CELLS and ENERGY
Supplying the cell with energy
To carry out their
day to day functions, cells require energy. The
ultimate source of all this energy is the sun.
Some organisms can trap energy directly from the sun, storing it away in the bonds
of organic molecules such as the simple sugar, glucose.
This process is called photosynthesis,
and organisms which are capable of photosynthesis are called autotrophs. Organisms which are not capable of
photosynthesis are called heterotrophs,
and must acquire their energy-containing organic molecules through their diet instead. To convert the energy stored in organic molecules
into a form that is usable, both autotrophs and heterotrophs must take large
molecules and break them down, and then recapture the energy released in the process and
store it the bonds of smaller, easier to use molecules.
This process is known as respiration.
Both photosynthesis
and respiration are really nothing more than series of chemical reactions resulting in the
transfer of energy (remember, every molecule contains potential energy) between reactants
and products. Remember those energetically
favorable reactions where the energy contained in the reactant molecules is greater
than the energy contained in the product molecules? Those
reactions, which as you remember will result in the release of excess energy, are given
the term exergonic. On the other hand, the reactions where the energy
contained in the reactant molecules is less than that in the product molecules
require the addition of extra energy in order to proceed, and are called endergonic. Of
course, since chemical reactions are generally reversible, a reaction that is
exergonic in one direction must be endergonic in the reverse direction.
During many of the
reactions of photosynthesis and respiration, the excess energy produced by an
exergonic reaction is used to "drive" an endergonic reaction. Through this coupling of the two reactions, the excess
energy produced by the exergonic reaction is not lost, but instead is stored away in the
products of the endergonic reaction. Later,
the reversal of the endergonic reaction (which will of course be exergonic) can be used to
re-release the stored energy, either to drive yet another reaction or for direct
use by the cell.
Oxidation-reduction and
phosphorylation reactions
There is one special
group of reactions that are particularly important in this transferring of energy within
the cell. Oxidation-reduction reactions are pairs of
reactions where one or more electrons are taken from one molecule (a process called
oxidation) and donated to another molecule (a process called reduction). Often, one or more H+ ions (which are
usually readily available inside the cell) are carried along with the electrons.
Generally, the oxidation part of an oxidation-reduction reaction is exergonic, and
supplies the energy to drive the endergonic reduction.
Oxidation-reduction reactions in the cell often involve the special
electron-carrying molecules NADP+, NAD+,
and FAD. Each of these
complex molecules can be reduced, accepting a pair of electrons along with a H+
ion to form the molecules NADPH, NADH, and FADH2. Phosphorylation
reactions are another important type of cellular reaction. In these reactions, a phosphate group (PO4)
is transferred from one molecule to another. These
reactions often involve the molecules ADP
(adenosine diphosphate) or ATP (adenosine triphosphate). Transfer of a phosphate group from another
molecule to ADP is generally endergonic, and results in the formation of ATP. Transfer of a phosphate group from ATP to
another molecule is generally exergonic, and results in the formation of ADP.
All chemical
reactions, even exergonic ones, need a little help in getting started, because in order to
react, the molecules involved first need to come into contact with each other. Although molecules can acquire enough kinetic
energy through their random motions to eventually collide and react, the process is slow
and impossible to regulate. Catalysts can speed up and control
reactions by helping to bring the reactants together.
Catalysts cannot, however, force reactions to happen that would not happen anyway,
or alter the equilibrium balance between reactants and products (they can participate in
the reverse reaction conversion of products to reactants as well as the conversion of
reactants to products). Catalysts participate
in reactions, but they themselves remain unchanged. Within
the cell, the role of catalyst is fulfilled by a group of proteins called enzymes.
Alberts,
Bruce, Bray, Dennis, Lewis, Julian, Raff, Martin, Roberts, Keith, and Watson, James D. 1994.
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Lodish, Harvey, Berk, Arnold, Zipursky, S. Lawrence,
Matsudaira, Paul, Baltimore, David, and Darnell, James. 2000. Molecular
Cell Biology. W.H. Freeman and Company, New York.