Example: Mitochondrial inner membrane has high amounts of functional electron transport system proteins. Plasma membrane, with fewer functions (mainly ion transport), has less protein.
Membranes with similar function (i.e. from the same organelle) are similar across species lines, but membranes with different function (i.e. from different organelles) may differ strikingly within a species.
Distribution of lipids in membranes. There can be large membrane to membrane differences in lipids (compare phosphoglycerides and cholesterol in plasma membrane vs. inner mitochondrial membrane). .
There can be large differences within classes of lipids (compare the cardiolipin of the inner mitochondrial membrane to other membranes). There are also patterns of differences among the fatty acyl groups of the lipids of various membranes.
c) Cell membranes act as barriers to most, but not all, molecules. Development of a cell membrane that could allow some materials to pass while constraining the movement of other molecules was a major step in the evolution of the cell. Cell membranes are differentially (or semi-) permeable barriers separating the inner cellular environment from the outer cellular (or external) environment. .
Proteins are suspended in the inner layer, although the more hydrophilic areas of these proteins "stick out" into the cells interior as well as the outside of the cell. These integral proteins are sometimes known as gateway proteins. Proteins also function in cellular recognition, as binding sites for substances to be brought into the cell, through channels that will allow materials into the cell via a passive transport mechanism, and as gates that open and close to facilitate active transport of large molecules.
The outer surface of the membrane will tend to be rich in glycolipids, which have their hydrophobic tails embedded in the hydrophobic region of the membrane and their heads exposed outside the cell.
