Mitochondria (singular – mitochondrion) are rod-shaped or kidney-shaped, membrane-enclosed organelles, ranging in size from 1 to 10 micrometers, that are found in the cytoplasm of most eukaryotic cells. Depending on organism, tissue type and level of cellular metabolic activity, a cell may contain just one mitochondrion or several thousand mitochondria; human cells normally contain 3000-5000 mitochondria.
A mitochondrion contains two main membranes composed of phosopholipid bilayers and proteins –
The smooth outer membrane – This encloses the entire organelle and contains porin protein molecules that serve as diffusion channels for minute protein molecules across the membrane. Larger molecules can enter only if their signaling sequence can bind to a large translocase protein in the outer membrane.
The folded inner membrane – This contains about 1/5th of the mitochondrion protein, but has no porin proteins, and includes several hundred polypeptides. The inner membrane is impermeable and ions and molecules require special membrane transporters to pass through it. The inner membrane folds are known as cristae.
The cristae – With their folds, the cristae increase the total surface area of the inner membrane. The cristae create two sections in the mitochondrion –
The intermembrane space – This lies between the outer and inner membranes.
The matrix – This is the inner space enclosed by the inner member. The matrix contains about 2/3rd of the total mitochondrion protein. It also contains several hundred enzymes, transfer RNA (tRNA) and copies of the mitochondrial DNA genome.
Gene Organization in the Mitochondria
Mitochondria contain their own DNA, manufacture their own RNA and proteins, and reproduce and function independently of the cell in which they are found.
A human mitochondrial DNA sequence contains 16,569 base pairs that encode 37 genes. These 37 genes are 22 transfer RNA, 2 ribosomal RNA (rRNA) and 13 polypeptides. The 22 tRNA help translate messenger RNAs into protein. The 13 polypeptides are a crucial part of the protein complex in the inner membrane.
Mitochondria transmit down the generations through the maternal lineage. In the fertilization process, the sperm is propelled towards the egg with energy it gets from the mitochondrion in its tail. Once it fertilizes the egg, the paternal mitochondria is destroyed and only the maternal mitochondria in the egg is retained and contributes to the formation of the new embryo. As the mitochondria is passed intact from mother to offspring, they are of great help to researchers in tracing lineages.
Function of Mitochondria
The enzymes in the matrix break down carbohydrates and sugars to produce adenosine triphosphate (ATP). ATP molecules store the chemical energy required by the cell to carry out its metabolic functions. It is for this reason that the mitochondria are called the power plants of the cell.
Other functions of the mitochondria include controlling the cell cycle – signaling, differentiation, growth and death – and assisting with cellular aerobic respiration.