Life is driven by energy. All the activities that organisms
carry out - the swimming of bacteria, the purring of a cat, our reading
these words - use energy. Even though the ways that organisms use energy
and many are varied, all of life's energy ultimately has the same beginning:
the sun. Plants, algae, and some bacteria harvest the energy of sunlight
by the process of photosynthesis, thus converting radiant energy to chemical
energy. These organisms, along with a few others that use chemical energy
in a similar way, are called autotrophs (self-feeders). All organisms live
on the energy produced by these autotrophs. Those that do not have the
ability to produce their own food are called heterotrophs (fed by others).
At least 95 % of the kinds of organisms on earth - - all animals, all fungi,
and most protists and bacteria - are heterotrophs; most of them live by
feeding on the chemical energy fixed by photosynthesis. All of us - plants
and bacteria, you and I - share the same ultimate dependency on the sun.
We are all children of light.
Using Chemical Energy to Drive Metabolism
Many of the reactions occur in sequences called biochemical
pathways. In biochemical pathways, exergonic reactions- those that involve
a release of free energy - occur without the net input of energy; endergonic
reactions - those that require the addition of energy - do not. Chemical
energy powers metabolism by driving endergonic reactions through the use
of ATP. Chemical energy is used to create ATP, and the splitting of ATP
is completed due to endergonic reactions, providing the necessary energy.
How Cells Make ATP
1. Substrate-level phosphorylation. Because the formation of
ATP from ADP + inorganic phosphate (Pi) requires an input of free energy,
ATP formation is endergonic - it does not occur spontaneously. When coupled
to an exergonic reaction that has a strong tendency to occur, however,
the synthesis of ATP from ADP + Pi does take place. The reaction occurs
because other release of energy from the exergonic reaction is greater
than the input of energy required to drive the synthesis of ATP. The generation
of ATP by coupling strongly exergonic reactions with the synthesis of ATP
from ADP and Pi is called substrate level phosphorylation. Many bacteria
subsist entirely on ATP generated in this way. 2. Chemiosmotic generation
of ATP. Almost all organisms posses transmembrane channels that function
in pumping protons out of cells. Proton-pumping channels use a flow of
exited electrons to induce a change in the shape of a transmembrane protein,
which in turn causes protons to pass outward. As the proton concentration
outside the membrane rises higher than that inside, the outer protons are
driven inward by diffusion, passing backward through special proton channels
that use their passage induce the formation of ATP from ADP + Pi. Because
the chemical formation of ATP is driven by a diffusion force similar to
osmosis, this process if referred to as a chemiosmotic one. The harvesting
of chemical energy can be considered to take place in one or more of two
stages: 1. A reshuffling of chemical bonds to couple ATP formation to highly
exergonic reaction, called substrate-level phosphorylation. 2. The transport
of high-energy electrons to a membrane where they drive a proton pump and
so power the chemiosmotic synthesis of ATP. 1. In photosynthetic organisms,
light energy boosts electrons to higher energy levels and these electrons
are channeled to proton pumps. 2. In all organisms photosynthetic and non-photosynthetic
alike, high-energy electrons are extracted from chemical bonds and carried
by coenzymes to proton pumps. This electron-harvesting process is given
a special name: cellular respiration.
An Overview of Cellular Respiration
Cellular respiration of glucose is carried out in three stages
in most organisms. The first stage is a biochemical pathway called glycolysis.
C6H12O6 + 6H2O+6O2 = 6CO2+12H2O +Energy In the presence of oxygen however,
two other pathways of cellular respiration also occur to your body, ones
that extract far more ATP. The second pathway is the oxidation of pyruvate.
The third pathway is called the citric acid cycle, after the six-carbon
citric acid molecule formed in its first step.
