Mitochondria Definition

The Mitochondrion (plural = mitochondria) is a membrane-bound cytoplasmic organelle in a cell concerned with the production of energy. It is a rod-shaped or oval-shaped structure with a diameter of 0.5 to 1. It is covered by a bilayered membrane.

The outer membrane is smooth and encloses the contents. This membrane contains various enzymes such as acetyl-CoA synthetase and glycerolphosphate acetyltransferase.

Mitochondria are the organelles that function as the cell “powerhouse” – generating ATP (the universal form of energy used by all cells). They convert food nutrients such as glucose, to a fuel (ATP) that the cells of the body can use. They are tiny saclike structures found near the nucleus.

Little shelves called cristae are formed from folds in the inner membrane. Cells that are metabolically active such as muscle, liver, and kidney cells have high energy requirements and therefore have more mitochondria.

What are Mitochondria?

The Mitochondria are sausage-shaped structures made of an outer membrane and an inner membrane that is folded to form shelves or cristae. The space between the two membranes is called the intracristal space (or inter-membrane space) and the space inside the inner membrane is called the matrix space.

The evolutionary origin of mitochondria suggests that they may have originally been free-living bacteria that became incorporated in cells in a symbiotic relationship (symbionts).

Mitochondria can replicate in cells and have their own protein-synthesizing apparatus. Their density can vary both up and down, e.g. up to a 10-fold increase in resting skeletal muscle if it is repeatedly stimulated to contract over a prolonged period.

They have their own mitochondrial genome, which suggests that they were once autonomous organisms that came to develop a symbiotic relationship with eukaryotic cells.

Compared to the nuclear genome, the mitochondrial genome contains much less DNA. The mitochondrial DNA codes certain key enzymes of oxidative phosphorylation. Just as in Lysosomes, mutations in mitochondrial DNA cause a large number of relatively rare mitochondrial diseases.

Mitochondria Structure 

The mitochondria may be globular, rod, thread, star-shaped or ring-shaped. The outer part is surrounded by a bi-layered membrane. The membrane is made of lipoprotein, which is enriched with lipid and protein.

The outer membrane is smooth but the inner one has a series of enfolding into the inner cavity. These enfolding are called cristae. Small stalked granular bodies are arranged on the cristae and they are called auxisome.

Size

Mitochondria vary in size, most of them being 0.5 to 2 m in length. They are numerous and larger in cells with a high oxidative metabolism.

Membrane

The Mitochondria are made up of a double-layered membrane. The outer membrane contains enzymes concerned with biologic oxidations, providing raw materials for the reactions occurring in the matrix space. The matrix contains enzymes of the citric acid cycle and enzymes required for the oxidation of fatty acids.

The inner membrane contains succinate dehydrogenase and adenosine triphosphate (ATP) synthetase. The intracristal space contains adenyl kinase and creatine kinase. The enzymes in the matrix, inner membrane, and intermembrane space are marker enzymes for mitochondria, that is, they are found nowhere else.

Where is Mitochondria found (Location)

The Mitochondria is located in the cytosol of the cytoplasm of both plant and animal cells

How many Mitochondria are in a cell?

Their number may vary according to the species. Normally each cell contains an average of 300-400 mitochondria. Whereas metabolically inactive cells have relatively few mitochondria, active cells such as hepatocytes contain approximately 1000 mitochondria.

Functions

1. Production of energy: commonly called the powerhouse or power plant of the cell because it produces the energy required for cellular functions. The energy is produced during the oxidation of digested food particles like proteins, carbohydrates, and lipids by the oxidative enzymes in cristae. During the oxidative process, water and carbon dioxide are produced with the release of energy. The released energy is stored in the mitochondrion and used later for the synthesis of ATP.
2. Synthesis of ATP: The components of the respiratory chain in mitochondrion are responsible for the synthesis of ATP by utilizing the energy by oxidative phosphorylation. ATP molecules diffuse throughout the cell from the mitochondrion. Whenever energy is needed for cellular activity, the ATP molecules are broken down.
3. Apoptosis: Some enzymes such as Cytochrome C and second mitochondria-derived activator of caspases (SMAC) are secreted in the mitochondrion and are involved in apoptosis.
4. TCA cycle, Electron Transport Chain (ETC), Oxidative phosphorylation all take place in the mitochondrion.
5. It aids in the storage of calcium and detoxification of ammonia in the liver.
6. Other metabolic roles include beta-oxidation of fats, synthesis of urea, haem synthesis.

Mitochondria in plant cells and in animal cells all perform the same function of cellular respiration. They are known as the Powerhouse of the cell.

Cellular respiration

The inner membrane of the mitochondria is folded in the form of shelf-like inward projections called cristae and it covers the inner matrix space. Cristae contain many enzymes and other protein molecules which are involved in the respiration and synthesis of adenosine triphosphate (ATP).

Because of these functions, the enzymes and other protein molecules in cristae are collectively known as respiratory chain or electron transport system.

Enzymes and other proteins of the respiratory chain include the following:

1. Succinic dehydrogenase
2. Dihydronicotinamide adenine dinucleotide (NADH) dehydrogenase
3. Cytochrome oxidase
4. Cytochrome C
5. ATP synthase