Figure 4.12: Most of the reactions of cellular respiration are carried out in the mitochondria.
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cycle is a series of oxidation steps that produce NADH and FADH 2 , another type of electron
carrier. These electron carriers will be used in the final step of cellular respiration. To begin
the Kreb’s Cycle, the 3-carbon pyruvate from glycolysis must be converted into a 2-carbon
molecule, which then can enter the cycle. During the cycle carbon dioxide is produced. Two
molecules of ATP are also produced per each initial glucose molecule. A graphic of the
mitochondria is shown inFigure4.12.
In the final steps of cellular respiration, theelectron transport chainaccepts the electrons
from glucose that are being carried by NADH and FADH 2. These electrons are passed along
the chain until they are finally combined with oxygen, which with the addition of hydrogen
ions, becomes water. That is the key reason why this process only occurs in the presence of
oxygen. Illustrated inFigure4.13.
As the electrons move down the electron transport chain, energy is released and later used
to synthesize ATP. The process of ATP synthesis is exactly the same as photosynthesis;
hydrogen ions are pumped across the cristae of the mitochondria, forming a chemiosmotic
gradient, and ATP synthase uses the energy of the movement of hydrogen ions back across
the membrane, from high to low concentration, to make ATP.
Because oxygen is the final electron acceptor in this process, the electron transport chain
can only occur in the presence of oxygen. This is known asaerobicrespiration. However,
there is not always enough oxygen present for aerobic respiration to occur. In this case, the
next step after glycolysis will be fermentation instead of the citric acid cycle.