The cell membrane – also called the plasma membrane or cytoplasmic membrane – is among the most fascinating and elegant constructs in the world of biology. The cell is considered the fundamental unit or "building block" of all living things on Earth; your own body has trillions of them, and different cells in different organs and tissues have different structures that correlate exquisitely with the functions of the tissues consisting of these cells.
While the nuclei of cells often draw the most attention since they contain the genetic material necessary for passing along information to subsequent generations of the organism, the cell membrane is the literal gatekeeper and guardian of the cell's contents. Far from a mere container or barrier, however, the membrane has evolved to maintain cellular equilibrium, or internal balance, through efficient and tireless transport mechanisms that make the membrane a sort of microscopic customs official, allowing and denying the entry and exit of ions and molecules in accordance with the cell's real-time needs.
Cell Membranes Across the Life Spectrum
All organisms have cell membranes of some sort. This includes prokaryotes, which are mostly bacteria and are believed to represent some of the oldest living species on Earth, as well as eukaryotes, which include animals and plants. Both the prokaryotic bacteria and the eukaryotic plants have a cell wall external to the cell membrane for additional protection; in plants, this wall has pores, and they are not especially selective in terms of what can pass through and what cannot. In addition, eukaryotes possess organelles, such as the nucleus and mitochondria, enclosed by membranes like the one surrounding the cell as a whole. Prokaryotes do not even have nuclei; their genetic material is dispersed, albeit somewhat tightly, throughout the cytoplasm.
Considerable molecular evidence suggests that eukaryotic cells are descended from prokaryotic cells, losing the cell wall at some point in their evolution. Although this made individual cells more vulnerable to insults, it also allowed them to become more complex and expand geometrically in the process. In fact, eukaryotic cells can be ten times as large as prokaryotic cells, a finding made all the more striking by the fact that a single cell is the entirety of a prokaryotic organism by definition. (Some eukaryotes are single-celled as well.)
Cell Membrane Structure
The cell membrane consists of a double-layered structure (sometimes called the "fluid mosaic model") composed mainly of phospholipids. One of these layers faces the interior of the cell, or cytoplasm, while the other faces the external environment. The outward- and inward-facing sides are considered "hydrophilic," or attracted to watery environments; the inner portion is "hydrophobic," or repelled by watery environments. In isolation, cell membranes are fluid at body temperatures, but at cooler temperatures, they take on a gel-like consistency.
The lipids in the bilayer account for about half of the total mass of the cell membrane. Cholesterol makes up about one-fifth of the lipids in animal cells, but not in plant cells, as cholesterol is not found anywhere in plants. Most of the remainder of the membrane is accounted for by proteins with a diverse variety of functions. Since most proteins are polar molecules, like the membrane itself, their hydrophilic ends jut to the cell exterior, and their hydrophobic ends point to the interior of the bilayer.
Some of these proteins have carbohydrate chains attached to them, making them glycoproteins. Many of the membrane proteins are involved in the selective transport of substances across the bilayer, which they can do either by creating protein channels across the membrane or by physically shuttling them across the membrane. Other proteins function as receptors on cell surfaces, providing binding sites for molecules that carry chemical signals; these proteins then relay this information to the interior of the cell. Still other membrane proteins act as enzymes catalyzing reactions particular to the plasma membrane itself.
Cell Membrane Functions
The critical aspect of the cell membrane is not that it is "waterproof" or impermeable to substances in general; if it were either, the cell would die. The key to understanding the cell membrane's main job is that it is selectively permeable. An analogy: Just as most nations on Earth do not completely forbid people from traveling across the nation's international borders, countries around the globe are not in the habit of letting anyone and everyone enter. Cell membranes attempt to do what these countries' governments do, on a much smaller scale: allow desirable entities to enter the cell after being "vetted" while barring entry to entities that are likely to prove toxic or destructive to the interior or the cell as a whole.
Overall, the membrane acts as a formal boundary, holding the various parts of the cell together the same way a fence around a farm keeps the livestock together even while allowing them to roam about and mingle. If you had to guess the kinds of molecules that are allowed to enter and exit most readily, you might say "sources of fuel" and "metabolic waste" respectively, given that this is essentially what bodies as a whole do. And you would be right. Very small molecules, such as gaseous oxygen (O2), gaseous carbon dioxide (CO2), and water (H2O), can pass freely across the membrane, but the passage of larger molecules, such as amino acids and sugars, is tightly controlled.
The Lipid Bilayer
The molecules that are almost universally called "phospholipids" that make up the cell membrane bilayer are more properly called "glycerophospholipids." They consist of a glycerol molecule, which is a three-carbon alcohol, attached to two long fatty acids on one side and a phosphate group on the other. This gives the molecule a long, cylindrical shape that is well-suited to the job of being a part of a wide sheet, which is what a single layer of the membrane bilayer resembles on cross-section.
The phosphate portion of the glycerophospholipid is hydrophilic. The specific kind of phosphate group varies from molecule to molecule; for example, it can be phosphatidylcholine, a which includes a nitrogen-containing component. It is hydrophilic because it has an uneven distribution of charge (i.e., is polar), just like water, so the two "get along" in close microscopic quarters.
The fatty acids on the interior of the membrane do not have an uneven distribution of charge anywhere in their structure, so they are nonpolar and hence hydrophobic.
Because of the electrochemical properties of phospholipids, the phospholipid bilayer arrangement requires no input of energy to create or maintain. In fact, phospholipids placed in water tend to spontaneously assume the bilayer configuration in much the same way fluids "seek their own level."
Cell Membrane Transport
Because the cell membrane is selectively permeable, it must provide a means of getting a variety of substances, some large and some small, from one side to the other. Think of the ways you might cross a river or a body of water. You might take a ferry; you might simply drift on a light breeze, or you may be carried by steady river or ocean currents. And you may only find yourself crossing the body of water in the first place because there is too high a concentration of people on your side and too low a concentration on the other, presenting a need to even things out.
Each of these scenarios bears some relationship to one of more of the ways molecules can pass through the cell membrane. These ways include:
Simple diffusion: In this process, molecules simply drift through the double membrane to pass either into or out of the cell. The key here is that molecules in most situations will move down a concentration gradient, meaning that they naturally drift from areas of higher concentration to areas of lower concentration. If you were to pour a can of paint into the middle of a swimming pool, the outward movement of the paint molecules would represent a form of simple diffusion. The molecules that can cross cell membranes in this way, as you may predict, are small molecules such as O2 and CO2.
Osmosis: Osmosis might be described as a "sucking pressure" that causes the movement of water when the movement of particles dissolved in the water is impossible. This occurs when a membrane allows water, but not the dissolved particles ("solutes") in question, to pass through it. The driving force is again a concentration gradient, because the entire local environment "seeks" an equilibrium state in which the amount of solute per unit water is the same throughout. If there are more solute particles on one side of a water-permeable, solute-impermeable membrane than the another, water will flow to the area of higher solute concentration. That is, if particles cannot change their concentration in water by moving, then the water itself will move to accomplish more or less the same job.
Facilitated diffusion: Again, this type of membrane transport sees particles move from areas of higher concentration to areas of lower concentration. Unlike the case with simple diffusion, however, the molecules move into or out of the cell via specialized protein channels, rather than simply drifting through the spaces between glycerophospholipid molecules. If you have ever watched what happens when something drifting down a river suddenly finds itself in a passageway between rocks, you know that the object (perhaps a friend on an inner tube!) speeds up considerably while in this passageway; so it is with protein channels. This is most common with polar or electrically charges molecules.
Active transport: The types of membrane transport previously discussed all involve movement down a concentration gradient. Sometimes, however, just as boats must move upstream and cars have to climb hills, substances most move against a concentration gradient – an energetically unfavorable situation. As a result, the process has to be powered by an outside source, and in this case that source is adenosine triphosphate (ATP), that widespread fuel for microscopic biological transactions. In this process, one of the three phosphate groups is removed from ATP to create adenosine diphosphate (ADP) and a free phosphate, and the energy liberated by the hydrolysis of the phosphate–phosphate bond is used to "pump" molecules up the gradient and across the membrane.
Active transport may also occur in an indirect or secondary fashion. For example, a membrane pump may move sodium across its concentration gradient from one side of the membrane to the other, out of the cell. When the sodium ion diffuses back through in the other direction, it might carry a glucose molecule with it against that molecule's own concentration gradient (glucose concentration is usually higher on the insides of cells than on the outside). Since the movement of glucose is against its concentration gradient, this is active transport, but because no ATP is directly involved, it this is an example of secondary active transport.