PLASMA MEMBRANE (PLASMALEMMA, CELL MEMBRANE, CYTOLEMMA, CYTOMEMBRANE)
Plasma Membrane consists of phospholipids and proteins. It serves as a semi-permeable molecular barrier controlling transport of small molecules and ions in and out of the cell. It limits the cavity in which cell components are placed. Some
organelles, such as, for example,
Golgi complex, are directly connected with the surface of the membrane. Others, like, for example, endoplasmatic reticulum (
rough and
smooth), do not contact immediately with the plasma membrane.
Structure. In an electronic microscope membranes look like about 7 nm thick, with clear three-layered structure. Lipid molecules are places in two layers in the way, that their non-polar hydrophobic components are in the depth of the membrane, and the polar hydrophilic components are oriented toward inner and outer water media. The membrane has various protein molecules included into it. Some of the protein molecules are on the inner or outer surface of the lipid part of the membrane; others pierce all the membrane thickness through. This kind of membrane model is called liquid-mosaic (as an effect of tight packing of molecules, tail to tail).
Function. Cell membrane is selectively permeable, that is, it admits some matters easier than others.