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
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
|⇒ Equation 
||6CO2 + 12H2O → C6H12O6 + 6O2 + 6H2O,
refers to a six-carbon sugar. This equation
in reverse describes the oxidative catabolism of a sixcarbon
sugar such as glucose.