Mitochondria are elongated cylinders, 0.5–1.0 m m in diameter (Figure 30.2). They are important in metabolism, carry out most cellular oxidation reactions and produce most of the animal cell’s ATP. The energy available in the form of NADH, NADH2 and FAD is harnessed by electron transport chains to the synthesis of ATP. Energy is provided by the oxidation of nutrients.
Mitochondria consist of internal membranes, a smooth outer and a folded inner membrane. The infoldings of the inner membrane are called cristae which increase the surface area of the inner membrane. The space within a crista connects to the intermembrane space through five to six tubular channels at the edges. The inner membrane separates the mitochondria into two distinct spaces:
• the internal or matrix space
• the intermembrane space
The outer membrane has few enzymatic activities, but is permeable and involved in transmembrane transport of molecules with a molecular weight of up to 5000 Da. The inner membrane is impermeable to ions and polar molecules and limits the movement of energy-rich compounds between the mitochondrial matrix and the intermembrane space. The inner membrane is rich in enzymes.
The number of mitochondria in a tissue reflects the tissue’s requirement for ATP; the more mitochondria, the more ATP is produced. The mitochondrial enzymes and enzyme complexes are organised in defined positions in the different compartments of the organelle.
1. Mitochondria are elongated cylinders, important in metabolism and cellular oxidation reactions and production of ATP The energy provided by nutrient oxygenation in the form of NADH, NADH2 and FAD is harnessed by electron transport chains to the synthesis of ATP.
2. Mitochondria contain internal membranes, a smooth outer and a folded inner membrane. The inner membrane separates the mitochondria into two distinct spaces, the internal or matrix space and the intermembrane space. The outer membrane has few enzymatic activities, but is permeable to molecules with a molecular weight of up to 5000 Da. The inner membrane is impermeable to ions and polar molecules, limits the movement of energy-rich compounds and is rich in enzymes.
3. The mitochondrion is unusual in that it contains its own DNA and the genes are derived entirely from the mother.
4. The number of mitochondria in a tissue reflects that tissue’s requirement for ATP.
5. Mitochondrial functions include the synthesis of ATP, terminal oxidation of pyruvate from carbohydrate and amino acid catabolism, b -oxidation of fatty acids and oxidation of acetate, fatty acid, protein and carbohydrate. Mitochondria are also involved in oxidation of branched chain amino acids, maintenance of nitrogen homeostasis and urea formation, oxidation of sulphite, activation of vitamin D3 and synthesis of many important compounds, e.g. bile acids.
6. The processes of glycolysis, b -oxidation and the tricarboxylic acid cycle generate NADH and NADH2. These undergo oxidation-reduction reactions which provide a mechanism whereby electrons move from reduced coenzymes (NADH and FADH2) to oxygen.
7. NADH dehydrogenase can only oxidize NADH within the mitochondrial matrix. Protons are bound and released on either side of the inner membrane of the mitochondria when electrons progress down the respiratory chain. This is known as chemiosmosis.
8. Mitochondria respond to metabolic and environmental conditions in ageing, training and pathological processes.