Respiration WORLD OF MICROBIOLOGY AND IMMUNOLOGY484•
Several male factors can influence the ability of suc-
cessful fertilization, including the presence of male anti-sperm
antibodies (IgG and IgM) that bind to the surface of the sper-
matozoa and may mask receptors or other functionally impor-
tant proteins, thus interfering with the sperm-egg interaction,
and reducing the probability for successful fertilization. Male
anti-sperm antibody production is more likely to occur after
vasectomy, or with undescended testicles, or epididymitis.See also Autoimmunity and autoimmune diseases;
Immunochemistry; Immunologic therapies; Immunological
analysis techniquesRRespirationESPIRATION
Respiration is the physiological process that produces high-
energy molecules such as adenosine triphosphate (ATP). The
high-energy compounds become the fuel for the various man-
ufacturing and growth processes of the cell. Respiration
involves the transfer of electrons in a chemically linked series
of reactions. The final electron acceptor in the respiration
process is oxygen.
Respiration occurs in all types of organisms, including
bacteria, protists, fungi, plants, and animals. In eukaryotes,
respiration is often separated into three separate components.
The first is known as external respiration, and is the exchange
of oxygen and carbon dioxide between the environment and
the organism (i.e., breathing). The second component of respi-
ration is internal respiration. This is the exchange of oxygen
and carbon dioxide between the internal body fluids, such as
blood, and individual cells. Thirdly, there is cellular respira-
tion, which is the biochemical oxidation of glucose and con-
sequent synthesis of ATP.
Cellular respiration in prokaryotes and eukaryotes is
similar. Cellular respiration is an intracellular process in
which glucose is oxidized and the energy is used to make the
high-energy ATP compound. ATP in turn drives energy-
requiring processes such as biosynthesis, transport, growth,
and movement.
In prokaryotes and eukaryotes, cellular respiration
occurs in three sequential series of reactions; glycolysis, the
citric acid cycle, and the electron transport chain. In prokary-
otes such as bacteria, respiration involves components that are
located in the cytoplasmof the cell as well as being mem-
brane-bound.
Glycolysis is the controlled breakdown of sugar (pre-
dominantly, glucose, a 6-carbon carbohydrate) into pyruvate,
a 3-carbon carbohydrate. Organisms frequently store complex
carbohydrates, such as glycogen or starch, and break these
down into glucose that can then enter into glycolysis. The
process involves the controlled breakdown of the 6-carbon
glucose into two molecules of the 3-carbon pyruvate. At least
10 enzymesare involved in glucose degradation. The oxida-
tion of glucose is controlled so that the energy in this molecule
can be used to manufacture other high-energy compounds.
Each round of glycolysis generates only a small amount of
ATP, in a process known as substrate-level phosphorylation.For each glucose molecule that is broken down by glycolysis,
there is a net gain of two molecules of ATP. Glycolysis pro-
duces reduced nicotinamide adenine dinucleotide (NADH), a
high-energy molecule that can subsequently used to make ATP
in the electron transfer chain. For each glucose molecule that
is broken down by glycolysis, there is a net gain of two mole-
cules of NADH. Finally, glycolysis produces compounds that
can be used to manufacture compounds that are called fatty
acids. Fatty acids are the major constituents of lipids, and are
important energy storage molecules.
Each pyruvate molecule is oxidized to form carbon diox-
ide (a 1-carbon molecule) and acetyl CoA (a two carbon mole-
cule). Cells can also make acetyl CoA from fats and amino
acids. Indeed, this is how cells often derive energy, in the form
of ATP, from molecules other than glucose or complex carbo-
hydrates. Acetyl CoA enters into a series of nine sequential
enzyme-catalyzed reactions, known as the citric acid cycle.
These reactions are so named because the first reaction makes
one molecule of citric acid (a 6-carbon molecule) from one
molecule of acetyl CoA (a 2-carbon molecule) and one mole-
cule of oxaloacetic acid (a 4-carbon molecule). A complete
round of the citric acid cycle expels two molecules of carbon
dioxide and regenerates one molecule of oxaloacetic acid.
The citric acid cycle produces two high-energy com-
pounds, NADH and reduced flavin adenine dinucleotide
(FADH 2 ), that are used to make ATP in the electron transfer
chain. One glucose molecule produces 6 molecules of NADH
and 2 molecules of FADH 2. The citric acid cycle also produces
guanosine triphosphate (GTP; a high-energy molecule that can
be easily used by cells to make ATP) by a process known as sub-
strate-level phosphorylation. Finally, some of the intermediates
of the citric acid cycle reactions are used to make other impor-
tant compounds, in particular amino acids (the building blocks
of proteins), and nucleotides (the building blocks of DNA).
The electron transfer chain is the final series of bio-
chemical reactions in respiration. The series of organic elec-
tron carriers are localized inside the mitochondrial membrane
of eukaryotes and the single membrane of Gram-positive bac-
teria or the inner membrane of Gram-negative bacteria.
Cytochromes are among the most important of these electron
carriers. Like hemoglobin, cytochromes are colored proteins,
which contain iron in a nitrogen-containing heme group. The
final electron acceptor of the electron transfer chain is oxygen,
which produces water as a final product of cellular respiration.
The main function of the electron transfer chain is the
synthesis of 32 molecules of ATP from the controlled oxida-
tion of the eight molecules of NADH and two molecules of
FADH 2 , made by the oxidation of one molecule of glucose in
glycolysis and the citric acid cycle. The electron transfer chain
slowly extracts the energy from NADH and FADH 2 by pass-
ing electrons from these high-energy molecules from one elec-
tron carrier to another, as if along a chain. As this occurs,
protons (H+) are pumped across the membrane, creating a pro-
ton gradient that is subsequently used to make ATP by a
process known as chemiosmosis.
Respiration is often referred to as aerobic respiration,
because the electron transfer chain utilizes oxygen as the final
electron acceptor. When oxygen is absent or in short supply,womi_R 5/7/03 8:17 AM Page 484