First 427 words of the document:
There are three main stages to cellular respiration where oxygen is present. These are Glycolysis,
Kreb's cycle and the Electron Transport Chain. Glycolysis and the Kreb's cycle are linked by a stage
called the link reaction; however this is not a major stage as no ATP is produced.
Glycolysis is the first stage of cellular respiration; it is the only stage of aerobic respiration which
does not require oxygen. It takes place in the cytosol of a cell. During glycolysis glucose is converted
into pyruvate, a 3-carbon molecule. To do this the glucose is first phosphorylated to make it more
reactive and impermeable, stopping it from leaving the cell. For this two phosphate groups are
donated from ATP molecules. Next the 6-carbon sugar phosphate is split through hydrolysis to give
two glycerate-3-phosphate molecules. These molecules are the oxidised by removing hydrogen
ions. The hydrogen ions are accepted by the electron carrier, NAD. Each of the 3-carbon sugar
phosphates loses their two phosphate groups to ADP molecules, forming 4 ATP molecules. This gives
a net gain of 2 ATP molecules and 2 NADH molecules.
In the link reaction the pyruvate is decarboxylated and picked up by Coenzyme A to form Acetyl CoA.
The Kreb's cycle takes place in the mitochondrial matrix and occurs twice per a molecule of glucose.
The Acetyl CoA from the link reaction is combines with oxaloacetate to form citrate. During 2 turns of
the Kreb's cycle 6 molecules of NAD are reduced to NADH, 2 molecules of FAD are reduced to
FADH2, 4 molecules of CO2 are formed and 2 molecules of ATP are formed by substrate level
The final stage of aerobic respiration is the Electron Transport Chain. This is where most ATP is
formed from the reoxidation of the electron carriers, NAD and FAD. The Electron Transport Chain
takes place in the inner mitochondrial matrix. The hydrogen from the NADH splits into H+, which is
pumped out into the intermembranal space and an electron, which is passed along a series of protein
carrier molecules. The protons are used to make ATP from ADP and free phosphate groups using the
enzyme ATP synthase. Oxygen is the final electron acceptor in the transport chain and is reduced to
form H2O. In the Electron Transport Chain 30 ATP molecules are made from the reoxidation of NADH,
and 4 molecules of ATP are made from the reoxidation of FADH2.