Glycolysis is a nearly universal process in which the six carbon
sugar glucose is anaerobically converted, through a series of enzymatically
catalyzed steps in the cytosol, the fluid portion of the cytoplasm,
into two molecules of the three carbon compound pyruvate. Two molecules
of ATP are expended early on in glycolysis, but four more are generated
later by substrate-level phosphorylation. Thus, there is a net production
of two ATP molecules per molecule of glucose. In addition, two
molecules of nicotinamide adenine dinucleotide (NAD) become reduced
by gaining two electrons.
Either fermentation or respiration may follow glycolysis (Figure
1-3). Fermentation is an oxygen-independent process, occuring in the
cytosol, which uses organic molecules as terminal electron acceptors.
Fermentation regenerates the supply of NAD+
for glycolysis and results
in the consumption of pyruvate and the release of molecules such as CO2
(gases); lactic, formic, acetic, succinic, butyric, or propionic acids;
and ethanol, butanol, or propanol (alcohols). The final product depends
on the species. No additional ATP is generated during fermentation.
Respiration involves the oxidation of molecules, the generation of
high-energy molecules, such as ATP, by passing pairs of electrons (and
hydrogen ions, or protons) through an electron transport system, and the
donation of these electrons to an inorganic electron acceptor. If the terminal
electron acceptor is oxygen, this process is termed aerobic respiration.
Anerobic respiration occurs when the terminal electron acceptor
is an inorganic molecule other than molecular oxygen (such as sulfate
or nitrate). Organisms vary in their oxygen requirements; some are strict
anaerobes and cannot survive in the presence of oxygen. Facultative
anaerobes can respire aerobically or anaerobically, and obligate aerobes
require oxygen for survival.
Pyruvate generated from glycolysis in the cytosol may enter the mitochondria
and, if oxygen is available, be enzymatically converted to
acetyl coenzyme α(acetyl CoA) and CO2
. Within the matrix of the mitochondria
or the cytosol of aerobic prokaryotes, the two-carbon acetyl CoA enters a circular set of enzymatic reactions known as the Krebs cycle, the
tricarboxylic acid cycle (TCA), or the citric acid cycle (Figure 1-3).
|Figure 1-3 Chemoheterotrophic metabolism.
During oxidation of a substrate, two major electron carriers, NAD+
and FAD, become reduced to NADH and FADH2
. One complete turn of
the TCAproduces three molecules of NADH, two molecules of CO2
molecule of FADH2
, and one molecule of guanosine triphosphate
(GTP). The electrons and H+
ions from NADH and FADH2
to the electron transport chains within the cristae of the mitochondria
or the plasma membrane of prokaryotes. These chains consist of series
of proteins that first serve as electron acceptors, then donors to the
next complex in the chain. This series of coupled oxidations and reductions
results in the terminal tranfer of electrons and H+
s to oxygen, forming
water as the end product.
The complete oxidation of glucose:
ATP can be generated by three different mechanisms. It can be
formed from adenosine diphosphate (ADP) by either substrate-level
phosphorylation or oxidative phosphorylation. In substrate-level phosphorylation,
an enzyme mediates the transfer of a phosphate group from
a phosphorylated organic molecule to ADP. Oxidative phosphorylation
occurs when molecules are oxidized and energy is extracted from the
electrons by passing them through an electron transport system, where
most of the resulting free enrgy is used to drive the phosphorylation of
ADP, producing ATP. Photophosphorylation also synthesizes ATP, but
uses the energy from sunlight rather than from the breakdown of organic