Mitochondria

Mitochondria were first described Richard Altman in 1890 and he called them as bioblasts. In 1897, Benda first coined the term ‘Mitochondria’ and then in 1920 Warburg, a biochemist, found that oxidative reactions take place in small compartments in almost every eukaryotic cell. Later it was discovered that these small compartments are actually mitochondria. Mitochondria are the membrane-bound cellular organelles which help to carry out a variety of cellular metabolic functions. These are found in almost all eukaryotic cells. Cells require energy, to survive and perform different functions, which is provided in the form of adenosine triphosphate (ATP) produced by mitochondria. Mitochondria are round to oval shaped ranging in size from 0.5-10μm. They not only produce energy for the cell but also generate heat, mediate cell growth, and death as well as store calcium for cell signaling. The number of mitochondria per cell is variable, some have a few some have many and some do not have any. For example in red blood cells (RBCs) in humans do not have any mitochondria.

Structure of Mitochondria

Mitochondria are double membrane-bound, rod-shaped organelles found in both plant and animal cells. The outer membrane is freely permeable to small molecules. It also contains special channels which are capable of transporting large molecules. However, the inner membrane is selectively permeable to very small molecules, which make up the mitochondria’s central mass. These membranes are made up of proteins and phospholipids. Components of mitochondria are described below:

Outer membrane

The outer membrane is a freely permeable smooth structure made up equal amounts of proteins and phospholipids. It contains special proteins known as porins in large amounts. Porins are the integral proteins which permit the movement of molecules less than or equal to 5000 daltons in weight.

Inner membrane

The inner membrane is a more complex structure which folds and doubles in on itself to form incomplete partitions called cristae. The cristae increase the surface area for the chemical reactions which produce energy in the form of ATP for the cell. The inner membrane is very strictly permeable to oxygen and ATP. It also helps in transferring the metabolites across the membrane.

Intermembrane space

There is space between the inner and outer membrane, called intermembrane space. Its composition is similar to the cytoplasm of the cell except for its protein content.

Matrix

The space between the cristae is called the matrix. It is a mixture of proteins and enzymes that are important for the synthesis of ATP, ribosomes, tRNA and mitochondrial DNA.

Function of Mitochondria

The function of the mitochondria depends on the type of cell in which they are present. However, their most important function is to produce energy. So they are referred to as the “powerhouse” of the cell. The chemical reactions take place on the membrane. They take in small nutrient molecules, break them down and convert them into energy. This energy is then used by the cell to carry out different vital functions.

Among other functions of mitochondria are generating heat and mediating cell growth and death. They also store calcium for cell-signaling. The number of mitochondria per cell vary and cells have the ability to produce more mitochondria as per requirement.

Mitochondrial DNA

Mitochondrial DNA is the small amount of DNA found in the mitochondria of eukaryotic cells. The mitochondrial DNA or mtDNA or mDNA contains 37 genes which are essesntial for the normal functioning of mitochondria. The mDNA is inherited from mother only so it is helpful in tracing back the maternal lineage.ThemDNA is a circular and double stranded DNA which is covalently closed. Mutations in mDNA leads to many illnesses.

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