Biological Membranes | Research & Encyclopedia Articles

Biological Membranes

Biological membranes are those membranes enclosing the cell or any of the organelles within it. Membranes serve not only to enclose and define the cell or organelle, but to regulate the flow of materials passing in and out of it. The membrane surrounding a cell is known as the plasma membrane, while those inside it are called internal membranes.

Biological membranes contain approximately equal weights of two different types of molecules: phospholipids and proteins. Phospholipids give the membrane its basic structure, while proteins perform most of the regulatory functions.

The structure of the membrane is best understood by looking at the structure of the phospholipid molecule. In a phospholipid, a glycerol molecule is bonded to two fatty acid chains and one phosphate group. The fatty acids are long and nonpolar, while the phosphate group is relatively short and polar. This gives the molecule as a whole a polar (hydrophilic) end that attracts water and a nonpolar (hydrophobic) end that repels it. Molecules with both hydrophilic and hydrophobic parts are known as amphipathic.

When placed in water, phospholipid molecules tend to spontaneously orient themselves in a layer at the surface one molecule thick (a monolayer) with the polar end touching the water's surface and the nonpolar end facing away from it. The long fatty acid tails bond weakly with van der Waals attractions. Under the right conditions, two monolayers can join together in water to form a spherical bilayer, with the nonpolar ends of each layer touching on the interior of the bilayer, and the polar ends facing the water both outside and within the sphere. This phospholipid bilayer is the fundamental structure of all biological membranes.

The weak attractions between phospholipid molecules allows individual molecules in the membrane to move in two dimensions, and biological membranes are sometimes called two-dimensional fluids. With the large number of proteins embedded in membranes, they are also called fluid mosaics. The fluidity of the membrane is stabilized by cholesterol, a steroidal lipid found in all animal cell membranes. Cholesterol helps prevent drastic changes of fluidity that would otherwise occur with large temperature changes.

Phosphate groups of phospholipids each have other small groups covalently attached to them. These groups differ between the internal and external lipid layers, with more positively charged groups to the outside and negatively charged groups on the inside. This provides each side with a unique chemical signature, which may be important for orienting and embedding membrane proteins, for instance.

Membrane proteins may span the entire membrane, or be exposed to one surface or the other. Proteins of either type rely on hydrophobic amino acids on their surfaces to embed them in the membrane. Some proteins act as open channels for the movement of ions through the membrane, while others act as carriers for specific molecules, often ones in very low concentration that must be pumped into the cell using energy.Glycoproteins on the external surface act as cellular identity tags, allowing different cells to recognize one another for the purposes of forming tissues, or for protection by the immune system.

Small, uncharged molecules can cross the membrane without any transporter. These molecules include water, oxygen, and carbon dioxide. The concentration of each within the cell tends to equalize with the concentration outside the cell, because of diffusion. Diffusion of water across a semipermeable membrane such as the cell's plasma membrane is known as diffusion.

Channel proteins form pores to allow the diffusion of certain ions, such as potassium. Although it would seem the potassium channel would also allow the flow of other, smaller ions such as sodium, in fact the channel can be selective for potassium, due to its structure. All ions in solution are hydrated, surrounded by water molecules that bond to it through ionic attractions. Ions are unlikely to shed this cage of water unless they are stabilized by attraction to other negatively charged groups. The potassium channel is lined with such groups, spaced to accommodate the large potassium ion, but too far apart to effectively stabilize smaller ions. As a result, potassiums easily shed their hydrating waters upon entering the channel and pass through rapidly, while sodium does not. Other channels exist for sodium, chloride, and calcium, with the distribution and concentration of each depending on cell type.

Sodiums and potassiums are also actively transported by one of the most important membrane proteins, the K⁺/Na⁺ ATPase. This protein pumps three sodiums out of the cell, and two potassiums in, using the energy of one adenosine triphosphate (ATP). This creates transmembrane gradients for both ions. The potassium gradient dissipates by diffusion of potassium through its channel, leaving the cell with a net negative charge inside. The combination of the charge gradient and the sodium gradient, known as an electrochemical gradient, is a powerful force promoting the movement of sodium back into the cell. In the intestine, this force is used to drive the transport of glucose out of the gut and into the intestinal cell, by linking its transport to that of sodium. This is accomplished by another membrane protein, the sodium/glucose cotransporter. Binding of both triggers a conformation change that transports both into the cell.

The Na⁺/K⁺ ATPase is also used to regulate cell volume. The concentration of water inside the cell is usually lower than outside, due to the high concentration of large molecules such as proteins and carbohydrates, as well as ions. Therefore, water tends to flow into the cell by osmosis, down its concentration gradient. To prevent this, the cell runs the Na⁺/K⁺ ATPase, whose net effect is to decrease the number of ions in the cell, and increase it outside. In addition, since it creates a positive charge outside the cell, chloride ions tend flow out of the cell as well, further decreasing the osmotic potential for water. The Na⁺/K⁺ ATPase is so important to the function of cells that its operation consumes approximately 30% of the entire energy output of the cell.

Larger molecules such as proteins cannot be transported across the membrane by carriers. Instead, the movement of these molecules is accomplished by endocytosis (taking within the cell) and exocytosis (releasing outside the cell). In endocytosis, the substance to be transported (the ligand) first attaches to receptor proteins in the plasma membrane. These receptor-ligand complexes move across the outer surface of the membrane until they become engaged in special regions known as coated pits. Proteins on the inner surface of the membrane then cause this region of the membrane to pull in, forming a pocket that eventually seals and pinches off from the plasma membrane. This small sphere, known as a vesicle, is transported to processing organelles within the cell, where its contents are directed to their final destination. Cholesterol enters cells from the blood stream via this mechanism. It is transported through the bloodstream attached to low-density lipoprotein, or LDL. LDL attaches to LDL receptors on the cell surface.

Exocytosis is the reverse of endocytosis, in that an internal vesicle fuses with the membrane, releasing the contents to the exterior surface. Many protozoans, such as the amoeba, use endocytosis for capturing and ingesting food, and exocytosis for releasing waste products.

As noted previously, all internal organelles have membranes as well. These include lysosomes, the Golgi complex, the endoplasmic reticulum, and vacuoles (not found in human cells). Other organelles have double membranes, consisting of two phospholipid bilayers. These organelles are the nucleus and the mitochondria. In plants and algae, chloroplasts have three membranes--two forming the outer surface, and a third forming the grana.

Internal membranes serve to divide the cell into functional compartments, allowing highly specialized reactions to occur in different parts of the cell, free from interference with other parts.

The endoplasmic reticulum is the largest single membrane system in the cell. Its functions include formation of phospholipids, modification of proteins, and creation of vesicles. Vesicles join with the Golgi complex for further processing, and then bud off for exocytosis. The Golgi also creates lysosomes, organelles responsible for digestion of worn out cell components and endocytosed substances.

The multiple membranes of the mitochondrion and chloroplasts are thought to reflect their origins as free-living bacteria that developed a symbiotic relationship with host cells hundreds of millions of years ago. It is speculated that these bacteria were endocytosed, but rather than being consumed by the host cell, they became permanent residents, maintaining their own membranes while remaining surrounded by a host membrane as well.