Structure
Mitochondria are about the size of bacteria varying between 0.5 and 10 micrometers. Their size as well as their shape is dependent on where in the body they are located. The organelle is compartmentalised into five distinct regions created by their double plasma membrane.
Mitochondrial structure image courtesy of wikimedia. This image is in the public domain and thus free of any copyright restrictions.
The Outer Mitochondrial Membrane (OMM)
Surrounding the entire Mitochondrion is the outer mitochondrial membrane which has a protein to phospholipid ratio of approximately 1:1 by weight. The proteins imbedded in the OMM control what proteins are able to cross and enter the mitochondrion. These proteins are either beta barrel proteins called porins which allow the passage of small molecules under 5000D, or membrane transport proteins which allow larger molecules over 5000 D to traverse using active transport. As well as acting as a barrier and controlling molecular transport, the OMM contains enzymes involved in processes such as the elongation of fatty acids and the oxidation of adrenaline.
The Intermembrane Space
The intermembrane space separates the outer and inner membranes and contains almost the same concentration of small molecules as the cell cytosol due to their unrestricted movement through porins. The space contains a low pH due to the proton gradient created when protons contained in the matrix are pumped in by the machinery of the electron transport chain.
The Inner Mitochondrial Membrane (IMM) and Cristae
The IMM is highly convoluted creating internal compartments called cristae. The large number of folds in the IMM produces increases the cristae surface area (5x more than the OMM) which optimises the production of ATP. These structures contain elements of the electron transport chain and can vary in number depending on the cell's energy demand. For example, there are more cristae in muscle cell mitochondria than those of a liver cell. A large range of polypeptides and proteins such as those required for the redox (reduction-oxidation) reactions of oxidative phosphorylation are found on the IMM producing a high protein to phospholipid ratio (3:1 by weight). This ratio, usually found on the membranes of prokaryotes, is indicative of the many functions carried out by this region. The IMM is largely impermeable as it contains no porins so molecules have to be transported in and out of the matrix specifically. Transport is mainly performed by a series of mitochondria specific carrier proteins collectively referred to as the translocase of the inner membrane (TIM) which ferry proteins across the barrier.
The Matrix
The inside of the mitochondrion is referred to as the matrix. This space, enclosed by the IMM, contains 2/3 of the total protein content of the organelle. It contains a mixture of hundreds of enzymes crucial for the oxidation of pyruvate and fatty acids (the starting substrates of the citric acid cycle). Along with enzymes, the matrix houses several copies of the mitochondrial genome and all the tools required for DNA processing such as mitochondrial ribosomes and tRNA.
Did you know?
The structure of mitochondria is a relic of its bacterial ancestor which lived 1.5 billion years ago. Its closest known relative today is the Rickketsia bacterium which causes a number of illnesses including Rocky Mountain spotted fever. To find out more about the origins of mitochondria click here.
Rickettsia images both courtesy of wikimedia. These images are in the public domain and thus free of any copyright restrictions.