Cell Membrane I PDF

Cell Membrane I Asst. Prof. Yalda Hekmatshoar Email: [email protected]  Despite their differing functions, all biological membranes have a common general structure: veach is a very thin film of lipid and protein molecules, held together mainly by noncovalent interaction...

Cell Membrane I Asst. Prof. Yalda Hekmatshoar Email: [email protected]  Despite their differing functions, all biological membranes have a common general structure: veach is a very thin film of lipid and protein molecules, held together mainly by noncovalent interactions.  Cell membranes ü are crucial to the life of the cell. ü The plasma membrane encloses the cell, defines its boundaries, and maintains the essential differences between the cytosol and the extracellular environment. ü Inside eukaryotic cells, the membranes of the nucleus, endoplasmic reticulum, Golgi apparatus, mitochondria, and other membrane-enclosed organelles maintain the characteristic differences between the contents of each organelle and the cytosol. ü Ion gradients across membranes, established by the activities of specialized membrane proteins, can be used to synthesize ATP, to drive the transport of selected solutes across the membrane, or, as in nerve and muscle cells, to produce and transmit electrical signals. ü In all cells, the plasma membrane also contains proteins that act as sensors of external signals, allowing the cell to change its behavior in response to environmental cues, including signals from other cells; these protein sensors, or receptors, transfer information—rather than molecules— across the membrane.  Cell membranes are, dynamic, fluid structures, most of their molecules move about in the plane of the membrane. The lipid molecules are arranged as a continuous double layer about 5 nm thick. This lipid bilayer provides the basic fluid structure of the membrane and serves as a relatively impermeable barrier to the passage of most water-soluble molecules. Most membrane proteins span the lipid bilayer and mediate nearly all of the other functions of the membrane, including the transport of specific molecules across it, and the catalysis of membrane-associated reactions such as ATP synthesis. In the plasma membrane, some transmembrane proteins serve as structural links that connect the cytoskeleton through the lipid bilayer to either the extracellular matrix or an adjacent cell, while others serve as receptors to detect and transduce chemical signals in the cell’s environment. It takes many kinds of membrane proteins to enable a cell to function and interact with its environment, and it is estimated that about 30% of the proteins encoded in an animal’s genome are membrane proteins. THE LIPID BILAYER ü Provides the basic structure for all cell membranes. ü It is easily seen by electron microscopy, and its bilayer structure is attributable exclusively to the special properties of the lipid molecules, which assemble spontaneously into bilayers even under simple artificial conditions. Phosphoglycerides, Sphingolipids, and Sterols Are the Major Lipids in Cell Membranes ü Lipid molecules constitute about 50% of the mass of most animal cell membranes, nearly all of the remainder being protein. ü All of the lipid molecules in cell membranes are amphiphilic —that is, they have a hydrophilic (“water-loving”) or polar end and a hydrophobic (“water-fearing”) or nonpolar end. Phospholipids ü The most abundant membrane lipids. ü These have a polar head group containing a phosphate group and two hydrophobic hydrocarbon tails. ü In animal, plant, and bacterial cells, the tails are usually fatty acids, and they can differ in length (they normally contain between 14 and 24 carbon atoms). One tail typically has one or more cis-double bonds (that is, it is unsaturated), while the other tail does not (that is, it is saturated). each cis-double bond creates a kink in the tail. Differences in the length and saturation of the fatty acid tails influence how phospholipid molecules pack against one another, thereby affecting the fluidity of the membrane. Phosphoglycerides üThe main phospholipids in most animal cell membranes, üwhich have a three-carbon glycerol backbone, üTwo long-chain fatty acids are linked through ester bonds to adjacent carbon atoms of the glycerol, and the third carbon atom of the glycerol is attached to a phosphate group, which in turn is linked to one of several types of head group. üBy combining several different fatty acids and head groups, cells make many different phosphoglycerides. üPhosphatidylethanolamine, phosphatidylserine, and phosphatidylcholine are the most abundant ones in mammalian cell membranes  sphingolipids ü Another important class of phospholipids ü They are built from sphingosine rather than glycerol, ü is a long acyl chain with an amino group (NH2) and two hydroxyl groups (OH) at one end. ü In sphingomyelin, the most common sphingolipid, a fatty acid tail is attached to the amino group, and a phosphocholine group is attached to the terminal hydroxyl group. ü Together, the phospholipids phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, and sphingomyelin constitute more than half the mass of lipid in most mammalian cell membranes.  In addition to phospholipids, the lipid bilayers in many cell membranes contain glycolipids and cholesterol. Glycolipids resemble sphingolipids, but, instead of a phosphate-linked head group, they have sugars attached. cholesterol ü Eukaryotic plasma membranes contain especially large amounts of cholesterol Ø up to one molecule for every phospholipid molecule. ü Cholesterol is a sterol. ü It contains a rigid ring structure, to which is attached a single polar hydroxyl group and a short nonpolar hydrocarbon chain. ü The cholesterol molecules orient themselves in the bilayer with their hydroxyl group close to the polar head groups of adjacent phospholipid molecules. Glycolipids üAre Found on the Surface of All Eukaryotic Plasma Membranes üSugar-containing lipid molecules called glycolipids ü have the most extreme asymmetry in their membrane distribution: these molecules, Øwhether in the plasma membrane or in intracellular membranes, are found exclusively in the monolayer facing away from the cytosol. Glycolipids probably occur in all eukaryotic cell plasma membranes, Øwhere they generally constitute about 5% of the lipid molecules in the outer monolayer. ü The shape and amphiphilic nature of the phospholipid molecules cause them to form bilayers spontaneously in aqueous environments. ü They spontaneously aggregate to bury their hydrophobic tails in the interior, where they are shielded from the water, and they expose their hydrophilic heads to water. ü Depending on their shape, they can do this in either of two ways: 1. They can form spherical micelles , with the tails inward, 2. They can form double-layered sheets, or bilayers , with the hydrophobic tails sandwiched between the hydrophilic head groups The Lipid Bilayer Is a Two-dimensional Fluid Around 1970, researchers first recognized that individual lipid molecules are able to diffuse freely within the plane of a lipid bilayer. The initial demonstration came from studies of synthetic (artificial) lipid bilayers, which can be made in the form of spherical vesicles, called liposomes ; or in the form of planar bilayers formed across a hole in a partition between two aqueous compartments or on a solid support.  Lipid asymmetry is functionally important, üEspecially in converting extracellular signals into intracellular ones. üMany cytosolic proteins bind to specific lipid head groups found in the cytosolic monolayer of the lipid bilayer. üThe enzyme protein kinase C (PKC), for example, which is activated in response to various extracellular signals, binds to the cytosolic face of the plasma membrane, where phosphatidylserine is concentrated, and requires this negatively charged phospholipid for its activity.  The translocation of the phosphatidylserine in apoptotic cells is thought to occur by two mechanisms: 1. The phospholipid translocator that normally transports this lipid from the outer monolayer to the inner monolayer is inactivated. 2. A “scramblase” that transfers phospholipids nonspecifically in both directions between the two monolayers is activated.