Mitochondria are the power houses’ of the cell. Their main job is to produce energy required for the functioning of our cells. They are double membrane structures, and their shape and number varies depending on the cell type. Their membranes are composed of phospholipid bilayer and proteins. The two membranes, however, have different properties. Within the mitochondria, there are five compartments, namely the inner and outer membranes, intermembrane space, cristae mitochondriales and the matrix. The outer membrane contains a large number of integral proteins called porins. Like the name suggests, porins are channels which allow molecules of 5000 Daltons or less to pour in, or diffuse freely into the mitochondrion. The inner membrane consists of proteins which have four main functions, performing redox reations, ATP generation in the matrix, regulation of metabolite passage into and out of the matrix and protein import. The inner membrane is compartmentalised into folded cup-like structures called the cristae. They exist, as they expand the surface area of the inner membrane for the activity of respiratory enzymes, thus, enhancing their ability to produce ATP. Cristae can affect the overall chemiosmotic function. Mitochondria of cells requiring more ATP have more cristae than others who do not need as much.
The area enclosed by the inner membrane is called the mitochondrial matrix. It contains a highly concentrated mixture of several enzymes, specialised mitochondrial ribosomes, tRNA and numerous copies of the mitochondrial genome. It is worth noting at this point that mitochondrial DNA is different to that of our chromosomal DNA. They have their own genetic machinery to manufacture RNAs and proteins. The male part of mitochondrial DNA is destroyed at fertilization and zygote formation as it is present in the sperm’s tail, which falls off once the head manages to fertilize the ovum. So any mitochondrial DNA in humans is female. This means that all generations of mitochondria have female DNA. Mitochondria can also self replicate during the mitotic cell division.
The main role of mitochondria is energy production, as we saw earlier. It facilitates aerobic and anaerobic respiration & ATP production. It is also important in the regulation of cellular metabolism. The enzymes in mitochondrial matrix carry out the oxidation of pyruvate and fatty acids and aid in the citric acid cycle. Mitochondria also have an important function in heat production and cell’s homeostasis of calcium. Mitochondria are also very good cytolitic buffers’ for calcium. In addition to these processes, they are also vital in many other metabolic tasks such as regulation of the excitotoxic neuronal injury, cellular proliferation regulation, some heme synthesis reactions, synthesis of steroids, etc.
However, it should be noted that not all these functions are carried out by Mitochondria in all cells, they are very cell specific.
Reference: Introduction to Human Physiology and Anatomy, by Tortora and other notes from my lectures
