Membrane Structure: How it Relates to Function
Over the last thirty five years, our concept of the plasma membrane has changed considerably from the simple three-layered structure surrounding the cell to that of a FLUID MOSAIC MODEL. This model of the membrane consists of a fluid phospholipid bilayer with a mosaic of interspersed proteins embedded in the lipid layers. Some proteins extend through both lipid layers, while others may extend only halfway or be loosely bound to the surface of the membrane. Chemical analysis of several kinds of membranes reveals that most contain about 50 percent protein and 50 percent lipid. Membrane surrounding cytoplasmic organelles or specialized cells may have percentages that vary from this 1:1 ratio. The inner membranes of the mitochondria, for example, contain 75 percent protein and only 25 percent lipid. Membranes of nerve cells often contain 90 percent lipid and 10 percent protein. The fluid mosaic model explains many of the known facts about membrane functions, but much remains unknown about both its structure and its interactions with the kinetic activity of ions and molecules in order to sustain the necessary conditions for the living cell (1).
ARCHITECTURE OF THE PLASMA MEMBRANE (2)
Cell membranes are assembled from four components:
1. A lipid bilayer foundation - every cell membrane has as its basic foundation a phospholipid bilayer. The lipid bilayer is a fluid like the "shell" of a soap bubble. It has the viscosity of olive oil. The other components of the membrane are enmeshed within the bilayer, which provides a flexible matrix and, at the same time, imposes a barrier to permeability. Within the lipid bilayer of animal cells, cholesterol molecules are inserted, adding fluidity.
2. Transmembrane proteins - a major component of every membrane is a collection of proteins that float within the lipid bilayer like icebergs on the sea. These proteins provide channels into the cell through which molecules and information pass. Membrane proteins are not fixed in position like headstones in a graveyard. Instead, they move about like boats on a lake. Some membranes are crowded with proteins, side by side, just as some lakes are so crowded with boats you can hardly see the water. In other membranes, the proteins are more sparsely dispersed. The nonpolar regions of the proteins serve to tie them to the membrane's nonpolar interior. The three principal classes of membrane protein are CHANNELS, RECEPTORS, and CELL SURFACE MARKERS.
3. Network of supporting fibers - membranes are structurally supported by proteins that reinforce the membrane's shape. This is why a red blood cell is shaped like a doughnut rather than being irregular. The membrane is held in that shape by a scaffold of protein on its inner surface. Membranes use networks of other proteins to control the lateral movements of some key membrane proteins, anchoring them to specific sites so that they do not simply drift away. Unanchored proteins have been observed to move as much as 10 micrometers in one minute.
4. Exterior glycolipids - a thicket of carbohydrate and lipid extends from the cell surface. These chains acts as cell identity markers. Different cell types exhibit different kinds of carbohydrate chains on their surfaces. Carbohydrate chains (strings of sugar molecules) are often bound to the proteins in the membrane. These chains, as mentioned, serve as distinctive identification tags, unique to particular types of cells.
A membrane, then, is a sheet of lipid and protein that is supported by other proteins and to which carbohydrates are attached. The key functional proteins acts as passages through the membrane, extending all the way across the bilayer. How do these transmembrane proteins manage to span the membrane, rather than just floating on the surface, in the way that a drop of water floats on oil? The part of the membrane that traverses the lipid bilayer is specially constructed, a spiral helix of nonpolar amino acids. Because water responds to nonpolar amino acids much as it does to nonpolar lipid chains, the nonpolar helical spiral is held within the interior of the lipid bilayer by the strong tendency of water to avoid contact with these amino acids. Although the polar ends of the protein protrude from both sides of the membrane, the protein itself is locked into the membrane by its nonpolar helical segment.
SOME WAYS A CELL'S MEMBRANES REGULATE ITS INTERACTIONS
WITH THE ENVIRONMENT (2)
1. Passage of water - membranes are freely permeable to water, but the spontaneous movement of water into and out of cells sometimes presents problems
2. Passage of bulk materials - cells sometimes engulf large hunks of other cells or gulp liquids
3. Selective transport of molecules - membranes are very picky about which molecules they allow to enter or leave the cell
4. Reception of information - membranes can identify chemical messages with exquisite sensitivity
5. Expression of cell identity - membranes carry name tags that tell other cells who they are
6. Physical connection with other cells - in forming tissues, membranes make special connections with each other
The movement of substances between cells and the surrounding environment occurs in three general ways: DIFFUSION, TRANSPORT BY PROTEIN CARRIERS, and ENDOCYTOSIS or EXOCYTOSIS. The movement of substances by diffusion does not require the input of cellular energy. The difference between active and passive (facilitated) transport by protein carriers may be distinguished on the basis of energy expenditure by the cell. Both endocytosis and exocytosis require energy and the formation of vacuoles.
MOVEMENT BY DIFFUSION
The establishment of a CONCENTRATION GRADIENT due to a higher concentration of molecules results in a diffusion of molecules into the cell. Movements through the membrane components depends on the size and chemical nature of the molecule. Oxygen, carbon dioxide, and other lipid soluble molecules, such as alcohol and benzene pass through the lipid portion of the thin 7.5 nm membrane (about 25X the thickness of a water molecule.) Water and other small ions are thought by some investigators to pass through protein - lined "pores." Small electrically charged ions would not be permitted such passage if these protein pores had a positive charge similar to the membrane itself. The positive ions would be repelled and negative ions would at first be attracted but then held at the surface (1).
TRANSPORT BY PROTEIN CARRIERS
The fact that some small molecules pass with ease, while others pass through with difficulty or not at all, has resulted in the membrane being regarded as SELECTIVELY PERMEABLE. When various sugars of identical molecular size are present both outside and inside the cell, certain ones cross the membrane hundreds of times faster than others. Observations also show that sugars and amino acids are able to diffuse through the membranes of cells that have been metabolically suppressed. This and other evidence indicates that the passage of these molecules is facilitated by a protein carrier without the expenditure of cellular energy. These carriers are thought to be highly specific and able to combine with only certain molecules. This FACILITATED DIFFUSION, assisted by protein carriers, operates under the same basic rules as does simple diffusion. Substances move only from an area of higher concentration to an area of lower concentration.
In contrast to facilitated diffusion, ACTIVE TRANSPORT requires the use of ATP energy by a carrier protein in order to move ions or molecules through the cell membrane against a concentration gradient. The molecules thus accumulate inside or outside a cell in a region of higher concentration. This process is considered to be responsible for the accumulation of an excess amount of solute within the cells. One example is an increase in the potassium ion concentration on the inside of the cell membrane (a thousand times greater than on the outside of the cell membrane). Other examples include the high concentration of iodine in the cells of the thyroid gland, the virtual removal of sodium from urine by cells lining the kidney tubules, and the total absorption of sugar by intestinal cell of the gut.
Subjecting cells to an increased ion concentration results in an increase in ion uptake, but only up to a point. Raising the concentration above a certain point does not increase the uptake. Graphically, the results of such experiments produce a saturation curve similar to the activity of an enzyme. The manner in which ions seemingly compete for a carrier suggests to some investigators a situation analogous to an enzyme-substrate complex. When a variety of similar ions, such as potassium, rubidium, and cesium are presented to the cell, the uptake of any ion in the group is slower than when the ion is alone. The same is true of calcium, strontium and barium. Interestingly, other close chemical relatives show no competition. Sodium, although similar to cesium, rubidium, and potassium, does not compete with these ions during uptake. The fact that active transport across a membrane can be thought of as an enzymatic activity has prompted some to give the protein carriers an enzymatic name - PERMEASES.
The energy used by the protein carriers might be necessary for the attachment of the molecule or ion, to change the shape of the carrier, to release the molecule, or a combination of all three. The expended energy results in the passage of the ion across the membrane. This renders the carrier useless for active transport until a high energy phosphate molecule is provided by another molecule of ATP. It is not surprising, therefore, that cells involved primarily with active transport, such as those of the kidney, have a large number of mitochondria near the surface membrane where active transport is occurring.
One type of active transport found in all cells, but especially important in muscle and nerve cells, involves the accumulation of sodium ions outside the cell and a corresponding accumulation of potassium ions within the cells. Since these two events are linked together, they are usually referred to as the sodium-potassium pump. The "pump" is thought to be a protein molecule carrier capable of combining with both the sodium and potassium ions. The external portion of the carrier allows the potassium ions to enter the cell while the internal part of the carrier assists the sodium ion to exit the membrane. The protein making up this channel changes its configuration by using energy from ATP molecules. The enzyme ATPase breaks the bond holding the terminal phosphate group in the molecule, thus releasing the bond energy. More than one third of all the energy expended by a cell that is not actively dividing is used to actively transport (Na+) sodium and (K+) potassium ions. Recent clinical investigations indicate a correlation between obesity and low levels of ATPase. This condition could result in a low metabolic rate since the Na+/K+ exchange is estimated to account for 20-50 percent of the body's total heat production and consumes considerable calories.(1,2)
ENDOCYTOSIS AND SURFACE RECEPTORS (1,2)
Endocytosis, although not directly related to active transport, can be more adequately explained using the fluid-mosaic model. At various locations over the surface of the membrane, receptor molecules with irregular protrusions extend beyond the cell. (1) These receptors are specific for larger required molecules. These receptor molecules are capable of diffusing over the surface of the cell by moving among the lipid molecules making up the outer layer of the membrane. This receptor molecule may contact the required molecule and form a receptor-molecule complex.
In general, the glycoprotein receptors are responsible for "self" recognition and identified as the major histocompatibility complex (MHC) proteins. Likewise, glycolipids are the cell surface markers that differentiate the various organs and tissues of the vertebrate body. The markers on the surface of red blood cells that distinguish different blood types are glycolipids. Recent studies have revealed new functions for mammalian glyco-proteins and glycolipids. New glycosylation processes have been discovered which may regulate macromolecular transport across the nuclear pore and may play a role in regulating gene expression. Carbohydrates play a key role in HIV and influenza virus infectivity. In cancer research, carbohydrates are being examined as tumor markers, and for their role in tumor metastasis (3).
Information transmitted between cells is carried by compounds that are often impermeable to the cell membrane. These compounds bind to cell surface receptors. This binding activates reaction pathways that terminate in the release of a compound into the cytoplasm that carries the original information. These compounds are called second messengers and can act either directly or indirectly. Second messengers transmit information regulating a variety of physiological and biochemical processes, including cell growth (4).
As the receptor-molecule complex continues to move over the undulating, irregular membrane surface, it will eventually reach an indented, pit-like region. After moving into the pit-like region, a type of invagination occurs around the receptor-molecule complex with some of the membrane's lipid molecules forming a lipid sphere. It should be noted that this sphere has a hydrophobic tail ends oriented outward making it soluble within the membrane. This sphere can move across the membrane to the cytoplasmic side. Again, the mosaic model provides an explanation of the spreading of the lipid molecules on the inner membrane surface. This in turn allows either for the foreign molecule to be released into the cytoplasm or for the vesicle to pass into the cytoplasm and transfer the molecule into a specific cell organelle. The lipid molecules that make up this transporting vesicle again become part of the membrane structure after the required molecule is released from the receptor-molecule complex.This is sometimes called receptor-mediatied endocytosis.
In summary, the fluid mosaic membrane model appears to resemble a flexible, undulating lipid layer with randomly positioned protein molecules extending partially or completely through the bilayer. Most biologically important organic molecules are insoluble in the center of the bilayer and thus the barrier properties of the membrane. To pass through this nearly impermeable barrier, many compounds require the aid of ATP and a protein molecule acting in some role as a carrier.
(1) Longenecker, N.E., and Hibbs, E.T. The American Biology
Teacher, Vol. 48, No. 5, 5/86, pages 304-306.
(2) Raven, P.H. and Johnson, G.B. Understanding Biology, Times Mirror/Mosby College Publishing, St. Louis, 1988, Chapter 6.
(3) Johnson, M.T. DuPont Biotech Update, Vol. 6, No. 3, August 1991, pages 41-49.
(4) Olechno, J.D. and Rohrer, J.S. American Biotechnology Labora
tory, September 1991, page 8.
(5) Postiglione, R. and Salvemini, J. The American Biology
Teacher, vol. 47, No. 3, 3/85, page 182
(6) Science, October 17, 1986, pages 286-288.