Cell plasma membrane phospholipid bilayer

Phospholipids are found in all living cells and typically constitute about half of the mass of animal cell plasma membranes (Cevc, 1992). The reason forthe variety of membrane lipids might simply be that these amphiphilic structures have in common the ability to arrange as bilayers in an aqueous environment (Paltauf and Hermetter, 1990). Thus, the use of endogenous phospholipids to form vesicles as drug carriers may have much less adverse effects in patients compared to synthetic drui carrier molecules. 

The stracture of the cell plasma membrane is illustrated in Figure 7.7. The phosphohpid molecules aggregate into a bilayer which serves to remove the hydrophobic chains from the aqueous environment and place a polar, hydrophilic head group at eaeh side of the bilayer which is exposed to water. The bilayer aggregate is liquid crystalline in nature, in that the head groups do not have any periodic ordering and the hydrocarbon chains are not rigid. The liquidity of the stracture aUows the movement of phospholipid molecules about the cell membrane. Of course, the associated proteins can also move within the cell membrane but they do so more slowly. This liquid crystalline stractrrre of the cell membrane provides form and allows the selective movement of materials in and out of the cells. 

In a cell (plasma) membrane, two layers of phospholipids are arranged with their hydrophilic heads at the outer and inner surfaces of the membrane, and their hydrophobic tails in the center. This double layer arrangement of phospholipids is called a lipid bilayer (see Figure 15.11). The outer layer of phosphohpids is in contact with the external fluids, and the inner layer is in contact with the internal contents of the ceU. 

The movement of substances between the blood and the extracellular fluid surrounding the cells in most tissues of the body occurs very readily. This exchange takes place at the level of the capillaries, the smallest blood vessels in the cardiovascular system whose walls are formed by a single layer of endothelial cells. Lipid-soluble substances are able to move across this layer of endothelial cells at any point because they can move directly through the plasma membrane by passing between the phospholipid molecules of the bilayer. The movement of water-soluble substances is limited to the multiple pores found between the cells however, it also takes place rapidly and efficiently. 

The plasma membrane, a phospholipid bilayer in which cholesterol and protein molecules are embedded. The bottom layer, which faces the cytoplasm, has a slightly different phospholipid composition from that of the top layer, which faces the external medium. While phospholipid molecules can readily exchange laterally within their own layer, random exchange across the bilayer is rare. Both globular and helical kinds of protein traverse the bilayer. Cholesterol molecules tend to keep the tails of the phospholipids relatively fixed and orderly in the regions closest to the hydrophilic heads the parts of the tails closer to the core of the membrane move about freely. This model is not believed to apply to blood or lymph capillaries. (Reprinted with permission from Bretscher MS. The molecules of the cell membrane. Sci Am 1985 253 104. Copyright 1985 by Scientific American, Inc. All rights reserved.)... 

The boundary between a living cell and its surroundings is the incredibly thin (7-10 ran) plasma membrane. This vital partition, which controls the flow of materials into and out of a cell and which senses and controls the response of cells to hormones and other external signals, consists largely of phospholipids together with embedded proteins. Tire nonpolar chains of the phospholipids stick together to form a double molecular layer or bilayer which provides the basic structure of almost all biological membranes. 

The idea of a lipid bilayer was first proposed by E. Gorter and F. Grendel, who showed in 1925 that the phospholipid content of the erythrocyte plasma membrane is approximately the amount needed to enclose the cell with a bilayer. Subsequent x-ray diffraction measurements confirmed this picture (fig. 17.8). 

We have not yet said much about the second major constituent of eukaryotic membrane lipids, cholesterol. Cholesterol broadens the melting transition of the phospholipid bilayer (see fig. 17.20). Below the Tm, cholesterol disorders the membrane because it is too bulky to fit well into the neatly packed arrangement of the fatty acid chains that is favored at low temperatures. Above the Tm, cholesterol restricts further disordering because it is too large and inflexible to join in the rapid fluctuations of the chains. If the amount of cholesterol in a phospholipid bilayer is increased to about 30%, roughly the amount in the plasma membranes of typical animal cells, the melting transition becomes so broad as to be almost undetectable. 

Whereas a major function of biological membranes is to maintain the status quo by preventing loss of vital materials and entry of harmful substances, membranes must also engage in selective transport processes. Living cells depend on an influx of phosphate and other ions, and of nutrients such as carbohydrates and amino acids. They extrude certain ions, such as Na+, and rid themselves of metabolic end products. How do these ionic or polar species traverse the phospholipid bilayer of the plasma membrane How do pyruvate, malate, the tricarboxylic acid citrate and even ATP move between the cytosol and the mitochondrial matrix (see figs. 13.15 and 14.1) The answer is that biological membranes contain proteins that act as specific transporters, or permeases. These proteins behave much like conventional enzymes They bind substrates and they release products. Their primary function, however, is not to catalyze chemical reactions but to move materials from one side of a membrane to the other. In this section we discuss the general features of membrane transport and examine the structures and activities of several transport proteins. 

The Singer and Nicholson (13) model for the plasma membrane, which now receives much support, is basically a smectic liquid crystal consisting of one bilayer of phospholipid (Figure 4a). The phospholipid bilayer contains cholesterol at a concentration which depends on cell type. Embedded in the lipid liquid crystal he protein molecules. Some of these protein molecules transverse the entire lipid bilayer and communicate both with the inside and the outside of the cells. Some of these may... 

Transmission of extracellular signals to the cell interior is based on receptor-induced recruitment and assembly of proteins into signaling complexes at the inner leaflet of the plasma membrane. Protein-protein and protein-lipid interactions play a crucial role in the process in which molecular proximity in specially formed membrane subdomains provides the special and temporal constraints that are required for proper signaling. The phospholipid bilayer is not merely a passive hydrophobic medium for this assembly process, but is also a site where the lipid and the protein components are enriched by a dynamic process (see Chapter 5). 

The bulk constituent of cells is water (H20). The cell membrane or plasma membrane (PM) that encloses the living cell is basically composed of a phospholipid bilayer, a 0.01 micrometre ( xm) (10 nm) thick bimolecular layer of hydrophobic (or water repelling) fatty molecules. In eukaryotes (organisms having a nucleus) there is a phospholipid bilayer PM enclosing the cell. Similar membranes bound specialized intracellular organelles, namely the endoplasmic reticulum (ER), ER-associated Golgi vesicles, lysosomes, vacuoles, peroxisomes, nucleus and mitochondria (and, additionally, the chloroplasts in plant cells). 

---