Section:General Biochemistry » Bioenergetics


From a purely thermodynamic standpoint, life is an improbable event. Consider, for example, the complex structures of organisms, not only at the macroscopic level, but also at the microscopic and atomic levels. These ordered structures can be formed and maintained only by the expenditure of energy. Within the ecosystem that we call the earth, the organic nutrients necessary to sustain the life of heterotrophs such as us are provided directly and indirectly by photosynthesis.

In both quantitative and qualitative terms photosynthesis is the most significant biological process on Earth. Approximately 2×1011 tons of carbon dioxide are converted to organic compounds each year. It is to photosynthesis in prehistoric times that we owe the reserves of fossil fuels. The oxygen that we breathe is a direct result of photosynthesis, now and in prehistory.

If the earth were an isolated system in a thermodynamic sense, life would be in jeopardy in that the energy reserves for life would be consumed. Without the input of energy from a source external to the earth, the planet must tend toward achieving equilibrium within its environment.

Fortunately, the earth is not an isolated system. The hydrogen fusion reactor of the Sun bathes our planet in electromagnetic radiation, including visible light. A fraction of the solar energy that impinges on Earth is converted by photosynthesis to chemical energy in the form of organic molecules that heterotrophic organisms use to satisfy their continued need for energy. The process by which light energy is used to drive the otherwise unfavorable synthesis of these organic molecules is called photosynthesis.

Although some bacteria carry out photosynthesis without the evolution of oxygen, this article deals solely with oxygenic photosynthesis that takes place in higher plants and algae. In a purely formal sense, oxygenic photosynthesis may be represented as the reverse of the oxidative breakdown of a six-carbon carbohydrate, such as glucose. An equation that describes photosynthesis in part illustrates this relationship:

⇒ Equation [5] 6CO2 + 12H2O → C6H12O6 + 6O2 + 6H2O,

where C6H12O6 refers to a six-carbon sugar. This equation in reverse describes the oxidative catabolism of a sixcarbon sugar such as glucose.
Under standard conditions, the complete oxidation of glucose liberates 686 kcal/mol; the synthesis of a mole of glucose from carbon dioxide and water thus minimally requires the input of an equivalent amount of energy. In photosynthesis, visible light provides this energy. When it is considered that the only source of carbon for the tens of thousands of organic compounds synthesized in green plants is from the assimilation of carbon dioxide by means of photosynthesis, the inadequacy of Eq. (5) to describe photosynthesis, despite its usefulness, is readily apparent.

Inspection of Eq. (5) reveals that photosynthesis is an oxidation–reduction process. Simply put, photosynthesis is the light-driven reduction of carbon dioxide to the oxidation–reduction level of a carbohydrate by using water as the electron and hydrogen donor. In the process, water is oxidized to molecular oxygen. As stated previously, water is a very poor reducing agent. However, water at an effective concentration of 55 M is readily available in the biosphere. Although organic compounds and inorganic molecules such as hydrogen sulfide are more powerful reducing agents than water is, their use in photosynthesis as the source of electrons for photosynthesis is restricted to certain species of bacteria. The thermodynamically very unfavorable reduction of carbon dioxide bywater is driven by light.