Mitochondria are bacteria-sized organelles (about 1 × 2 μm in size), which are found in large numbers in almost all eukaryotic cells. Typically, there are about 2000 mitochondria per cell, representing around 25% of the cell volume.
Structure of Mitochondria
- Mitochondria are enclosed by two membranes—a smooth outer membrane and a markedly folded or tubular inner mitochondrial membrane, which has a large surface and encloses the matrix space.
- The folds of the inner membrane are known as cristae, and tube-like protrusions are called tubules.
- The intermembrane space is located between the inner and the outer membranes.
- The number and shape of the mitochondria, as well as the numbers of cristae they have, can differ widely from cell type to cell type.
- Tissues with intensive oxidative metabolism— e. g., heart muscle—have mitochondria with particularly large numbers of cristae.
- Even within one type of tissue, the shape of the mitochondria can vary depending on their functional status.
- Mitochondria are mobile, plastic organelles.
- Mitochondria probably developed during an early phase of evolution from aerobic bacteria that entered into symbiosis with primeval anaerobic eukaryotes.
- This endo-symbiont theory is supported by many findings. For example, mitochondria have a ringshaped DNA (four molecules per mitochondrion) and have their own ribosomes. The mitochondrial genome became smaller and smaller during the course of evolution.
- In humans, it still contains 16,569 base pairs, which code for two rRNAs, 22 tRNAs, and 13 proteins.
- Only these 13 proteins (mostly subunits of respiratory chain complexes) are produced in the mitochondrion.
- The mitochondrial envelope consisting of two membranes also supports the endosymbiont theory.
- The inner membrane, derived from the former symbiont, has a structure reminiscent of prokaryotes.
- It contains the unusual lipid cardiolipin, but hardly any cholesterol.
- Both mitochondrial membranes are very rich in proteins.
- Porins in the outer membrane allow small molecules to be exchanged between the cytoplasm and the intermembrane space.
- The inner mitochondrial membrane is completely impermeable even to small molecules (with the exception of O2, CO2, and H2O).
- Numerous transporters in the inner membrane ensure the import and export of important metabolites.
- The inner membrane also transports respiratory chain complexes, ATP synthase, and other enzymes.
- The matrix is also rich in enzymes.
Functions of Mitochondria
- The most important function of the mitochondria is to produce energy.
- The simpler molecules of nutrition are sent to the mitochondria to be processed and to produce charged molecules.
- These charged molecules combine with oxygen and produce ATP molecules. This process is known as oxidative phosphorylation.
- Mitochondria help the cells to maintain proper concentration of calcium ions within the compartments of the cell.
- The mitochondria also help in building certain parts of blood and hormones like testosterone and estrogen.
- The liver cells mitochondria have enzymes that detoxify ammonia.
- The mitochondria also play important role in the process of apoptosis or programmed cell death.
- Abnormal death of cells due to the dysfunction of mitochondria can affect the function of organ.
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