Page 11 - Academic Press Encyclopedia of Physical Science and Technology 3rd BioChemistry

P1: FYD Revised Pages
Encyclopedia of Physical Science and Technology EN002H-54 May 17, 2001 20:22

Bioenergetics 107

satisfied, the excess amino acids in the diet are catabo- Although some bacteria carry out photosynthesis with-
lized to CO 2 and water as a source of energy. Some amino out the evolution of oxygen, this article deals solely with
acids are degraded to molecules that feed directly into oxygenic photosynthesis that takes place in higher plants
glycolysis, and others result in the production of acetyl and algae. In a purely formal sense, oxygenic photosyn-
CoA. Excess nitrogen resulting from the catabolism of thesis may be represented as the reverse of the oxidative
amino acids and other compounds that contain nitrogen is breakdown of a six-carbon carbohydrate, such as glucose.
excreted in mammals in urine in the form of the simple An equation that describes photosynthesis in part illus-
organic compound urea. Some amino acids are precursors trates this relationship:
in the biosynthesis of other organic molecules.
6CO 2 + 12H 2 O → C 6 H 12 O 6 + 6O 2 + 6H 2 O, (5)
where C 6 H 12 O 6 refers to a six-carbon sugar. This equa-
E. Summary
tion in reverse describes the oxidative catabolism of a six-
In summary, organisms such as humans and other animals, carbon sugar such as glucose. Under standard conditions,
many bacteria, fungi, and nongreen plants derive the en- the complete oxidation of glucose liberates 686 kcal/mol;
ergy they must have to power life from foodstuffs they the synthesis of a mole of glucose from carbon dioxide
obtain from their environment. The degradation of carbo- and water thus minimally requires the input of an equiv-
hydrates and the oxidation of fats are the major sources of alent amount of energy. In photosynthesis, visible light
energy for heterotrophic (other feeding) organisms. How provides this energy. When it is considered that the only
these molecules that are essential to life are generated is source of carbon for the tens of thousands of organic com-
the next subject considered. poundssynthesizedingreenplantsisfromtheassimilation
of carbon dioxide by means of photosynthesis, the inad-
equacy of Eq. (5) to describe photosynthesis, despite its
II. PHOTOSYNTHESIS usefulness, is readily apparent.
Inspection of Eq. (5) reveals that photosynthesis is an
From a purely thermodynamic standpoint, life is an im- oxidation–reduction process. Simply put, photosynthe-
probable event. Consider, for example, the complex struc- sis is the light-driven reduction of carbon dioxide to the
tures of organisms, not only at the macroscopic level, but oxidation–reduction level of a carbohydrate by using wa-
also at the microscopic and atomic levels. These ordered ter as the electron and hydrogen donor. In the process, wa-
structures can be formed and maintained only by the ex- ter is oxidized to molecular oxygen. As stated previously,
penditure of energy. Within the ecosystem that we call water is a very poor reducing agent. However, water at
the earth, the organic nutrients necessary to sustain the an effective concentration of 55 M is readily available in
life of heterotrophs such as us are provided directly and thebiosphere.Althoughorganiccompoundsandinorganic
indirectly by photosynthesis. molecules such as hydrogen sulfide are more powerful re-
In both quantitative and qualitative terms photosynthe- ducing agents than water is, their use in photosynthesis as
sis is the most significant biological process on Earth. Ap- the source of electrons for photosynthesis is restricted to
11
proximately 2 × 10 tons of carbon dioxide are converted certain species of bacteria. The thermodynamically very
to organic compounds each year. It is to photosynthesis in unfavorable reduction of carbon dioxide by water is driven
prehistoric times that we owe the reserves of fossil fuels. by light.
The oxygen that we breathe is a direct result of photosyn-
thesis, now and in prehistory.
A. Light Reactions
If the earth were an isolated system in a thermodynamic
sense, life would be in jeopardy in that the energy reserves How the electromagnetic energy of light is converted to
for life would be consumed. Without the input of energy chemical energy in the form of reduced organic molecules
from a source external to the earth, the planet must tend is complex. Nonetheless, the first principles of energy con-
toward achieving equilibrium within its environment. servation and conversions in photosynthesis may be sim-
Fortunately, the earth is not an isolated system. The hy- ply depicted. All higher photosynthetic organisms contain
drogen fusion reactor of the Sun bathes our planet in elec- two forms of the green pigment chlorophyll. More than
tromagnetic radiation, including visible light. A fraction 99% of the chlorophyll in chloroplasts, the organelles in
of the solar energy that impinges on Earth is converted by which photosynthesis takes place, functions in a passive,
photosynthesis to chemical energy in the form of organic purely physical manner. Organized in specific pigment–
molecules that heterotrophic organisms use to satisfy their protein complexes within the photosynthetic membrane,
continued need for energy. The process by which light en- these chlorophylls absorb visible light and transfer excita-
ergy is used to drive the otherwise unfavorable synthesis tion energy to nearby chlorophylls with efficiencies very
of these organic molecules is called photosynthesis. close to 100%. In a real sense, more than 99% of the