are reactions in which a molecule loses electrons and releases energy. During photosynthesis, CO₂ molecules are reduced to form higher-energy carbohydrate molecules. This requires both an input of energy from ATP and the transfer of electrons from an electron donor. In photosynthesis, energy from sunlight is used to produce ATP and electron donor molecules capable of reducing CO₂ (Fig. 8.2).

Where do the electrons used to reduce CO₂ come from? In photosynthesis carried out by plants and many algae, the ultimate electron donor is water. The oxidation of water results in the production of electrons, protons, and O₂. Thus, oxygen is formed in photosynthesis as a by-product of water’s role as a source of electrons. We can demonstrate that water is the source of the oxygen released during photosynthesis using isotopes, molecules that can be distinguished on the basis of their molecular mass (Fig. 8.3).

Overall, then, the equation for photosynthesis leading to the synthesis of glucose (C₆H₁₂O₆) can be described as follows:

The oxidation of water is linked with the reduction of CO₂ through a series of redox reactions in which electrons are passed from one compound to another. This series of reactions constitutes the photosynthetic electron transport chain. The process begins with the absorption of sunlight by protein–pigment complexes. The absorbed sunlight provides the energy that drives electrons through the photosynthetic electron transport chain. In turn, the movement of electrons through this transport chain is used to produce ATP and NADPH. And finally, ATP and NADPH are the energy sources needed to synthesize carbohydrates using CO₂ in a process called the Calvin cycle (see Fig. 8.2).

Quick Check 1 If you want to produce carbohydrates containing the heavy oxygen (¹⁸O) isotope, should you water your plants with H₂¹⁸O or inject C¹⁸O₂ into the air?