Membranes

Each cell is bounded on the outside by a very thin, delicate cell membrane of about 7.5nm in thickness. The membrane is an integral part of the cytoplasm. Different kinds of cell membrane models have been proposed, and one of the most useful is the Fluid-mosaic model. In this model the membrane is seen as a bilayer of phospholipids in which protein molecules are embedded.

An illustration of the Fluid mosaic model

• the plasma membrane (flexible bilayer of phospholipids and proteins that the defines the boundary of the cell) and all membranes have a similar basic structure;
• each membrane is referred to as a unit membrane and is composed of two layers of protein and lipid molecules; there is an intervening gap between these two layers;
• minute pores are formed in a cell membrane, through which molecules can pass through the membrane; it is said to be selectively, or differentially, or semi- permeable;
• on the cell surface of animal cells is an adhesive, cement-like substance that holds neighbouring cells together;
• the surface membrane of certain cells , e.g. the kidney cells, can be extended into minute finger-like processes called microvilli that increase the surface area for the functioning of these cells;
• cilia are thread-like projections of certain cells that beat in a regular fashion to create currents that sweep materials along;
• the two membranes that form the nuclear envelope, are typical cell membranes, with pits or pores that are larger than those of the plasmalemma, to allow for the free movement of RNA and other large molecules into and out of the nucleus;
• the mitochondria and chloroplasts are surrounded by double membranes; the lysosomes by single unit membranes; the endoplasmic reticulum is formed by a complex system of membranes forming hollow sheets, or tubes, branching through the cytoplasm; and the Golgi body is also formed by branching tubes formed by membranes.

• the cell membrane functions as a physiological boundary separating the cell from surrounding cells and the extracellular medium;
• all other membranes within cells keep materials in various compartments and allow the movement of selected molecules through them;
• the plasma membrane can also enclose small quantities of fluid in inward projected vesicles, in a process call pinocytosis ("cell-drinking"); particles may be enclosed in the same way in a process called phagocytosis ("cell-eating").