Membrane Structure & Function cont. I. Membrane Protein Function II. Cellular Transport

Integral proteins – span lipid bilayer – called transmembrane proteins – hydrophobic regions consist of one or more stretches of nonpolar amino acids –often coiled into alpha helices –Visualize and draw membrane with transmembrane protein containing 2 helices

LE 7-8 EXTRACELLULAR SIDE N-terminus C-terminus CYTOPLASMIC SIDE  Helix

Six major functions of membrane proteins: –Transport –Enzymatic activity –Signal transduction –Cell-cell recognition –Intercellular joining –Attachment to the cytoskeleton and extracellular matrix (ECM)

LE 7-9a Enzymes Signal Receptor ATP Transport Enzymatic activity Signal transduction

LE 7-9b Glyco- protein Cell-cell recognition Intercellular joining Attachment to the cytoskeleton and extra- cellular matrix (ECM)

The Role of Membrane Carbohydrates in Cell- Cell Recognition Cells recognize each other by binding to surface molecules, often carbohydrates, on the plasma membrane Carbohydrates covalently bonded to lipids (glycolipids) or more often to proteins (glycoproteins) Much variability of extracellular carbohydrates among species, individuals, cell types in an individual Example of Pneumococcus

Synthesis and Sidedness of Membranes Membranes distinct inside and outside faces Plasma membrane is added to by vesicles from ER & Golgi. Secreted and integral membrane proteins, lipids and associated carbohydrates transported to membrane by these vesicles.

LE 7-10 Plasma membrane: Cytoplasmic face Extracellular face Transmembrane glycoprotein Plasma membrane: Secreted protein Vesicle Golgi apparatus Glycolipid Secretory protein Transmembrane glycoproteins ER

Transport across cellular membranes To exchange materials with surroundings in part to take in nutrients and give off waste Exchange(or transport) regulated: selective permeability

Structure Dictates Membrane Permeability Hydrophobic (nonpolar) molecules cross membrane rapidly e.g., hydrocarbons, oxygen, CO 2 can dissolve in the lipid bilayer and pass through the membrane rapidly Polar molecules cross slowly e.g. sugars, charged proteins, water

How do hydrophilic substances cross membranes? Transport proteins Some create hydrophilic channels across membranes for polar molecules or ions to pass through Example: Aquaporin water channel protein With Help!

Carrier proteins binds solutes & change the shape of carrier help to facilitate passage across membrane highly specific for transported solutes Examples: glucose transporter is a carrier protein for glucose only

Transport Can be Passive or Active

LE 7-11a Molecules of dyeMembrane (cross section) WATER Net diffusion Equilibrium Diffusion of one solute Passive Transport: Diffusion

Substances diffuse down their concentration gradient High to low Substances reach dynamic equilibrium No work (no added energy) required

LE 7-11b Net diffusion Equilibrium Diffusion of two solutes Net diffusion Equilibrium

Effects of Osmosis on Water Balance Osmosis –diffusion of water across a selectively permeable membrane Diffuses across a membrane from the region of lower solute (such as an ion) concentration to the region of higher solute concentration The direction of osmosis is determined only by a difference in total solute concentration

LE 7-12 Lower concentration of solute (sugar) Higher concentration of sugar Same concentration of sugar Selectively permeable mem- brane: sugar mole- cules cannot pass through pores, but water molecules can H2OH2O Osmosis

Water Balance of Cells Without Walls Tonicity –ability of a solution to cause a cell to gain or lose water Isotonic solution solute concentration is equal inside and outside the cell --> no net water movement cell remains same size

Hypertonic solution external solute concentration is greater than that inside the cell--> cell loses water

Hypotonic solution external solute concentration is less than that inside the cell--> cell gains water May expand enough to burst!

LE 7-13 Animal cell Lysed H2OH2O H2OH2O H2OH2O Normal Hypotonic solution Isotonic solutionHypertonic solution H2OH2O Shriveled H2OH2O H2OH2O H2OH2O H2OH2O Plant cell Turgid (normal) FlaccidPlasmolyzed

Water Balance of Cells with Walls vs No Walls Cell walls help maintain water balance Plant cell in hypotonic solution swells -->turgid (firm) Animal cell? Plant cell and its surroundings isotonic--> no net water movemen; the cell becomes flaccid (limp), and the plant may wilt Animal cell? In hypertonic environment, plant cells lose water--> membrane pulls away from the wall: plasmolysis Lethal Animal cell?

Passive Transport Aided by Proteins Facilitated diffusion – transport proteins speed movement of molecules across the plasma membrane –Channel proteins –Carrier proteins

LE 7-15a EXTRACELLULAR FLUID Channel protein Solute CYTOPLASM

LE 7-15b Carrier protein Solute

Active transport uses energy to move solutes against their gradients Requires energy, usually ATP Performed by specific membrane proteins Example sodium-potassium pump

LE 7-16 Cytoplasmic Na + bonds to the sodium-potassium pump CYTOPLASM Na + [Na + ] low [K + ] high Na + EXTRACELLULAR FLUID [Na+] high [K+] low Na + ATP ADP P Na + binding stimulates phosphorylation by ATP. Na + K+K+ Phosphorylation causes the protein to change its conformation, expelling Na + to the outside. P Extracellular K + binds to the protein, triggering release of the phosphate group. P P Loss of the phosphate restores the protein’s original conformation. K + is released and Na + sites are receptive again; the cycle repeats. K+K+ K+K+ K+K+ K+K+ K+K+

LE 7-17 Diffusion Facilitated diffusion Passive transport ATP Active transport

Electrogenic pumps is a transport protein that generates a voltage across a membrane--> opposite charges across membrane (membrane potential) Example: In animals, Na-K pump In plant fungi and bacteria, proton pump –Requires ATP (active transport)

LE 7-18 H+H+ ATP CYTOPLASM EXTRACELLULAR FLUID Proton pump H+H+ H+H+ H+H+ H+H+ H+H – – – – –

Cotransport Coupled Transport by a Membrane Protein When active transport of one solute indirectly drives transport of another Example Plants commonly use the proton gradient generated by proton pumps to drive transport of nutrients into the cell

LE 7-19 H+H+ ATP Proton pump Sucrose-H + cotransporter Diffusion of H + Sucrose H+H+ H+H+ H+H+ H+H+ H+H+ H+H – – – – – –

How do large molecules move in and out of cells? 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 via vesicles

Exocytosis Transport vesicles with cargo migrate to the membrane, fuse with it, and are release contents Example: –Many secretory cells use exocytosis to export their products – Pancreatic cells (beta-cells) secrete insulin

LE 7-10 Plasma membrane: Cytoplasmic face Extracellular face Transmembrane glycoprotein Plasma membrane: Secreted protein Vesicle Golgi apparatus Glycolipid Secretory protein Transmembrane glycoproteins ER

Endocytosis Cell takes in macromolecules by forming vesicles at the plasma membrane Reversal of exocytosis, involving different proteins

Three types of endocytosis –Phagocytosis (“cellular eating”): Cell engulfs particle in a vacuole –Pinocytosis (“cellular drinking”): Cell creates vesicle around fluid –Receptor-mediated endocytosis: Binding of ligands to receptors triggers vesicle formation

LE 7-20c Receptor RECEPTOR-MEDIATED ENDOCYTOSIS Ligand Coated pit Coated vesicle Coat protein Coat protein Plasma membrane 0.25 µm A coated pit and a coated vesicle formed during receptor- mediated endocytosis (TEMs).