Glycolysis is the formation of pyruvate from glucose.
It takes place in the cytoplasm of the cell.
There is a net gain of 2 ATP from glycolysis.
To break down glucose into Triose phosphate, 2 molecules of ATP must be converted into 2ADP + 2Pi for energy.
The 2 Triose phosphate molecules are then broken down into 2 Pyruvate molecules.
When the Triose phosphate molecules are being converted into pyruvate, 1 molecule of NAD is reduced and 4 molecules of ATP are formed.
Glucose (6C) --> 2x Triose phosphate (3C) --> 2x Pyruvae (3C)
This is the the furthest respiration can go without oxygen.
The link reaction is the production of acetyl coenzyme A and carbon dioxide from coenzyme A and pyruvate:
Pyruvate + Coenzyme A --> Acetyl coenzyme A + CO2
This takes place in the matrix of the mitochondria of the cell.
When the link reaction takes place 1 molecule of NAD is reduced.
Kreb's cycle occurs in the matrix of the mitochondria of the cell.
Acetyl coenzyme A (2C) joins together with oxaloacetate (4C) to form citrate (6C).
The citrate molecule then loses 2 of its carbon atoms in turn, turning back into oxaloacetate.
Oxygen then joins with the 2 carbon molecules that are lost, forming 2 carbon dioxide molecules.
1 molecule of FAD and 3 molecules of NAD are reduced when citrate is turned into oxaloacetate.
A molecule of ATP is also formed.
The whole cycle then repeats with more acetyl coenzyme A being added.
Oxidative Phosphorylation takes place in the inter-membrane space of the mitochondria.
All of the reduced NAD's and FAD's from glycolysis, the link reaction and Kreb's cycle become oxidised (drop off their hydrogens).
The hydrogens then split up into a proton and an electron:
H --> H+ & e-
The electrons then pass through the electron transport chain and give off energy as they do so.
This energy is then used to actively transport the H+ ions into the inter-membrane space of the mitochondria, causing a high diffusion gradient.
The H+ ions then move back into the matrix by facilitated diffusion through the ATPase channel.
This channel uses the kinetic energy of the H+ ions moving to join together ADP + Pi molecules to create ATP.
The H+ and e- ions then re-join and bond to oxygen to make water.
Anaerobic Respiration (animals)
If no oxygen is available then glycolysis is the furthest that respiration can go.
The pyruvate bonds with the reduced NAD from glycolysis to form lactate and regenerates the NAD:
Pyruvate + NADH --> Lactate + NAD
This happens in the cytoplasm of the cell.
If lactate is allowed to build up in the muscle tissue it will cause cramp and muscle fatigue.
So it is taken in the bloodstream to the liver to be converted to glycogen.
Anaerobic Respiration (Plants)
Like in animals, If no oxygen is available then glycolysis is the furthest that respiration can go.
The pyruvate and reduced NAD make regenerated NAD, carbon dioxide and ethanol as opposed to lactate.
Pyruvate + NADH --> NADH + CO2 + Ethanol