Delphinidin

The outer boundary of the living part of the cell, the plasma membrane, is roughly eight-millionths of a millimeter thick. To get an idea of how incredibly thin that is, consider that it would take 12,500 such membranes neatly stacked in a pile to achieve the thickness of an ordinary piece of writing paper. Yet this delicate, semipermeable structure is of vital importance in regulating the movement of substances into and out of the cell. While the plasma membrane may inhibit movement of some substances, it can otherwise allow free movement and can even control movement of other substances into and out of the cell. As a result, the proportions and makeup of chemicals within a cell become quite different from those outside the cell. The plasma membrane is also involved in the production and assembly of cellulose for cell walls.

Evidence obtained since the early 1970s indicates that the plasma membrane and other cell membranes are composed of phospholipids arranged in two layers, with proteins interspersed throughout. This fluid mosaic model for the plasma membrane implies a dynamic structure with numerous components, some of which can migrate and interact directly with each other. Covalent bonds link carbohydrates to both lipids and proteins on the outer surfaces of membranes. Some proteins extend across the entire width of the membrane, while others are embedded or apparently are loosely bound to the outer surface.

The remainder of cell contents usually push the plasma membrane up against the cell wall because of pressures developed by osmosis, but the membrane is quite flexible and often forms folds, which may, in turn, become little hollow spheres or vesicles that float off into the cell. In fact, experiments have shown that by adding detergents to a continuous membrane, it can be broken up and dispersed, yet it can partially reform when the detergents are removed. The membrane may even shrink away from the wall temporarily, but if it ever ruptures, the cell soon dies.