Overview Life at the Edge The plasma membrane is the boundary that separates the living cell from its surroundings The plasma membrane exhibits selective permeability allowing some substances to cross it more easily than others Hydrophilic head WATER Hydrophobic tail WATER Fig 7 7 Fibers of extracellular matrix ECM Glycoprotein Carbohydrate Glycolipid EXTRACELLULAR SIDE OF MEMBRANE Cholesterol Microfilaments of cytoskeleton Peripheral proteins Integral protein CYTOPLASMIC SIDE OF MEMBRANE Concept 7 1 Cellular membranes are fluid mosaics of lipids and proteins Phospholipids are the most abundant lipid in the plasma membrane Phospholipids are amphipathic molecules containing hydrophobic and hydrophilic regions The fluid mosaic model states that a membrane is a fluid structure with a mosaic of various proteins embedded in it Phospholipids in the plasma membrane can move within the bilayer Membranes must be fluid to work properly they are usually about as fluid as salad oil Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Fig 7 3 Phospholipid bilayer Hydrophobic regions of protein Hydrophilic regions of protein Fig 7 5 Most of the lipids and some Lateral movement proteins drift 10 times per second a Movement of phospholipids laterally and Fluid rarely they go flip flop transversely across the Unsaturated hydrocarbon tails with kinks membrane 7 Flip flop once per month Viscous Saturated hydrocarbon tails b Membrane fluidity Cholesterol c Cholesterol within the animal cell membrane Fig 7 6 RESULTS Membrane proteins Mouse cell Mixed proteins after 1 hour Human cell Hybrid cell Membrane Proteins and Their Functions A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer Proteins determine most of the membrane s specific functions Peripheral proteins are bound to the surface of the membrane Integral proteins penetrate the hydrophobic core The hydrophobic regions of an integral protein consist of one or more stretches of nonpolar amino acids often coiled into alpha helices Fig 7 8 N terminus C terminus Helix EXTRACELLULAR SIDE CYTOPLASMIC SIDE Six major functions of membrane proteins 1 Transport 2 Enzymatic activity 3 Signal transduction 4 Cell cell recognition 5 Intercellular joining 6 Attachment to the cytoskeleton and extracellular matrix ECM Fig 7 9 Signaling molecule Enzymes ATP a Transport Receptor Signal transduction b Enzymatic activity c Signal transduction e Intercellular joining f Attachment to the cytoskeleton and extracellular matrix ECM Glycoprotein d Cell cell recognition The Role of Membrane Carbohydrates in Cell Cell Recognition Cells recognize each other by binding to surface molecules often carbohydrates on the plasma membrane Membrane carbohydrates may be covalently bonded to lipids forming glycolipids or more commonly to proteins forming glycoproteins Carbohydrates on the external side of the plasma membrane vary among species individuals and even cell types in an individual Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Concept 7 2 Membrane structure results in selective permeability A cell must exchange materials with its surroundings a process controlled by the plasma membrane Plasma membranes are selectively permeable regulating the cell s molecular traffic Hydrophobic nonpolar molecules such as hydrocarbons can dissolve in the lipid bilayer and pass through the membrane rapidly Polar molecules such as sugars do not cross the membrane easily and for them we need the transport protein Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Transport Proteins Transport proteins allow passage of hydrophilic substances across the membrane Some transport proteins called channel proteins have a hydrophilic channel that certain molecules e g water or ions can use as a tunnel Channel proteins called aquaporins facilitate the passage of water Other transport proteins called carrier proteins bind to molecules and change shape to shuttle them across the membrane Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Concept 7 3 Passive transport is diffusion of a substance across a membrane with no energy investment Diffusion is the tendency for molecules to spread out evenly into the available space Although each molecule moves randomly diffusion of a population of molecules may exhibit a net movement in one direction Substances diffuse down their concentration gradient the difference in concentration of a substance from one area to another The diffusion of a substance across a biological membrane is passive transport because it requires no energy from the cell to make it happen Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Fig 7 11 Molecules of dye Membrane cross section WATER Net diffusion Net diffusion Equilibrium a Diffusion of one solute Net diffusion Net diffusion Equilibrium Net diffusion Equilibrium Net diffusion At dynamic equilibrium as many molecules cross one way as b the Diffusion of twodirection solutes cross in other Effects of Osmosis on Water Balance Osmosis is the diffusion of water across a selectively permeable membrane Water diffuses across a membrane from the region of higher water concentration to the region of lower water concentration like the case with diffusion Another way of saying that would be Water diffuses across a membrane from the region of lower solute concentration to the region of higher solute concentration Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Fig 7 12 Lower concentration of solute sugar Higher concentration of sugar H2O Selectively permeable membrane Osmosis Same concentration of sugar Water Balance of Cells Without Walls Tonicity is the ability of a solution to cause a cell to gain or lose water Isotonic solution Solute concentration is the same as that inside the cell no net water movement across the plasma membrane Hypertonic solution Solute concentration is greater than that inside the cell cell loses water Hypotonic solution Solute concentration is less than that inside the cell cell gains water Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Fig 7 13 Hypotonic solution H2O Isotonic solution H2O H2O Hypertonic solution H2O a Animal cell Lysed H2 O Normal Shriveled H2O H2O H2O b Plant cell Turgid normal Flaccid Plasmolyzed Hypertonic or hypotonic environments create osmotic problems for