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4.2 The first stage of respiration of glucose, occurring in cellular cytoplasm
There are two variations of the respiration of glucose: aerobic and anaerobic. Each of these types of respiration has only
one `stage' in common, and that is glycolysis. This is a metabolic pathway which is the first stage in cellular respiration,
occurring in both eukaryotic and prokaryotic cells. The chains of reactions which are known collectively as glycolysis take
place in the cytoplasm, not the mitochondrion. The entire process consists of ten unique reactions, but it can be
simplified to a simple three stage process.
ADP + Pi
glucose phosphate
rearrangement of hexose sugar glucose phosphate
using the enzyme isomerase gives fructose phosphate
fructose phosphate
ADP + Pi
fructose biphosphate
(3C) (3C)
triose phosphate triose phosphate
2ADP + 2Pi 2ADP + 2Pi
NAD (ox) NAD (ox)
NADH2 (red) NADH2 (red)
(3C) (3C)
pyruvate pyruvate
The first few steps are collectively known as phosphorylation. The process begins with glucose, a hexose sugar (or six-
carbon sugar). One molecule of ATP is hydrolysed, which releases one phosphate group, which attaches to the glucose
molecule. The molecule then becomes glucose phosphate (still a 6-carbon sugar). This molecule is then rearranged to give
fructose phosphate (which has the same chemical formula, but a slightly different biochemical structure). This happens
using the enzyme isomerase. Next, another molecule of ATP is hydrolysed, and the phosphate group released attaches to
the fructose at carbon-1 to become fructose biphosphate.
Splitting of fructose biphosphate
Note that, at this point, the process has used up two molecules of ATP, but none have been produced. Therefore, there is
an energy deficit by the end of the phosphorylation stage. However, by the end of the glycolysis process, there will be an
energy surplus (and further energy is released in the various other stages of respiration). The second stage of glycolysis
involves splitting the fructose biphosphate into two separate molecules. Two triose phosphate molecules are produced
(triose as they are a 3-carbon sugar, and they each get one phosphate group from the two accumulated so far).

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Forming the pyruvate
Next, two hydrogen atoms are removed from each triose phosphate using dehydrogenase enzymes, which are aided by
NAD (nicotinamide adenine dinucleotide), a hydrogen acceptor, so one molecule of NAD combines with the two hydrogen
atoms from each triose phosphate molecule, resulting in one reduced NAD molecule for each triose phosphate (reduced
NAD is NADH2). At this stage, two molecules of ADP are phosphorylated (one for each triose phosphate molecule), so two
molecules of ATP are produced.…read more


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