CHAPTER SEVEN Membrane Structure and Function PLASMA MEMBRANE edge of life Separates living cells from surroundings Selective permeability allows some substances to cross more easily than others Molecular makeup of each membrane allows compartimentalized specialization in cells CELL MEMBRANE FLUID MOSAICS OF LIPIDS AND PROTEINS Membranes Lipids Proteins o Phospholipids form membranes o Amphipathic Hydrophobic hydrophilic region FLUID MOSAIC MODEL membrane is a fluid structure w a mosaic of various proteins attached to a double layer of phospholipids membranes are not static sheets of molecules locked rigidly in place membranes are held together by hydrophobic interactions weaker than covalent bonds lipids and some proteins shift laterally like elbowing across a room RAPID MOVEMENT rare for molecules to flip flop transversely switching from one phospholipid layer to another proteins are larger than lipids and move slowly membrane fluid temperature decreases until the phospholipids settle into closely packed arrangements and the membrane solidifies the temp at which a membrane solidifies depends on the types of lipids its made up of membrane remains fluid lower temps if it is rich in phospholipids with unsaturated hydrocarbon tails unsaturated hydrocarbon tails cannot pack closely together bc of kinks in tails where double bonds are located unsaturated hydrocarbon tails more fluid membranes must be fluid to work properly membrane solidifies permeability changes enzymatic proteins may become inactive lipid comp of cell membranes can change as an adjustment to changing temp FLUID MOSAIC membrane is a collage of different proteins embedded in lipid bilayer diff types of cells contain diff sets of membrane proteins various membranes w I a cell each have a unique collection of proteins MEMBRANE PROTEINS Integral proteins penetrate hydrophobic core of the lipid bilayer o Some are transmembrane o integrins Peripheral proteins not embedded in lipid bilayer at all o Loosely bound to surface of membrane o Exposed to parts of integral proteins Membrane proteins are held in place by attachment to the cytoskeleton stronger framework than just plasma membrane alone Functions of membrane proteins Transport Enzymatic activity Signal transduction Cell cell recognition o Cells ability to distinguish one type of cell from another o Important in sorting cells into tissues and organs o Basis for rejection of foreign cells o Recognize by binding to surface molecules on plasma memrbane Intercellular joining Attachment to cytoskeleton and ECM membranes have distinct inside outside faces when a vesicle fuses to the plasma membrane the outside layer of the vesicle becomes continuous w the cytoplasmic layer of the plasma membrane SYNTHESIS OF MEMBRANE COMPONENTS 1 Synthesis of membrane proteins and lipids in the ER a Carbs are added to proteins glycoproteins 2 In the golgi the glycoproteins undergo carbohydrate modification acquire carbohydrates glycolipids 3 Transmembrane proteins membrane glycolipids secretory proteins are transported in vesicles to the plasma membrane 4 Vesicles fuse w the membrane releasing secretory proteins from the cell a Vesicle fusion positions glycoproteins glycolipids on the outside of the plasma membrane MEMBRANE STRUCTURE SELECTIVE PERMEABILITY Nonpolar molecules hydrocarbons CO2 Oxygen hydrophobic Can dissolve easily in the lipid bilayer cross it easily Hydrophobic core of membranes delay direct passage of ions polar molecules which are hydrophilic through the membrane Charged atoms or ions are very hard to pass through the plasma membrane lipid bilayer is the only aspect of the gatekeeper system responsible for the selective permeability of a cell proteins built into the membrane play key roles in regulating transport Cell membranes are permeable to specific ions polar molecules These hydrophilic substances can avoid contact w lipid bilayer by passing through transport proteins that span the membrane TRANSPORT PROTEINS CHANNEL PROTEINS Hydrophilic channel that certain molecules or ions use as a tunnel through the membrane o Provide corridors that allow specific molecules or ions cross AQUAPORINS facilitating the passage of water molecules through the membrane Channel proteins increase rate of transport CARRIER PROTEINS hold onto their passengers and change shape in a way that shuttles them across the membrane o Changes shape to move the molecule across the membrane a transport protein is specific for the substance it moves allowing only certain substances to cross the membrane SELECTIVE PERMEABILITY DEPENDS ON BOTH THE BARRIER OF THE LIPID BIPLAYER THE SPECIFIC TRANSPORT PROTEINS BUILT INTO THE MEMBRANE PASSIVE TRANSPORT NO ENERGY Diffusion movement of molecules so that they spread out evenly throughout a space o Molecules move from less concentrated more concentrated o Diffuse down concentration gradient o Spontaneous process o No energy Osmosis diffusion of water across a selectively permeable membrane o Tonicity ability to lose or gain water Isotonic no net movement of water same rate water movement inside and outside cell Hypertonic cell will lose water to the environment Hypotonic water enters cell faster than it leaves Shrivel probably die Swell lyse burst Facilitated Diffusion polar molecules ions delayed by the lipid bilayer of the membrane diffuse passively w the help of transport proteins that span the membrane Cells w o rigid walls can tolerate neither excessive uptake of loss of water Osmoregulation control of water balance in animals WATER BALANCE IN PLANTS Turgid uptake of water firm Flaccid not enough water limp Channel Proteins ION CHANNELS open and close in response to a stimulus o Electrical or chemical Ex stimulus of a nerve cell by a neurotransmitter ACTIVE TRANSPORT ENERGY Solute movement against the gradient Carrier proteins only Makes sense bc channel proteins are open allow solutes to flow down concentration gradient Allows cells to maintain internal concentrations of small solutes that differ from concentrations in its environment ATP energy for active transport Sodium potassium pump exchanges Na for K across membranes of animal cells Membrane potential o Cells have voltages across their plasma membrane Cytoplasm charge neg Unequal dist of anions and cations on opposite sides of the membrane o Inside of cell is neg compared to outside the cell membrane favors passive transport of cations into cell and anions out of the cell o Electroneg gradient
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