Cell Membrane & Transport PDF

CELL MEMBRANE & MEMBRANE TRANSPORT THIS LECTURE  Cell Membrane – Structure, Function  Types of Transport  CELL MEMBRANE  The plasma membrane is the edge of life, the boundary that separates the living cell from its surroundings (8 nm thick).  Plasma membrane...

CELL MEMBRANE & MEMBRANE TRANSPORT THIS LECTURE  Cell Membrane – Structure, Function  Types of Transport  CELL MEMBRANE  The plasma membrane is the edge of life, the boundary that separates the living cell from its surroundings (8 nm thick).  Plasma membrane exhibits selective permeability (it allows some substances to cross it more easily than others).  A eukaryotic plasma membrane protein plays a crucial role in nerve cell signaling.  Lipids and proteins are the staple ingredients of membranes, although carbohydrates are also important.  The most abundant lipids in most membranes are phospholipids.  A phospholipid is an amphipathic molecule, meaning it has both a hydrophilic region and a hydrophobic region.  WATER Hydrophilic head Hydrophobic tail WATER  CELL MEMBRANE  Membrane proteins reside in the phospholipid bilayer with their hydrophilic regions on the outside.  This molecular arrangement would maximize contact of hydrophilic regions of proteins and phospholipids with water in the cytosol and extracellular fluid, while providing their hydrophobic parts with a nonaqueous environment.  In this fluid mosaic model, the membrane is a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids.  The fluid mosaic model states that a membrane is a fluid structure with a “mosaic” of various proteins embedded in it. Phospholipid bilayer Hydrophobic regions Hydrophilic of protein regions of protein  CELL MEMBRANE  The Fluidity of Membranes  Membranes are not static sheets of molecules locked rigidly in place.  A membrane is held together primarily by hydrophobic interactions, which are much weaker than covalent bonds.  Most of the lipids and some of the proteins can shift about laterally.  It is quite rare, however, for a molecule to flip-flop transversely across the membrane, switching from one phospholipid layer to the other.  Lateral movement Flip-flop (~107 times per second) (~ once per month) (a) Movement of phospholipids Fluid Viscous Unsaturated hydrocarbon Saturated hydro- tails with kinks carbon tails (b) Membrane fluidity Cholesterol (c) Cholesterol within the animal cell membrane  CELL MEMBRANE  The Fluidity of Membranes  Some membrane proteins seem to move in a highly directed manner, perhaps driven along cytoskeletal fibers by motor proteins connected to the membrane proteins’ cytoplasmic regions.  However, many other membrane proteins seem to be held immobile by their attachment to the cytoskeleton or to the extracellular matrix.  CELL MEMBRANE  The Fluidity of Membranes  A membrane remains fluid as temperature decreases until finally the phospholipids settle into a closely packed arrangement and the membrane solidifies, much as bacon grease forms lard when it cools. The temperature at which a membrane solidifies depends on the types of lipids it is made of.  The membrane remains fluid to a lower temperature if it is rich in phospholipids with unsaturated hydrocarbon tails.  Because of kinks in the tails where double bonds are located, unsaturated hydrocarbon tails cannot pack together as closely as saturated hydrocarbon tails, and this makes the membrane more fluid.  Fluid Viscous Unsaturated hydrocarbon Saturated hydro- tails with kinks carbon tails (b) Membrane fluidity  CELL MEMBRANE  The steroid cholesterol has different effects on membrane fluidity at different temperatures.  At warm temperatures (such as 37°C), cholesterol prevents the movement of phospholipids.  At cool temperatures, it maintains fluidity by preventing tight packing.  Cholesterol can be thought of as a “fluidity buffer” for the membrane, resisting changes in membrane fluidity that can be caused by changes in temperature.  Cholesterol High Temperature Low Temperature (c) Cholesterol within the animal cell membrane  CELL MEMBRANE  Membrane Proteins  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.  There are two major populations of membrane proteins: integral proteins and peripheral proteins.  CELL MEMBRANE  Membrane Proteins  Peripheral proteins are 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  Integral proteins penetrate the hydrophobic interior of the lipid bilayer.  CELL MEMBRANE  Membrane Proteins  The majority of integral proteins are transmembrane proteins, which span the membrane; other integral proteins extend only partway into the hydrophobic interior.  The hydrophobic regions of an integral protein consist of one or more stretches of nonpolar amino acids usually coiled into α helices.  The hydrophilic parts of the molecule are exposed to the aqueous solutions on either side of the membrane. Some proteins also have a hydrophilic channel through their center that allows passage of hydrophilic substances.  N-terminus EXTRACELLULAR SIDE C-terminus CYTOPLASMIC  Helix SIDE  Six major functions of membrane proteins:  Transport  Enzymatic activity  Signal transduction  Cell-cell recognition  Intercellular joining  Attachment to the cytoskeleton and extracellular matrix (ECM)  Major functions of membrane proteins:  Transport: A protein that spans the membrane may provide a hydrophilic channel across the membrane that is selective for a particular solute. Other transport proteins shuttle a substance from one side to the other by changing shape. Some of ATP these proteins hydrolyze ATP as an energy source to actively pump substances across the membrane.  Enzymes  Major functions of membrane proteins:  Enzymatic activity: A protein built into the membrane may be an enzyme with its active site exposed to substances in the adjacent solution. In some cases, several enzymes in a membrane are organized as a team that carries out sequential steps of a metabolic pathway.  Signaling molecule  Major functions of membrane Receptor proteins:  Signal transduction: A membrane protein (receptor) may have a binding site with a specific shape that fits the shape of a chemical messenger, such as a hormone. The external messenger (signaling molecule) may cause the protein to change shape, allowing it to relay the message to the inside of the cell, usually by binding to a cytoplasmic Signal transduction protein.  Major functions of membrane proteins:  Cell-cell recognition: Some glycoproteins serve as identification tags that are specifically recognized by Glyco- membrane proteins of other cells. This protein type of cell-cell binding is usually short-lived compared to intercellular joining.  Major functions of membrane proteins:  Intercellular joining: Membrane proteins of adjacent cells may hook together in various kinds of junctions, such as gap junctions or tight junctions. This type of binding is more long- lasting than the cell-cell recognition.  Major functions of membrane proteins:  Attachment to the cytoskeleton and extracellular matrix (ECM): Microfilaments or other elements of the cytoskeleton may be noncovalently bound to membrane proteins (integrins), a function that helps maintain cell shape and stabilizes the location of certain membrane proteins. Proteins that can bind to ECM molecules can coordinate extracellular and intracellular changes.  Signaling molecule Enzymes Receptor ATP Signal transduction (a) Transport (b) Enzymatic activity (c) Signal transduction Glyco- protein (d) Cell-cell recognition (e) Intercellular joining (f) Attachment to the cytoskeleton and extracellular matrix (ECM)  CELL MEMBRANE  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 e.g. Blood types!  CELL MEMBRANE  The Permeability of the Lipid Bilayer  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.  CELL MEMBRANE  The Permeability of the Lipid Bilayer – 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 or ions can use as a tunnel.  Channel proteins called aquaporins facilitate the passage of water.  CELL MEMBRANE  The Permeability of the Lipid Bilayer – Transport Proteins  Other transport proteins, called carrier proteins, bind to molecules and change shape to shuttle them across the membrane. ATP  A transport protein is specific for the substance it moves. Channel Carrier  CELL MEMBRANE  Types of Transport – Passive Transport  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.  At dynamic equilibrium, as many molecules cross one way as cross in the other direction. Molecules of dye Membrane (cross section) WATER Net diffusion Net diffusion Equilibrium (a) Diffusion of one solute  CELL MEMBRANE  Types of Transport – Passive Transport  A substance will diffuse from where it is more concentrated to where it is less concentrated.  Substances diffuse down their concentration gradient, the difference in concentration of a substance from one area to another.  No work must be done to move substances down the concentration gradient.  The diffusion of a substance across a biological membrane is passive transport because it requires no energy from the cell to make it happen.  Net diffusion Net diffusion Equilibrium Net diffusion Net diffusion Equilibrium (b) Diffusion of two solutes  CELL MEMBRANE  Types of Transport – Passive Transport  Osmosis is the diffusion of water across a selectively permeable membrane.  Water diffuses across a membrane from the region of lower solute concentration to the region of higher solute concentration. Lower Higher Same concentration concentration concentration of sugar of solute (sugar) of sugar H2O Selectively permeable membrane Osmosis  CELL MEMBRANE  Types of Transport – Passive Transport  Water balance in Cells without wall  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.  Hypotonic solution Isotonic solution Hypertonic solution H2O H2O H2O H2O Erythrocyte swell Lysed Normal Shriveled  CELL MEMBRANE  Types of Transport – Passive Transport  In facilitated diffusion, transport proteins speed the passive movement of molecules across the plasma membrane.  Channel proteins provide corridors that allow a specific molecule or ion to cross the membrane  Channel proteins include  Aquaporins, for facilitated diffusion of water  Ion channels that open or close in response to a stimulus (gated channels) EXTRACELLULAR FLUID Channel protein Solute CYTOPLASM (a) A channel protein  Carrier proteins undergo a subtle change in shape that translocates the solute-binding site across the membrane Carrier protein Solute (b) A carrier protein  CELL MEMBRANE  Types of Transport – Passive Transport  Facilitated diffusion is still passive because the solute moves down its concentration gradient.  Some transport proteins, however, can move solutes against their concentration gradients.  CELL MEMBRANE  Types of Transport – Active Transport  Active transport moves substances against their concentration gradient.  Active transport requires energy, usually in the form of ATP.  Active transport is performed by specific proteins embedded in the membranes.  Active transport allows cells to maintain concentration gradients that differ from their surroundings.  The sodium-potassium pump is one type of active transport system.  EXTRACELLULAR Na+ [Na+] high FLUID Na+ [K+] low Na+ Na+ Na+ Na+ Na+ Na+ [Na+] low ATP Na+ P [K+] high P CYTOPLASM ADP 1 Cytoplasmic Na+ binds to 2 Na+ binding stimulates 3 Phosphorylation causes the phosphorylation by ATP. protein to change its shape. Na+ the sodium-potassium pump. is expelled to the outside. P P 6 K+ is released, and the 5 Loss of the phosphate restores 4 K+ binds on the extracellular side and cycle repeats. the protein’s original shape. triggers release of the phosphate group.  Passive transport Active transport ATP Diffusion Facilitated diffusion  CELL MEMBRANE  Types of Transport – Bulk Transport  Small molecules and water enter or leave the cell through the lipid bilayer or by transport proteins.  Large molecules, such as polysaccharides and proteins, cross the membrane in bulk via vesicles.  Bulk transport requires energy.  CELL MEMBRANE  Types of Transport – Bulk Transport – Exocytosis  In exocytosis, transport vesicles migrate to the membrane, fuse with it, and release their contents.  Many secretory cells use exocytosis to export their products.  ER Transmembrane glycoproteins Secretory protein Glycolipid Golgi apparatus Vesicle Exocytosis Plasma membrane: Cytoplasmic face Extracellular face Transmembrane Secreted glycoprotein protein Membrane glycolipid  CELL MEMBRANE  Types of Transport – Bulk Transport – Endocytosis  In endocytosis, the cell takes in macromolecules by forming vesicles from the plasma membrane  Endocytosis is a reversal of exocytosis, involving different proteins  There are three types of endocytosis:  Phagocytosis (“cellular eating”)  Pinocytosis (“cellular drinking”)  Receptor-mediated endocytosis In phagocytosis a cell engulfs a particle in a vacuole The vacuole fuses with a lysosome to digest the particle PHAGOCYTOSIS EXTRACELLULAR CYTOPLASM 1 µm FLUID Pseudopodium Pseudopodium of amoeba “Food” or other particle Bacterium Food vacuole Food vacuole An amoeba engulfing a bacterium via phagocytosis (TEM)  In pinocytosis, molecules are taken up when extracellular fluid is “gulped” into tiny vesicles PINOCYTOSIS 0.5 µm Plasma membrane Pinocytosis vesicles forming (arrows) in a cell lining a small blood vessel (TEM) Vesicle  RECEPTOR-MEDIATED ENDOCYTOSIS Coat protein Receptor Coated vesicle Coated pit Ligand  In receptor-mediated endocytosis, binding of ligands to receptors triggers vesicle formation  A ligand is any molecule that binds specifically to a receptor site of another molecule  References & Thank You!