The Cell Membrane
Boundaries are important. Good fences keep cattle from straying from grazing land onto arable crops and shopping bags keep groceries from spilling all over the supermarket parking lot. Between the interior of a cell and the exterior environment is a boundary every bit as important as a good fence, but far more versatile than a plastic shopping bag.
Inside or out, most of a cell and its surroundings consists of water in which is dissolved ions, molecules, compounds and macromolecules. What makes the inside of a cell different to the outside of a cell is the quantity and quality of these ions and molecules. If nothing separated them, the molecules inside the cell would slowly diffuse to the outside and the ions outside would eventually penetrate and fill the inside - soon there would be no difference between a cell and its surroundings and there would be no life.
In a very real way, therefore, what keeps a living amoeba cell different from pond water is the boundary between it and its environment. Like a shopping bag it must keep vital materials from leaking out and unwanted material from leaking in. But a cell membrane or plasma membrane is more than an inert bag or container. Just separation is not enough, as well as keeping the inner and outer zones apart, the plasma membrane must also act as a border guard and customs inspector all in one, regulating the flow of needed materials into the cell and unwanted materials out of the cell. No static barrier, the cell membrane is a dynamic regulator of exchange.
Under the light microscope the cell membrane appears to be a very fragile organelle less than 0.1 micrometer in thickness, but nothing else can be distinguished about its structure or composition. Even using modern electron microscopes, the composition and makeup of a membrane is elusive because these more powerful instruments only give a static, snap shot picture of an organelle that is very dynamic and constantly changing. Taken together with molecular studies, however, a picture of a typical cell membrane is emerging.
The main constituent of membranes, the part that forms the boundary or barrier, is a molecule called a phospholipid which has the special property of being both hydrophobic and hydrophilic at the same time. A phospholipid consists of a molecule of glycerol to which is attached two fatty acid molecules (the hydrophobic part) and one modified phosphate group (the hydrophilic part). When mixed with water, these molecules align themselves in double layers the hydrophobic fatty acids pointing inwards away from the water, while the phosphate groups point outwards into the water. In this lipid bilayer arrangement the molecules are at their lowest energy and therefore most stable.
It is this bilayer of lipid molecules that forms the barrier between the inside and outside of a cell and keeps hydrophilic substances, such as sugar molecules, from leaking out. Located in, out and through this bilayer are large protein molecules which drift like icebergs in the lipid sea. These protein molecules and complexes of proteins are the agents which give the plasma membrane its dynamic and regulatory properties. Many of the proteins studded in the membrane are selective channels from one side of the barrier to the other. Under appropriate conditions these channels open and allow selected molecules or ions to pass from one side to the other. In some cases this passage is passive (by diffusion) and sometimes it is active (a pump). Teams of these regulated channels control and direct the flow of most materials across the cell membrane.
Far from being a uniform barrier, the cell membrane is a mosaic of materials (sometimes called a fluid mosaic) that is selectively or semipermeable to a wide variety of substances. Strangely, water seems to pass at will from one side of the membrane to the other, and cells must constantly fight this diffusion to stay alive. Osmosis is the name given to the phenomenon that occurs when a barrier like a cell membrane separates two solutions. Water diffuses freely across the semipermiable membrane, but other molecules cannot. Consequently there is a net movement of water from the least concentrated to the most concentrated side of the barrier. In fresh water (low concentration) amoeba constantly fight the movement of water into their cell body, while in salt water (fish cells) constantly have to fight the loss of water out from their cells.
Endocytosis and Exocytosis are the names given to methods of getting materials into and out of cells without having to cross the cell membrane. An amoeba, for example, coming across a large particle of food, simply surrounds it. As it comes in contact with the food the membrane of the cell folds inwards, wraps around the food particle and eventually fuses at the opposite side forming a pocket or vesicle inside the cell which contains the food and a small amount of the outside water. Exocytosis is just the opposite. The membrane of an internal vesicle fuses with the external plasma membrane and the internal contents of the vesicle are expelled to the outside. The membrane of the vesicle then becomes part of the plasma membrane.