The first theory of membrane structure proposed that cells are surrounded by a double layer of lipid molecules, a lipid bilayer. This theory was based on the known tendency of lipid molecules to form lipid bilayers with low permeability to water-soluble molecules. However, the lipid bilayer theory did not explain the selective movement of certain water-soluble compounds, such as glucose and amino acids, across the plasma membrane. In 1972, Singer and Nicolson proposed the fluid mosaic model of the plasma membrane (Fig. 2.1). With minor modifications, this model is still accepted as the correct picture of the structure of the plasma membrane.
The Plasma Membrane Has Proteins Inserted in the Lipid Bilayer
Proteins and lipids are the two major components of the plasma membrane, present in about equal proportions by weight. The various lipids are arranged in a lipid bilayer, and two different types of proteins are associated with this bilayer. Integral proteins (or intrinsic proteins) are embedded in the lipid bilayer,- many span it completely, being accessible from the inside and outside of the membrane. The polypeptide chain of these proteins may cross the lipid bi-layer once or may make multiple passes across it. The membrane-spanning segments usually contain amino acids with nonpolar side chains and are arranged in an ordered a-helical conformation. Peripheral proteins (or extrinsic proteins) do not penetrate the lipid bilayer. They are in contact with the outer side of only one of the lipid layers—either the layer facing the cytoplasm or the layer facing the extracellular fluid (see Fig. 2.1). Many membrane proteins have carbohydrate molecules, in the form of specific sugars, attached to the parts of the proteins that are exposed to the extracellular fluid. These molecules are known as glycoproteins. Some of the integral membrane proteins can move in the plane of the membrane, like small boats floating in the "sea" formed by the bilayer arrangement of the lipids. Other membrane proteins are anchored to the cytoskeleton inside the cell or to proteins of the extracellular matrix.
The proteins in the plasma membrane play a variety of roles. Many peripheral membrane proteins are enzymes, and many membrane-spanning integral proteins are carriers or channels for the movement of water-soluble molecules and ions into and out of the cell. Another important role of membrane proteins is structural,- for example, certain membrane proteins in the erythrocyte help maintain the biconcave shape of the cell. Finally, some membrane proteins serve as highly specific recognition sites or receptors on the outside of the cell membrane to which extracellular molecules, such as hormones, can bind. If the receptor is a membrane-spanning protein, it provides a mechanism for converting an extracellular signal into an intracellular response.
Lipids found in cell membranes can be classified into two broad groups: those that contain fatty acids as part of the lipid molecule and those that do not. Phospholipids are an example of the first group, and cholesterol is the most important example of the second group.
The fluid mosaic model of the plasma "membrane. Lipids are arranged in a bilayer. Integral proteins are embedded in the bilayer and often span it. Some membrane-spanning proteins form channels. Peripheral proteins do not penetrate the bilayer.
Phospholipids. The fatty acids present in phospholipids are molecules with a long hydrocarbon chain and a car-boxyl terminal group. The hydrocarbon chain can be saturated (no double bonds between the carbon atoms) or unsaturated (one or more double bonds present). The composition of fatty acids gives them some peculiar characteristics. The long hydrocarbon chain tends to avoid contact with water and is described as hydrophobic. The carboxyl group at the other end is compatible with water and is termed hydrophilic. Fatty acids are said to be amphi-pathic because both hydrophobic and hydrophilic regions are present in the same molecule.
Phospholipids are the most abundant complex lipids found in cell membranes. They are amphipathic molecules formed by two fatty acids (normally, one saturated and one unsaturated) and one phosphoric acid group substituted on the backbone of a glycerol or sphingosine molecule. This arrangement produces a hydrophobic area formed by the two fatty acids and a polar hydrophilic head. When phos-pholipids are arranged in a bilayer, the polar heads are on the outside and the hydrophobic fatty acids on the inside. It is difficult for water-soluble molecules and ions to pass directly through the hydrophobic interior of the lipid bilayer.
The phospholipids, with a backbone of sphingosine (a long amino alcohol), are usually called sphingolipids and are present in all plasma membranes in small amounts. They are especially abundant in brain and nerve cells.
Glycolipids are lipid molecules that contain sugars and sugar derivatives (instead of phosphoric acid) in the polar head. They are located mainly in the outer half of the lipid bilayer, with the sugar molecules facing the extracellular fluid.
Cholesterol. Cholesterol is an important component of mammalian plasma membranes. The proportion of cholesterol in plasma membranes varies from 10% to 50% of total lipids. Cholesterol has a rigid structure that stabilizes the cell membrane and reduces the natural mobility of the complex lipids in the plane of the membrane. Increasing amounts of cholesterol make it more difficult for lipids and proteins to move in the membrane. Some cell functions, such as the response of immune system cells to the presence of an antigen, depend on the ability of membrane proteins to move in the plane of the membrane to bind the antigen. A decrease in membrane fluidity resulting from an increase in cholesterol will impair these functions.
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