Respiration in Humans
The stages of respiration
Glycolysis is the first stage of respiration, and it is a series of reactions that extract energy
from glucose by breaking it into two three-carbon molecules called pyruvate. It takes place
in the cytoplasm and is common to anaerobic and aerobic respiration. Glycolysis is the
metabolic process through which glucose produces cellular energy. Glucose is important for
the proper functioning of every organ system. Glycolysis provides instant energy and
contributes to the production of more ATP in later stages of respiration. Aerobic respiration
produces more efficient energy than anaerobic respiration but is a slower process.
Anaerobic respiration does not require energy and release less of it but its quicker.
In the first phase of glycolysis, glucose is broken down to 2 molecules of glyceraldehyde-3-
phosphate in a series of 5 reactions. In the second stage, another series of 5 reactions
convert the two molecules of glyceraldehyde-3-phosphate into two molecules of pyruvate.
In the third stage, a phosphate group is transferred to fructose-6-phosphate producing
fructose-1,6-biphosphate. This step is catalysed by phosphofructokinase, which can be
controlled to speed up or slow down glycolysis pathway. In the fourth stage, fructose-1,6-
biphosphate splits to make 2 three-carbon sugars: dihydroxyacetone phosphate and
glyceraldehyde-3-phosphate. Although they are isomers of each other, only one can
continue through the next stage of glycolysis. In the fifth stage, an isomerase transforms the
dihydroxyacetone phosphate into its isomer, glyceraldehyde-3-phosphate. The 6-carbon
glucose has now been converted into two phosphorylated three-carbon molecules of G3P.
The sixth stage oxidises glyceraldehyde-3-phosphate, extracting high-energy electrons which
are picked up by electron carrier NAD+, making NADH. In the final stage, the sugar is
, phosphorylated by the addition of a second phosphate group, producing 1,3-
biphosphoglycerate.
Link reaction
It takes place in the mitochondrial matrix and is the second stage of respiration. During the
reaction, a carbon atom is removed from pyruvate, forming carbon dioxide. Pyruvate is then
converted into acetate by the removal of carbon dioxide (decarboxylated). Two hydrogen
atoms are also removed (dehydrogenated), and the acetate then joins with co-enzyme A to
form acetyl-CoA. The link reaction acts as a bridge between glycolysis and the Krebs cycle.
Link reaction is essential for transitioning the molecules from cytoplasmic glycolysis to the
mitochondrial Krebs cycle marking a crucial shift in respiration. In the link reaction, the
energy released prepares pyruvate, a product of glycolysis, for the Krebs cycle. The energy
released is not directly captured by ATP.
Krebs` cycle
It takes place in the mitochondrial matrix and is the third of respiration. It is used for
oxidising nutrients to CO2 and to produce energy. Acetyl-CoA enters the cycle, combines
with oxaloacetate, and make a six-carbon compound. Carbon dioxide than gets lost and
NAD+ is reduced to NADH, and it turns into a five-carbon compound. Another CO2 molecule
is lost, and NAD+ and ADP is converted into ATP and turns into a four-carbon compound.
Hydrogen is lost and is passed to FAD to make FADH2. We lose more reduction of NAD+ to
make NADH. Its regenerates so the cycle can continue.
The stages of respiration
Glycolysis is the first stage of respiration, and it is a series of reactions that extract energy
from glucose by breaking it into two three-carbon molecules called pyruvate. It takes place
in the cytoplasm and is common to anaerobic and aerobic respiration. Glycolysis is the
metabolic process through which glucose produces cellular energy. Glucose is important for
the proper functioning of every organ system. Glycolysis provides instant energy and
contributes to the production of more ATP in later stages of respiration. Aerobic respiration
produces more efficient energy than anaerobic respiration but is a slower process.
Anaerobic respiration does not require energy and release less of it but its quicker.
In the first phase of glycolysis, glucose is broken down to 2 molecules of glyceraldehyde-3-
phosphate in a series of 5 reactions. In the second stage, another series of 5 reactions
convert the two molecules of glyceraldehyde-3-phosphate into two molecules of pyruvate.
In the third stage, a phosphate group is transferred to fructose-6-phosphate producing
fructose-1,6-biphosphate. This step is catalysed by phosphofructokinase, which can be
controlled to speed up or slow down glycolysis pathway. In the fourth stage, fructose-1,6-
biphosphate splits to make 2 three-carbon sugars: dihydroxyacetone phosphate and
glyceraldehyde-3-phosphate. Although they are isomers of each other, only one can
continue through the next stage of glycolysis. In the fifth stage, an isomerase transforms the
dihydroxyacetone phosphate into its isomer, glyceraldehyde-3-phosphate. The 6-carbon
glucose has now been converted into two phosphorylated three-carbon molecules of G3P.
The sixth stage oxidises glyceraldehyde-3-phosphate, extracting high-energy electrons which
are picked up by electron carrier NAD+, making NADH. In the final stage, the sugar is
, phosphorylated by the addition of a second phosphate group, producing 1,3-
biphosphoglycerate.
Link reaction
It takes place in the mitochondrial matrix and is the second stage of respiration. During the
reaction, a carbon atom is removed from pyruvate, forming carbon dioxide. Pyruvate is then
converted into acetate by the removal of carbon dioxide (decarboxylated). Two hydrogen
atoms are also removed (dehydrogenated), and the acetate then joins with co-enzyme A to
form acetyl-CoA. The link reaction acts as a bridge between glycolysis and the Krebs cycle.
Link reaction is essential for transitioning the molecules from cytoplasmic glycolysis to the
mitochondrial Krebs cycle marking a crucial shift in respiration. In the link reaction, the
energy released prepares pyruvate, a product of glycolysis, for the Krebs cycle. The energy
released is not directly captured by ATP.
Krebs` cycle
It takes place in the mitochondrial matrix and is the third of respiration. It is used for
oxidising nutrients to CO2 and to produce energy. Acetyl-CoA enters the cycle, combines
with oxaloacetate, and make a six-carbon compound. Carbon dioxide than gets lost and
NAD+ is reduced to NADH, and it turns into a five-carbon compound. Another CO2 molecule
is lost, and NAD+ and ADP is converted into ATP and turns into a four-carbon compound.
Hydrogen is lost and is passed to FAD to make FADH2. We lose more reduction of NAD+ to
make NADH. Its regenerates so the cycle can continue.
