Aerobic Respiration

Aerobic Respiration is a series of reactions in which organic molecules, the fuel, are broken down to release chemical potential energy, which is used to synthesize ATP. The main fuel consists of the carbohydrate Glucose, but fatty acids, glycerol or amino acids may be used too.

The Breakdown of Glucose is done in four stages.

1. Glycolysis – Takes place in the cytoplasm. The glycolytic pathway is as following:

At first, glucose is phosphorylated (loaded with more energy) using energy. Two molecules of ATP are used to make Hexose Biphosphate. This molecule will be then broken into two molecules of Triose Phosphate.

Both of this molecules are further dehydrogenated, and the two Hydrogen atoms are transferred to NAD.

NAD (Nicotiamide Adenine Dinucleotide) is made of two linked nucleotides which both contain ribose. One of the nucleotides contains the nicoamide ring, which can accept a Hydrogen ion and an electron, thereby becoming reduced (reduced NAD).


The Hydrogen ions can easily be transferred to other molecules. NAD is used in the fourth stage, Oxidative Phosphorylation.

Two molecules of reduced NAD are produced for each molecule entering glycolysis. The end-product is Pyruvate, and 4 molecules of ATP (Net ATP production is 2)


2. Link Reaction – Intermediary reaction between Glycolysis and the Krebs Cycle.

Pyruvate molecules are passed by active transport from the cytoplasm into the mitochondrial matrix. There the pyruvate is decarboxylated and dehydrogenated, so it can then be combined to Coenzyme A. This forms Acetyl Coenzyme A (Acetyl CoA).

Acetyl (2C) CoA can also be produced from fatty acids.


3. Krebs Cycle – Closed pathway of enzyme-controlled reactions, that take place in the mitochondrion matrix.


Krebs Cycle, obviously discovered by Krebs, is not so important in producing ATP (One per cycle) as it is in making reducing NAD and FAD.
The cycle is as follows:

A) Acetyl CoA combines with a 4C molecule, Oxaloacetate, to form a 6C molecule, Citrate.

B) Citrate is then decarboxylated and dehydrogenated to yield Carbon Dioxide, and hydrogens which are accepted by the carriers NAD and FAD

C) Oxaloacetate is then regenerated, so that the cycle can repeat.


The reduced NAD and reduced FAD are then carried further to the site of Oxidative Phosphorylation.

4. Oxidative Phosphorylation

  • The energy for this process comes from the activity of the electron transport chain
  • Oxidative phosphorylation takes place in the inner mitochondrial membrane

1) Reduced NAD and reduced FAD are passed to the electron transport chain, where they are dehydrogenated. The removed hydrogen atoms are then split into H+ ion and an electron e-.

2) The electron is then taken up by a series of  electron carriers, which are associated with four different types of proteins, all being part of the respiratory complex.

3) Electrons will move down these carriers due to a gradient in energy (high energy to lower energy), and give off energy in the process. This energy is harvested and used to pump H+ ions into the membrane, so that a concentration gradient is set up.

4) The protons will re-enter the mitochondrial matrix through a protein which harvests the electrical potential energy of these protons. For every three protons that come across this protein, enough energy is harvested to carry chemiosmosis once.

5) Every H+ ions that comes through the protein channel, one molecule of water is formed (as waste product) together with oxygen and an electron.

Number five can be said in different ways:

  1. H+, e- and Oxygen are recycled into water
  2. Hydrogen is reduced to water.


At the end of oxidative phosphorylation there should be 3 molec. of ATP coming out of one molec. of reduced NAD, and 2 molec. of ATP coming out of one molec. of reduced FAD, but instead there are only 2.5 for one molec. of NAD, and 1.5 for one molec of FAD. This is because 25% of the ATP is used to transport ATP into and out of the mitochondrion.

The net gain of ATP is +32 molec. of ATP, while the total number of molecules of ATP made are 34. Two have been used in glycolysis.


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