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Functions of Mitochondria

Mitochondria are the powerhouses of the cell. They supply nearly all the required biological energy. Only the mitochondria are fully capable of converting pyruvic acid to dioxide and water.

1. Cell respiration

Mitochondria are the respiratory centers of the cell. Cell respiration can be divided into four phases: (1) glycolysis, (2) oxidation of pyruvic acid, (3) Kerbs citric acid cycle and (4) oxidative phosphorylation through the hydrogen/electron transport system.

(1) Glycolysis involves the breakdown of glucose to pyruvic acid. The enzymes for glycolysis are found in the cytosol, outside the mitochondria.

(2) Oxidation of pyruvic acid. Pyruvic acid is degraded to acetyl-CoA, with the liberation of a pair to hydrogens (2H).

(3) Kerbs citric acid cycle. Acetyl CoA condenses with oxalo-acetic acid to form citric acid. After several steps, oxaloacetic acid is generated. Four dehydrogenation reactions take place during the cycle, in each of which a pair of hydrogens (2H) is liberated. On ATP molecule is formed at substrate level during Kerbs cycle. The enzymes for Kerbs cycle are found in the matrix of the mitochondrion.

(4) Oxidative phosphorylation. It has been seen that pairs of hydrogen (2H) are liberated during aerobic glycolysis, oxidation of pyruvic acid and the Kerbs cycle. These hydrogen pairs are passed down the hydrogen/electron transport chain (respiratory chain), and oxidative phosphrylation takes place.

2. ATP transport

The ATP molecules produced as a result of cellular respiration accumulate in the mitochondria. The mitochondria collect at sites where energy requirement is high. As a result of membrane contraction, and increase in internal hydrostatic pressure of the mitochondrion, water and ATP are squeezed out. This result in a lowering of the ATP concentration and the mitochondrial membrane relax. The thyroid hormone thyroxine causes the mitochondria membrane to swell, while ATP brings about contraction.

3. Lipid synthesis

A set of enzymes that control synthesis of lecithin and phosphatidyl ethanolamine from fatty acids, glycerol and nitrogenous bases is present in most mitochondria.

4. Elongation of fatty acids

Mammalian mitochondria have a group of enzymes that carry out elongation of fatty acids by adding acetyl-CoA, and subsequently reducing the keto acid produced. By this method myristate is elongated to palmitate; palmitate to stearate, etc.  

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