Molecular Biology of the Cell: Membrane Structure PDF

Molecular Biology of the Cell Membrane Structure Mayank Gururani, PhD.  CHAPTER CONTENTS THE LIPID BILAYER MEMBRANE PROTEINS  Cell Membrane Cell membranes are crucial to the life of the cell ✓ The plasma membrane encloses the cell, ✓ Defin...

Molecular Biology of the Cell Membrane Structure Mayank Gururani, PhD.  CHAPTER CONTENTS THE LIPID BILAYER MEMBRANE PROTEINS  Cell Membrane Cell membranes are crucial to the life of the cell ✓ The plasma membrane encloses the cell, ✓ Defines its boundaries, ✓ Maintains the essential differences between the cytosol and the extracellular environment, ✓ Contains proteins that act as sensors of extracellular signals, allowing the cell to change its behavior in response to environmental signals, including signals from other cells. Biological membranes: different functions BUT all have common structure A three dimensional schematic view of a cell membrane and the general deposition of its lipid and protein constituents  THE LIPID BILAYER The lipid bilayer provides the basic structure for all cell membranes Lipid molecules constitute about 50% of the mass of most animal cell membranes, nearly all of the remainder being protein. Introduction It is easily seen by electron microscopy An electron micrograph of a segment of the plasma membrane of a human red blood cell seen in cross section, showing its bilayer structure  THE LIPID BILAYER Membrane bilayer structure is attributable exclusively to the special properties of the lipid molecules, which assemble spontaneously into bilayers even under simple artificial conditions All of the lipid molecules in cell membranes are amphiphilic Amphiphilic, that is, they have a hydrophilic (“water-loving”) or polar end and a hydrophobic (“water-fearing”) or nonpolar end.  THE LIPID BILAYER Phospholipids Spontaneously Form Bilayers The shape and amphiphilic nature of the phospholipid molecules cause them to form bilayers spontaneously in aqueous environment Hydrophilic molecules dissolve readily in water because they contain charged groups or uncharged polar groups that can form either favorable electrostatic interactions or hydrogen bonds with water molecules Hydrophobic molecules are insoluble in water because all, or almost all, of their atoms are uncharged and nonpolar and therefore cannot form energetically favorable interactions with water molecules How hydrophilic and hydrophobic molecules interact differently with water Packing arrangements of amphiphilic molecules in an aqueous environment A micelle and a bilayer seen in cross section Phosphoglycerides, Sphingolipids, and Sterols Are the Major Lipids in Cell Membranes Ester bond The parts of a typical phospholipid molecule Phosphoglycerides, Sphingolipids, and Sterols Are the Major Lipids in Cell Membranes A polar head group The most abundant containing membrane lipids are the phosphate group phospholipids Two hydrophobic hydrocarbon tails In animal, plant, and bacterial cells, the tails are usually fatty acids Phosphoglycerides, Sphingolipids, and Sterols Are the Major Lipids in Cell Membranes ▪ The main phospholipids in most animal cell membranes are the phosphoglycerides, which have a three- carbon glycerol backbone ▪ Two 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 ▪ By combining several different fatty acids and head groups, cells make many different phosphoglycerides Phosphoglycerides, Sphingolipids, and Sterols Are the Major Lipids in Cell Membranes 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, Sphingolipids, and Sterols Are the Major Lipids in Cell Membranes Phosphoglycerides, Sphingolipids, and Sterols Are the Major Lipids in Cell Membranes Sphingolipids, which are built from sphingosine rather than glycerol significant players in multiple biological processes such as signal transduction, stress responses, immune reaction, membrane structure, and brain development. Sphingosine is a long acyl chain with an amino group and two hydroxyl groups at one end (arrows in orange) 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 o The phospholipids Phosphatidylethanolamine, phosphatidylserine, phosphatidylcholine, and sphingomyelin are: ✓ the most abundant ones in mammalian cell membranes ✓ constitute more than half the mass of lipid in most mammalian cell membranes Glycosylation Glycosylation refers to the process of covalently attaching a carbohydrate to a protein or lipid Glycolipid: carbohydrate attached to lipid Glycoprotein: carbohydrate attached to protein Glycosylation has functional consequences: - Carbohydrates attached to lipids and proteins can serve as recognition signals for other cellular proteins. Example: glycoproteins carrying a mannose-6-phosphate sugar a recognized as protein to be targeted to the lysosome. - Membrane glycoproteins and glycolipids often play a role in cell surface recognition - Carbohydrates have a protective effect. Cell coat or glycocalyx - carbohydrate-rich zone on the cell surface shielding cell from mechanical and physical damage. Glycolipids and Cholesterol ▪ 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 The structure of cholesterol Eukaryotic plasma membranes contain especially large amount of cholesterol—up to one molecule for every phospholipid molecule The structure of cholesterol Cholesterol is a sterol It contains a rigid ring structure, to which is attached a single polar hydroxyl group and a short nonpolar hydrocarbon chain The structure of cholesterol The cholesterol molecules orient themselves in the bilayer with their hydroxyl group (OH) close to the polar head groups of adjacent phospholipid molecules Phospholipids Spontaneously Form Bilayers A lipid bilayer also has other characteristics that make it an ideal structure for cell membranes One of the most important of these is its fluidity, which is crucial to many membrane functions  THE LIPID BILAYER The Lipid Bilayer Is a Two-dimensional Fluid Membrane fluidity refers to the viscosity of the lipid bilayer of a cell membrane Viscosity of the membrane can affect the rotation and diffusion of proteins and other bio-molecules within the membrane, thereby affecting their functions.  Membranes are semifluid Biomembranes are Semifluid: most lipids can rotate freely around their long axes and move laterally within the membrane leaflet. (two-dimensional motion). Rotational and lateral movement keep the fatty acid tails within the hydrophobic interior. Such movement does not need energy. At 37 C lipid molecules exchanges places with its neighbors about 107 times/sec. In contrast to rotational and lateral movements, the “Flipflop” of lipids from one leaflet to the opposite leaflet does not occur spontaneously. The transport of lipids from one leaflet to another requires the action of the enzyme flippase, which uses energy from the hydrolysis of ATP to flip a lipid from one leaflet to another Lateral movement Rotational movement  THE LIPID BILAYER The Fluidity of a Lipid Bilayer Depends on Its Composition and Its Temperature The fluidity of cell membrane has to be precisely regulated Certain membrane transport processes and enzyme activities, for example, cease when the bilayer viscosity is experimentally increased beyond a threshold level.  THE LIPID BILAYER Cell membrane fluidity can be influenced by: Length of fatty acyl tails: range from 14 to 24 carbon atoms Shorter acyl tails are less likely to interact, which makes the membrane more fluid Presence of double bonds in the acyl tails (unsaturated): Double bond creates a kink in the fatty acyl tail, making it more difficult for neighboring tails to interact and making the bilayer more fluid Presence of cholesterol (short and rigid molecule produced by animal cells): Cholesterol tends to stabilize membranes. At higher temperature (mammals), cholesterol makes the membrane less fluid.. At lower temperature, cholesterol makes the membrane more fluid and prevents it from freezing. The influence of cis-double bonds in hydrocarbon chains The Fluidity of a Lipid Bilayer Depends on Its Composition Bacteria, yeast, and other organisms whose temperature fluctuates with that of their environment adjust the fatty acid composition of their membrane lipids to maintain a relatively constant fluidity As the temperature falls, for instance, the cells of those organisms synthesize fatty acids with more cis-double bonds, and they avoid the decrease in bilayer fluidity The Fluidity of a Lipid Bilayer Depends on Its Composition  THE LIPID BILAYER The lipid compositions of the two monolayers of the lipid bilayer in many membranes are strikingly different (asymmetrical) Membrane-bound phospholipid translocators generate and maintain lipid asymmetry The Asymmetry of the Lipid Bilayer Is Functionally Important The asymmetrical distribution of phospholipids and glycolipids in the lipid bilayer of human red blood cells The Asymmetry of the Lipid Bilayer Is Functionally Important In human red blood cell (erythrocyte) membrane, almost all of the phospholipid molecules that have choline—(CH3)3N+CH2CH2OH—in their head group (phosphatidylecholine and sphingomyeline) are in the outer monolayer, whereas, almost all that contain a terminal primary amino group (phosphatidylethanolamine and phosphatidylserine) are in the inner monolayer  THE LIPID BILAYER The Asymmetry of the Lipid Bilayer Is Functionally Important Converting extracellular signals into intracellular ones 1. Signal perception 2. Intracellular 3. Signal cellular signal transduction response Extracellular cytosol environment The Asymmetry of the Lipid Bilayer Is Functionally Important  THE LIPID BILAYER Glycolipids occur in all eukaryotic cell plasma membranes, where they generally constitute about 5% of the lipid molecules in the outer monolayer The most complex of the glycolipid, the gangliosides Gangliosides contain oligosaccharides with one or more sialic acid moieties, which give gangliosides a net negative charge The most abundant of the more than 40 different gangliosides that have been identified are in the plasma membrane of nerve cells (5-10% of the lipid mass)  THE LIPID BILAYER The function of glycolipids come from their localization In the plasma membranes of Epithelial cells, glycolipids may help to protect the plasma membrane against the harsh conditions frequently found there Charged glycolipids may be important because of their electrical effects Glycolipids also function in cell-recognition processes Some glycolipids provide entry points for certain bacterial toxins and viruses (e.g. Vibrio cholerae and polyomaviruses.