Page 133 - Demo 1

AEROBIC RESPIRATION

If oxygen is present following glycolysis, the cell will enter the aerobic phase
of cellular respiraon that takes place inside the mitochondrion. Aerobic
respiraon takes place in the presence of oxygen. It occurs in all plants, animals,
and some prokaryoc organisms. When a glucose molecule is broken down in
the presence of oxygen, energy is released, along with carbon dioxide and
water as the by-products of the reacon.

1. Preparatory Reaction
Each pyruvate from glycolysis goes into the mitochondrial matrix—the
innermost compartment of mitochondria. In the matrix, pyruvate is
oxidized, and a CO2 molecule is released. NAD accepts electrons and
+
hydrogen ions, forming NADH. The product of this reacon is a two-carbon
molecule bound to Coenzyme A, known as acetyl-CoA. Therefore, per
glucose molecule, the outputs are two CO2, two NADH, and two acetyl-
CoA.

2. The Krebs Cycle

The Krebs cycle, also called citric acid cycle, is a cyclical metabolic
pathway that takes place in the matrix of the mitochondria. It was so
named to honor Sir Hans Adolf Krebs, the biochemist who first studied
it.
Each acetyl CoA made in the last step combines with a four-carbon
molecule, oxaloacetate, to produce citric acid, a six-carbon molecule.
Twice, oxidaon reacons produce NADH, and CO2 is released. The loss of
two CO2 molecules results in a new four-carbon molecule. The remaining
four-carbon molecule undergoes a series of addional reacons -- first
making an ATP molecule, then reducing the electron carrier FAD to FADH2,
and finally generang another NADH. This set of reacons regenerates the
starng molecule, oxaloacetate, so the cycle can repeat.
Overall, one turn of the citric acid cycle releases two carbon dioxide
molecules and produces three NADH, one FADH2, and one ATP. The citric
acid cycle goes around twice for each molecule of glucose that enters
cellular respiraon because there are two pyruvates. Thus, two acetyl-CoA
molecules are produced per glucose.

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