Aerobic respiration occurs within cells.
It starts with glycolysis in the cytosol of the cell.
It then goes through the link reaction moving into the matrix of the mitochondria.
It then goes through the citric acid cycle within the mitochondria matrix.
Then it goes through the electron transport chain and chemiosmosis in the inner mitochondrial
membrane producing energy in the form of ATP.
In this video we are going to look at the third process involved in Aerobic Respiration,
The Citric Acid Cycle, also known as the Tricarboxylic Acid Cycle (or TCA Cycle), it is also known
as the Kreb’s Cycle.
At the end of the Glycolysis and the Link Reaction we were left with 2 molecules of
Acetyl-CoA within the mitochondria matrix that can now enter the citric acid cycle.
As we move through the Citric Acid Cycle we will take note of how many, NADH, FADH2 and
ATP are used or produced.
There are 8 steps in the Citric Acid Cycle.
The first step involves the joining of one of the two carbon Acetyl-CoA molecules with
the four carbon molecule Oxaloacetate forming the 6 carbon molecule Citrate using the enzyme
Citrate Synthase and a water (H2O) molecule.
Coenzyme A is also released.
The second step involves the dehydration and then rehydration of Citrate into Isocitrate
using the enzyme Aconitase, where a water molecule is released, and then a water molecule
is added back on.
The third step involves the decarboxylation and oxidation of Isocitrate a six carbon molecule
into alpha-ketoglutarate a five carbon molecule using the enzyme Isocitrate dehydrogenase.
This step involves a carbon dioxide (CO2) molecule being removed.
NAD+ is also reduced into NADH + H+.
This gives us one NADH, giving us a net total so far of 1 NADH for one Acetyl-CoA molecule.
The fourth step involves the conversion of alpha-ketoglutarate a 5 carbon molecule into
Succinyl-CoA a four carbon molecule using the enzyme alpha-ketoglutarate dehydrogenase.
This step involves the removal of a CO2 molecule.
A Coenzyme A (CoA) group is added and NAD+ is reduced into NADH and H+.
This step creates another NADH, taking our net total to 2 NADH for one Acetyl-CoA molecule.
The fifth step involved the conversion to Succinyl-CoA into Succinate using the enzyme
Succinyl-CoA synthetase.
In this step the Coenzyme A (CoA) is removed.
It also involves substrate level phosphorylation of GDP and an inorganic phosphate into GTP.
This GTP can then give a phosphate group to ADP to form ATP.
This step produces an ATP, taking our net total so far to 2 NADH and 1 ATP for one Acetyl-CoA
molecule The sixth step involves the oxidation of Succinate
into Fumarate using the enzyme Succinate dehydrogenase.
This step involves the reduction of FAD into FADH2.
This step produces an FADH2, taking our net total so far for one Acetyl-CoA molecule to
2NADH, 1 FADH2 and 1 ATP.
The seventh step involves the hydration of Fumarate into Malate using the enzyme Fumarase.
This step involves the addition of a water (H2O) molecule.
The eighth and final step involves the oxidation of Malate into Oxaloacetate using the enzyme
Malate Dehydrogenase.
This step reduces NAD+ into NADH and H+.
This step produces an NADH, taking our net total for one Acetyl-CoA molecule to 3NADH,
1 FADH2 and 1 ATP.
So far we have had one Acetyl-CoA go through the Citric Acid Cycle, but we have 2 Acetyl-CoA
molecules per glucose molecule, so this cycle would go through twice.
This doubles our net total for the citric acid cycle, giving us a final net total of
6NADH, 2 FADH2, and 2ATP.
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