Mitochondria: Structure and Functions

Be it the beating of the heart or moving of our hands, every action requires energy. This energy is stored in ATP (adenosine triphosphate) molecules that are produced in the mitochondria by the process of oxidative phosphorylation. Although mitochondria are present in every cell, they are found in high concentrations in the muscle cells that require more energy. Though the primary function of mitochondria is to produce energy, they also play an important role in the metabolism and synthesis of certain other substances in the body.

Structure

Mitochondria are rod-shaped structures that are enclosed within two membranes - the outer membrane and the inner membrane. The membranes are made up of phospholipids and proteins. The space in between the two membranes is called the inter-membrane space which has the same composition as the cytoplasm of the cell. However, the protein content in this space differs from that in the cytoplasm. The structure of the various components of mitochondria are as follows:

• Outer Membrane
  The outer membrane is smooth unlike the inner membrane and has almost the same amount of phospholipids as proteins. It has a large number of special proteins called porins, that allow molecules of 5000 daltons or less in weight to pass through it. The outer membrane is completely permeable to nutrient molecules, ions, ATP and ADP molecules.
• Inner Membrane
  The inner membrane is more complex in structure than the outer membrane as it contains the complexes of the electron transport chain and the ATP synthetase complex. It is permeable only to oxygen, carbon dioxide and water. It is made up of a large number of proteins that play an important role in producing ATP, and also helps in regulating transfer of metabolites across the membrane. The inner membrane has infoldings called the cristae that increase the surface area for the complexes and proteins that aid in the production of ATP, the energy rich molecules.
• Matrix
  The matrix is a complex mixture of enzymes that are important for the synthesis of ATP molecules, special mitochondrial ribosomes, tRNAs and the mitochondrial DNA. Besides these, it has oxygen, carbon dioxide and other recyclable intermediates.

Functions

• The most important function of the mitochondria is to produce energy. The food that we eat is broken into simpler molecules like carbohydrates, fats, etc., in our bodies. These are sent to the mitochondrion where they are further processed to produce charged molecules that combine with oxygen and produce ATP molecules. This entire process is known as oxidative phosphorylation.
• It is important to maintain proper concentration of calcium ions within the various compartments of the cell. Mitochondria help the cells to achieve this goal by serving as storage tanks of calcium ions.
• Mitochondria helps in the building of certain parts of the blood, and hormones like testosterone and estrogen.
• Mitochondria in the liver cells have enzymes that detoxify ammonia.

Although most of the genetic material of a cell is contained within the nucleus, the mitochondria have their own DNA. They have their own machinery for protein synthesis and reproduce by the process of fission like bacteria do. Due to their independence from the nuclear DNA and similarities with bacteria, it is believed that mitochondria have originated from bacteria by endosymbiosis.