(plasmalemma) The outer layer of the protoplasm below the cell wall. Like most cell membranes it is formed by the orderly orientation of protein and phosphoglyceride molecules. Plasma membranes range from 7.5 nm to 10 nm in thickness and are composed of approximately 60% protein and 40% phosphoglyceride. Membranes of different species contain characteristic types of polar lipids in proportions that are probably genetically determined. Davson and Danielli, in 1935, proposed that membranes are made up of a central region consisting of phosphoglycerides and an outer denser region composed of proteins. The phosphoglyceride molecules were believed to be arranged in two rows with their hydrophilic polar heads towards the outer edges and their hydrophobic hydrocarbon tails in the centre. Although it is still accepted that the two rows of phosphoglyceride molecules form the backbone of the membrane it is believed that globular proteins, rather than forming a distinct outer layer, actually penetrate the whole width of the membrane in places (see diagram). Both the phosphoglyceride molecules and the proteins are thought to be able to move laterally giving the membrane fluid-like properties. This hypothesis of membrane structure is termed the fluid-mosaic model. Plasma membranes are selectively permeable, controlling the passage of materials into and out of the cell. The proteins of the membrane include enzymes and compounds of the active transport system. Water and nonpolar molecules that dissolve in the phosphoglyceride layer pass readily through the membrane. The membrane is relatively impermeable to charged ions, which enter the cell by means of the active transport system. Permeability varies in different parts of the plant and at different stages of development. Time-lapse photography of living membranes reveals almost constant movement and it is probable that there is continual replenishment of membrane constituents.  |
