This type of movement is called lateral diffusion and can be measured by the technique called FRAP Figure 3. In this method, a laser strikes and stains a section of the lipid bilayer of a cell, leaving a spot as shown in B. Over time, the stain diffuses out ultimately across the entire lipid bilayer, much like a drop of ink will diffuse throughout when added to a glass of water. A measurement of the rate of diffusion gives an indication of the fluidity of a membrane.
While the movement in lateral diffusion occurs rapidly, movement of molecules from one leaflet over to the other leaflet occurs much more slowly. This type of molecular movement is called transverse diffusion and is almost nonexistent in the absence of enzyme action.
Remember that there is a bias of distribution of molecules between leaflets of the membrane, which means that something must be moving them.
There are three enzymes that catalyze movement of compounds in transverse diffusion. Floppases move membrane lipids in the opposite direction. Scramblases move in either direction. Besides glycerophospholipids and sphingolipids, there are other materials commonly found in lipid bilayers of cellular membranes. Two important prominent ones are cholesterol Figure 3. The flatness and hydrophobicity of the sterol rings allow cholesterol to interact with the nonpolar portions of the lipid bilayer while the hydroxyl group on the end can interact with the hydrophilic part.
It influences membrane fluidity. Figure 3. The mid-point of this transition, referred to as the Tm, is influenced by the fatty acid composition of the lipid bilayer compounds. Longer and more saturated fatty acids will favor higher Tm values, whereas unsaturation and short fatty acids will favor lower Tm values. It is for this reason that fish, which live in cool environments, have a higher level of unsaturated fatty acids in them - to use to make membrane lipids that will remain fluid at ocean temperatures.
Interestingly, cholesterol does not change the Tm value, but instead widens the transition range between frozen and fluid forms of the membrane, allowing it to have a wider range of fluidity. Cholesterol is also abundantly found in membrane structures called lipid rafts.
Depicted in Figure 3. Lipid rafts affect membrane fluidity, neurotransmission, and trafficking of receptors and membrane proteins. Distinguishing features of the rafts is that they are more ordered than the bilayers surrounding them, containing more saturated fatty acids tighter packing and less disorganization, as a result and up to 5 times as much cholesterol. The higher concentration of cholesterol in the rafts may be due to its greater ability to associate with sphingolipids Figure 3.
Some groups, such as prenylated proteins, like RAS, may be excluded from lipid rafts. Lipid rafts may provide concentrating platforms after individual protein receptors bind to ligands in signaling.
After receptor activation takes place at a lipid raft, the signaling complex would provide protection from nonraft enzymes that could inactivate the signal. For example, a common feature of signaling systems is phosphorylation, so lipid rafts might provide protection against dephosphorylation by enzymes called phosphatases. Lipid rafts appear to be involved in many signal transduction processes, such as T cell antigen receptor signaling, B cell antigen receptor signaling, EGF receptor signaling, immunoglobulin E signaling, insulin receptor signaling and others.
For more on signaling, see HERE. Transport of materials across membranes is essential for a cell to exist. The lipid bilayer is an effective barrier to the entry of most molecules and without a means of allowing food molecules to enter a cell, it would die.
The primary molecules that move freely across the lipid bilayer are small, uncharged ones, such as H2O, CO2, CO, and O2, so larger molecules, like glucose, that the cell needs for energy, would be effectively excluded if there were not proteins to facilitate its movement across the membrane. Potential energy from charge and concentration differences are harvested by cells for purposes that include synthesis of ATP, and moving materials against a concentration gradient in a process called active transport.
Proteins in a lipid bilayer can vary in quantity enormously, depending on the membrane. Proteins linked to and associated with membranes come in several types. Transmembrane proteins are integral membrane proteins that completely span from one side of a biological membrane to the other and are firmly embedded in the membrane Figure 3.
Transmembrane proteins can function as docking sites for attachment to the extracellular matrix, for example , as receptors in the cellular signaling system, or facilitate the specific transport of molecules into or out of the cell.
Peripheral membrane proteins interact with part of the bilayer usually does not involve hydrophobic interactions , but do not project through it. A good example is phospholipase A2, which cleaves fatty acids from glycerophospholipids in membranes. Associated membrane proteins typically do not have external hydrophobic regions, so they cannot embed in a portion of the lipid bilayer, but are found near them.
Such association may arise as a result of interaction with other proteins or molecules in the lipid bilayer. A good example is ribonuclease. Anchored membrane proteins are not themselves embedded in the lipid bilayer, but instead are attached to a molecule typically a fatty acid that is embedded in the membrane Figure 3. The oncogene family of proteins known as ras are good examples. These proteins are anchored to the lipid bilayer by attachment to non-polar farnesyl groups catalyzed by the enzyme farnesyltransferase.
A more detailed classification scheme further categorizes the integral and anchored proteins into six different types Figure 3. The plasma membrane protects the cell from its external environment, mediates cellular transport, and transmits cellular signals. The plasma membrane also known as the cell membrane or cytoplasmic membrane is a biological membrane that separates the interior of a cell from its outside environment. The primary function of the plasma membrane is to protect the cell from its surroundings.
Composed of a phospholipid bilayer with embedded proteins, the plasma membrane is selectively permeable to ions and organic molecules and regulates the movement of substances in and out of cells. Plasma membranes must be very flexible in order to allow certain cells, such as red blood cells and white blood cells, to change shape as they pass through narrow capillaries.
The plasma membrane also plays a role in anchoring the cytoskeleton to provide shape to the cell, and in attaching to the extracellular matrix and other cells to help group cells together to form tissues.
The membrane also maintains the cell potential. In short, if the cell is represented by a castle, the plasma membrane is the wall that provides structure for the buildings inside the wall, regulates which people leave and enter the castle, and conveys messages to and from neighboring castles.
Just as a hole in the wall can be a disaster for the castle, a rupture in the plasma membrane causes the cell to lyse and die. The plasma membrane : The plasma membrane is composed of phospholipids and proteins that provide a barrier between the external environment and the cell, regulate the transportation of molecules across the membrane, and communicate with other cells via protein receptors.
Among the most sophisticated functions of the plasma membrane is its ability to transmit signals via complex proteins. These proteins can be receptors, which work as receivers of extracellular inputs and as activators of intracellular processes, or markers, which allow cells to recognize each other.
Membrane receptors provide extracellular attachment sites for effectors like hormones and growth factors, which then trigger intracellular responses.
Some viruses, such as Human Immunodeficiency Virus HIV , can hijack these receptors to gain entry into the cells, causing infections. Membrane markers allow cells to recognize one another, which is vital for cellular signaling processes that influence tissue and organ formation during early development.
The fluid mosaic model describes the plasma membrane structure as a mosaic of phospholipids, cholesterol, proteins, and carbohydrates. The fluid mosaic model was first proposed by S. Singer and Garth L. Nicolson in to explain the structure of the plasma membrane. The model has evolved somewhat over time, but it still best accounts for the structure and functions of the plasma membrane as we now understand them.
The fluid mosaic model describes the structure of the plasma membrane as a mosaic of components —including phospholipids, cholesterol, proteins, and carbohydrates—that gives the membrane a fluid character. Plasma membranes range from 5 to 10 nm in thickness. The proportions of proteins, lipids, and carbohydrates in the plasma membrane vary with cell type. The principal components of a plasma membrane are lipids phospholipids and cholesterol , proteins, and carbohydrates attached to some of the lipids and some of the proteins.
The fluid mosaic model of the plasma membrane : The fluid mosaic model of the plasma membrane describes the plasma membrane as a fluid combination of phospholipids, cholesterol, and proteins. Carbohydrates attached to lipids glycolipids and to proteins glycoproteins extend from the outward-facing surface of the membrane. The main fabric of the membrane is composed of amphiphilic or dual-loving, phospholipid molecules.
The hydrophilic or water-loving areas of these molecules are in contact with the aqueous fluid both inside and outside the cell. Hydrophobic, or water-hating molecules, tend to be non- polar. A phospholipid molecule consists of a three-carbon glycerol backbone with two fatty acid molecules attached to carbons 1 and 2, and a phosphate-containing group attached to the third carbon.
This arrangement gives the overall molecule an area described as its head the phosphate-containing group , which has a polar character or negative charge, and an area called the tail the fatty acids , which has no charge.
They interact with other non-polar molecules in chemical reactions, but generally do not interact with polar molecules.
When placed in water, hydrophobic molecules tend to form a ball or cluster. The hydrophilic regions of the phospholipids tend to form hydrogen bonds with water and other polar molecules on both the exterior and interior of the cell. Thus, the membrane surfaces that face the interior and exterior of the cell are hydrophilic.
Most cells are surrounded by a dilute aqueous medium, which means that key compounds would, if not prevented, constantly leave the cell.
The answer to this problem is to enclose the cytoplasm with a membrane that prevents the free movement of molecules. As described above, a nonpolar material would work well for this membrane because it would not wash away in the surrounding water and would not dissolve water-soluble substances out of the cell.
Figure Detail Early researchers studying cells recognized that there was a boundary layer, but little was known about its structure until, in the s, Charles Overton started a series of studies to determine which molecules were able to cross this boundary layer. Until this time, it was accepted that water was the only material that could easily move into and out of the cell.
Overton showed that nonpolar chemicals were usually able to cross the boundary quite easily, and he published an account of his work Overton in which he explicitly suggested that the boundary layer was a lipid and that other lipids were able to freely enter and pass through.
For a more complete description of Overton's work, see Tanford So, if the cell membrane were a lipid, how would it be organized? In , Rayleigh, working on simple oils, showed that they tend to spread over the surface of water. By measuring the original volume of oil and the final area it covered, he was able to calculate the thickness of the film. This initial observation was improved on by the work of Agnes Pockels. Working in her kitchen, and with no formal training, she devised a simple apparatus to quantify the area covered by the oil film.
Her apparatus was refined by Langmuir and is now generally referred to as a Langmuir trough Figure 2 , although it really should be a Pockels trough. They were interested in determining the amount of lipid in the membranes of red blood cells.
Why use red blood cells? These cells were an excellent choice for this experiment because they have no nucleus or other membrane-bound organelles in the cytoplasm; therefore, any membrane lipids that are found must be those that make up the plasma membrane.
First, the scientists extracted the lipids with a variety of solvents, including acetone, from a known number of cells. Then they used the Langmuir trough to determine how large an area the lipids could cover.
Because they could measure the actual size surface area of a red blood cell and knew approximately how many of those cells they had in their sample , they could calculate the total surface area that would have to be covered by membrane.
When the two numbers were compared, it was clear that the amount of lipid they had extracted could cover twice the area needed to enclose all the cells. Why would there be so much? Additional experiments showed that lipids could spontaneously form a bilayer when mixed with water Figure 1.
Together, these observations suggested that there may be a simple explanation for the results with the red blood cells. The plasma membrane of these cells likely consists of a double layer of lipid surrounding each cell. As it happens, Gortner and Grendel made some errors in their experiment. They failed to completely extract all the lipids from the cells, and they also underestimated the total surface area of the individual red blood cells.
However, because these two errors canceled each other out, their final conclusions turned out to be correct, regardless of their miscalculations. Thereafter, the idea of a lipid bilayer became the basis for future models of membrane structure.
Sadava When the use of electron microscopy started to allow examination of the plasma membrane at high resolution, people noticed that the image clearly showed three layers, not two. In a key paper, Stoeckenius provided clear pictures of the three-layer structure.
He then described in both words and diagrams how the lipid bilayer results in a three-layer image. As it turns out, the inner and outer edges of the bilayer have a different composition than the interior. Under the view of the electron microscope, the outsides of the lipid bilayer show up as two darker layers, whereas the hydrophobic interior stains less densely, thus showing three apparent "layers" outside layers are represented as blue in Figure 1C. The first clues to lipid bilayer structure came from results with red blood cell membranes.
The ultimate discovery that the plasma membrane is a lipid bilayer with hydrophobic and hydrophilic properties changed the way this structure was viewed. Its semipermeable and liquid nature provided the groundwork for understanding both its physical and biological properties. Edidin, M. Lipids on the frontier: a century of cell-membrane lipids Nature Reviews : Molecular Cell Biology 4 : — Gortner, E. On bimolecular layers of lipoids on the chromacytes of blood.
Journal of Experimental Medicine 41 , — Langmuir, I. The constitution and fundamental properties of solids and liquids II: Liquids. Journal of the American Chemical Society 39 , — Overton, E. The probable origin and physiological significance of cellular osmotic properties.
Vierteljahrschrift der Naturforschende gesselschaft 44 , 88— In Biological Membrane Structure , trans.
0コメント