MICROBIAL METABOLISM
Living cells are in a state of ceaseless activity. To maintain its “life,” each cell
depends on highly coordinated biochemical reactions. Carbohydrates are an
important source of the energy that drives these reactions. This chapter discusses
the energy-generating pathways of carbohydrate metabolism are discussed. During
glycolysis, an ancient pathway found in almost all organisms, a small amount of
energy is captured as a glucose molecule is converted to two molecules of pyruvate.
Glycogen, a storage form of glucose in vertebrates, is synthesized by glycogenesis
when glucose levels are high and degraded by glycogenolysis when glucose is
in short supply. Glucose can also be synthesized from noncarbohydrate precursors
by reactions referred to as gluconeogenesis. The pentose phosphate pathway
enables cells to convert glucose-6-phosphate, a derivative of glucose, to ribose-
5-phosphate (the sugar used to synthesize nucleotides and nucleic acids) and other
types of monosaccharides. NADPH, an important cellular reducing agent, is
also produced by this pathway. In the glyoxylate cycle, used by some
organisms (primarily plants) to manufacture carbohydrate from fatty acids, is
considered. Photosynthesis, a process in which light energy is captured to driveLiving cells are in a state of ceaseless activity. To maintain its “life,” each cell
depends on highly coordinated biochemical reactions. Carbohydrates are an
important source of the energy that drives these reactions. This chapter discusses
the energy-generating pathways of carbohydrate metabolism are discussed. During
glycolysis, an ancient pathway found in almost all organisms, a small amount of
energy is captured as a glucose molecule is converted to two molecules of pyruvate.
Glycogen, a storage form of glucose in vertebrates, is synthesized by glycogenesis
when glucose levels are high and degraded by glycogenolysis when glucose is
in short supply. Glucose can also be synthesized from noncarbohydrate precursors
by reactions referred to as gluconeogenesis. The pentose phosphate pathway
enables cells to convert glucose-6-phosphate, a derivative of glucose, to ribose-
5-phosphate (the sugar used to synthesize nucleotides and nucleic acids) and other
types of monosaccharides. NADPH, an important cellular reducing agent, is
also produced by this pathway. In Chapter 9, the glyoxylate cycle, used by some
organisms (primarily plants) to manufacture carbohydrate from fatty acids, is
considered. Photosynthesis, a process in which light energy is captured to drive carbohydrate synthesis.
GLYCOLYSIS : Glycolysis, occurs, at least in part, in almost every living cell. This series of
reactions is believed to be among the oldest of all the biochemical pathways. Both
the enzymes and the number and mechanisms of the steps in the pathway are
highly conserved in prokaryotes and eukaryotes. Also, glycolysis is an anaerobic
process, which would have been necessary in the oxygen-poor atmosphere
of pre-eukaryotic Earth.
In glycolysis, also referred to as the Embden-Meyerhof-Parnas pathway, each
glucose molecule is split and converted to two three-carbon units (pyruvate).
During this process several carbon atoms are oxidized. The small amount of energy
captured during glycolytic reactions (about 5% of the total available) is stored
temporarily in two molecules each of ATP and NADH (the reduced form of the
coenzyme NAD+). The subsequent metabolic fate of pyruvate depends on the
organism being considered and its metabolic circumstances. In anaerobic organisms (those that do not use oxygen to generate energy), pyruvate may be converted
to waste products such as ethanol, lactic acid, acetic acid, and similar molecules.
Using oxygen as a terminal electron acceptor, aerobic organisms such as animals
Fig: Steps Of Glycolysis
1. Glucose is phosphorylated twice and cleaved to form two molecules of
glyceraldehyde-3-phosphate (G-3-P). The two ATP molecules consumed
during this stage are like an investment, because this stage creates the actual
substrates for oxidation in a form that is trapped inside the cell.
2. Glyceraldehyde-3-phosphate is converted to pyruvate. Four ATP and two
NADH molecules are produced. Because two ATP were consumed in stage
1, the net production of ATP per glucose molecule is 2.
The glycolytic pathway can be summed up in the following equation:
D-Glucose 2 ADP 2 Pi 2 NAD → 2 pyruvate 2 ATP
2 NADH 2H 2H2O.
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