Chapter 7 Membrane Structure and Function Life at the Edge The plasma membrane is the boundary that separates the living cell from its surroundings and controls traffic into and out of the cell it surrounds Selective permeability allows some substances to cross it more easily than others The ability of the cell to discriminate in its chemical exchanges with its environment is fundamental to life 7 1 Cellular Membranes are Fluid Mosaics of Lipids and Proteins Lipids and proteins are the staple ingredients of membranes The most abundant lipids in most membranes are phospholipids Amphipathic both a hydrophilic region and a hydrophobic region A phospholipid bilayer can exist as a stable boundary between two aqueous compartments because the molecular arrangement shelters the hydrophobic tails of the phospholipids from water while exposing the hydrophilic heads to water Most membrane proteins are amphipathic Proteins can reside in the phospholipid bilayer with their hydrophilic regions protruding This molecular orientation maximizes contact of hydrophilic regions of proteins with water in the cytosol and extracellular fluid while providing their hydrophobic parts with a nonaqueous environment Fluid mosaic model membrane is a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids Proteins are not randomly distributed in the membrane A The Fluidity of Membranes Membrane is held together primarily by hydrophobic interactions Weaker than covalent bonds A membrane remains fluid as temperature decreases until the phospholipids settle into a closely Lateral movement of phospholipids within the membrane is rapid packed arrangement and the membrane solidifies Some proteins move in a highly directed manner but some are immobile The membrane remains fluid to a lower temperature if it is rich in phospholipids with unsaturated Unsaturated hydrocarbon tails cannot pack together as closely as saturated hydrocarbon tails hydrocarbon tails making the membrane more fluid Cholesterol makes the membrane less fluid by restraining phospholipid movement Cholesterol hinders the close packing of phospholipids it lowers the temperature required for the membrane to solidify Cholesterol can be though of as a fluidity buffer for the membrane resisting changes in membrane fluidity that can be caused by changes in temperature Membranes must be fluid to work properly the fluidity affects both its permeability and the ability of membrane proteins to move to where their function is needed About as fluid as salad oil When a membrane solidifies its permeability changes and enzymatic proteins may become inactive Membranes too fluid cannot support protein function either B Evolution of Differences in Membrane Lipid Composition Variations in the cell membrane lipid compositions of many species appear to be evolutionary adaptations that maintain the appropriate membrane fluidity under specific environmental conditions C Membrane Proteins and Their Functions fluid matrix of the lipid bilayer A membrane is a collage of different proteins often clustered together in groups embedded in the Phospholipids form the main fabric of the membrane but proteins determine most of the membranes functions Integral Proteins penetrate the hydrophobic interior of the lipid bilayer The majority are trans membrane proteins which span the membrane Other integral proteins extend only partway into the hydrophobic interior hydrophobic regions of an integral protein consist of one or more stretches of nonpolar amino acids usually coiled into alpha helices Peripheral protein not embedded in the lipid bilayer at all they are appendages loosely bound to the surface of the membrane often to exposed parts of integral proteins On the cytoplasmic side of the plasma membrane some membrane proteins are held in place by attachment to the cytoskeleton matrix On the extracellular side certain membrane proteins are attached to fibers of the extracellular A single cell may have cell surface membrane proteins that carry out several different functions such as transport through the cell membrane enzymatic activity or attaching a cell to either a neighboring cell or the extracellular matrix The membrane is not only a structural mosaic but also a functional one Functions of membrane proteins o Transport o Enzymatic Activity o Signal Transduction o Cell Cell Recognition o Intercellular Joining o Attachment to the Cytoskeleton and Extracellular Matrix ECM Proteins on a cell s surface are important in the medical field D The Role of Membrane Carbohydrates in Cell Cell Recognition Cell Cell Recognition a cell s ability to distinguish on type of neighboring cell from another is crucial to the functioning of an organism Cell to Cell is the basis for the rejection of foreign cells by the immune system Cells recognize other cells by binding to molecules often containing carbohydrates on the extracellular surface of the plasma membrane Some are covalently bonded to lipids forming glycolipids Membrane carbohydrates are usually short branched chains of fewer than 15 sugar units Most are covalently bonded to proteins which are glycoproteins The diversity of the molecules and their location o the cell s surface enable membrane carbohydrates to function as markers that distinguish one cell from another E Synthesis and Sidedness of Membranes The asymmetrical arrangement of proteins lipids and their associated carbohydrates in the plasma membrane is determined as the membrane is being built by the endoplasmic reticulum ER and Golgi apparatus components of the endomembrane system 7 2 Membrane Structure Results in Selective Permeability The biological membrane is an exquisite example of a supramolecular structure many molecules ordered into a higher level of organization with emergent properties beyond those of the individual molecules The fluid mosaic model helps explain how membranes regulate the cell s molecular traffic A steady traffic of small molecules and ions moves across the plasma membrane in both directions Sugars amino acids and other nutrients enter the cell and metabolic waste products leave it A The Permeability of the Lipid Bilayer Nonpolar molecules such as hydrocarbons CO2 and O2 are hydrophobic They can therefore dissolve in the lipid bilayer of the membrane and cross it easily without the aid of membrane proteins Hydrophobic interior of the membrane impedes direct passage through the membrane of ions and polar molecules which are hydrophilic B
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