For definition purposes, the term mitochondrial membrane can mean either the inner or the outer membrane of a mitochondrial cell, depending on the context in which the term is used. As such, the membranes associated with mitochondria are identified as either the inner mitochondrial membrane or the outer mitochondrial membrane. The outer membrane of a mitochondrion is smooth and surrounds the entire organelle. Comparatively, the inner mitochondrial membrane forms the cristae, a twisted, turning pathway resembling the folds of a brain.
To fully understand mitochondrial membranes, it is necessary to first understand eukaryotes and how their cellular structure relates to mitochondria. Organisms with complex, membrane-based cellular structures, including plants, animals, and humans, are known as eukaryotes. All oxygen-dependent eukaryotes have mitochondria. Mitochondria appear inside each of the eukaryote's cells and are responsible for producing adenosine triphosphate (ATP), a chemical energy source derived from oxygen and various nutrients.
Under high-power magnification, mitochondria present as oval or oblong in shape, with a double membrane and an inner area known as the matrix. Numerous mitochondria exist within a single cell, with exact shapes and sizes dependent on the specific type of cell. Additionally, the mitochondria of one organism have a different appearance from those of another organism.
It is the job of mitochondria to perform aerobic respiration, the process of turning oxygen and nutrients into ATP. This process occurs along the cristae or inner mitochondrial membrane using enzymes embedded in the membrane and proteins from the matrix. Once ATP is produced, the cell uses the chemical as a power source to enable cell movement, cell division, and other metabolic functions.
While the inner mitochondrial membrane helps in the production of ATP, the outer mitochondrial membrane serves as a filter. Using a protein called porin, the outer membrane forms channels that only allow molecules of a certain size to enter the mitochondria. Once inside, molecules are further filtered by the inner membrane. Only those molecules that are pre-determined as crucial to the production of ATP are allowed to pass through the inner membrane.
Both inner and outer mitochondrial membranes serve an important purpose in the overall function of mitochondria. Although both serve a purpose, the majority of organelle activity involves the inner mitochondrial membrane and the matrix. When considering size comparisons, the cristae is considerably larger than the outer membrane, owing to its twisted, convoluted nature. With the cristae's larger surface area, each mitochondrion is able to host more of the enzymes needed to produce ATP, thus supplying the cell with more energy to function.