Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings PowerPoint ® Lecture Presentations for Biology Eighth Edition Neil Campbell and Jane Reece Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp Chapter 7 Membrane Structure and Function

I. Cell Membrane Structure Phospholipid bilayer (hydrophobic and hydrophilic regions) with proteins, cholesterol, carbohydrates inbedded in it

Fig. 7-2 Hydrophilic head WATER Hydrophobic tail WATER

Fig. 7-3 Phospholipid bilayer Hydrophobic regions of protein Hydrophilic regions of protein

Fig. 7-7 Fibers of extracellular matrix (ECM) Glyco- protein Microfilaments of cytoskeleton Cholesterol Peripheral proteins Integral protein CYTOPLASMIC SIDE OF MEMBRANE Glycolipid EXTRACELLULAR SIDE OF MEMBRANE Carbohydrate

II. The Fluidity of Membranes Phospholipids can move within the bilayer (laterally) Rarely does a molecule flip-flop transversely across the membrane

Fig. 7-5a (a) Movement of phospholipids Lateral movement (  10 7 times per second) Flip-flop (  once per month)

Membranes rich in unsaturated fatty acids are more fluid that those rich in saturated fatty acids Membranes must be fluid to work properly; they are usually about as fluid as salad oil

Fig. 7-5b (b) Membrane fluidity Fluid Unsaturated hydrocarbon tails with kinks Viscous Saturated hydro- carbon tails

Cholesterol (steroid) moderates the fluidity of the membrane Warm temperatures (such as 37°C), cholesterol restrains movement of phospholipids Cool temperatures, maintains fluidity by preventing tight packing

Fig. 7-5c Cholesterol (c) Cholesterol within the animal cell membrane

III. Membrane Proteins and Their Functions Peripheral proteins: bound to the surface of the membrane Integral proteins penetrate the hydrophobic core Transmembrane proteins: Int proteins that span the membrane Hydrophobic regions of integral proteins consist of stretches of nonpolar amino acids, often coiled into alpha helices

Fig. 7-8 N-terminus C-terminus  Helix CYTOPLASMIC SIDE EXTRACELLULAR SIDE

Fig. 7-9ac (a) Transport (b) Enzymatic activity (c) Signal transduction ATP Enzymes Signal transduction Signaling molecule Receptor

Fig. 7-9df (d) Cell-cell recognition Glyco- protein (e) Intercellular joining (f) Attachment to the cytoskeleton and extracellular matrix (ECM)

IV. Membrane Carbohydrates Cells recognize each other by binding to surface molecules, often carbs Carbs may be covalently bonded to lipids (glycolipids) or more commonly to proteins (glycoproteins) Carbs on external side of plasma membrane vary among species, individuals, and cell types

V. Synthesis and Sidedness of Membranes Membranes have inside and outside faces The distribution of proteins, lipids, and associated carbs in memb is determined when the memb is built by the ER and Golgi apparatus

Fig ER 1 Transmembrane glycoproteins Secretory protein Glycolipid 2 Golgi apparatus Vesicle 3 4 Secreted protein Transmembrane glycoprotein Plasma membrane: Cytoplasmic face Extracellular face Membrane glycolipid

VI. The Permeability of the Lipid Bilayer Hydrophobic (nonpolar) molecules, such as hydrocarbons and steroids, can dissolve in the lipid bilayer and pass through the membrane rapidly Polar molecules, such as sugars, do not cross the membrane easily

VII. Transport Proteins Transport proteins allow passage of hydrophilic substances across memb Channel proteins, have a hydrophilic channel that certain molecules or ions can use as a tunnel (ex. aquaporins facilitate passage of water)

Carrier proteins: bind to molecules and change shape to shuttle them across the membrane A transport protein is specific for the substance it moves

VIII. Diffusion Molecules spread out evenly into available space Substances diffuse down their concentration gradient (difference in concentration of a substance from one area to another) Passive transport: requires no energy from the cell to make it happen

Molecules of dye Fig. 7-11a Membrane (cross section) WATER Net diffusion (a) Diffusion of one solute Equilibrium

(b) Diffusion of two solutes Fig. 7-11b Net diffusion Equilibrium

IX. Effects of Osmosis on Water Balance Osmosis: diffusion of water across a selectively permeable membrane Water diffuses across a membrane from the lower solute concentration to the higher solute concentration

Lower concentration of solute (sugar) Fig H2OH2O Higher concentration of sugar Selectively permeable membrane Same concentration of sugar Osmosis

A. Water Balance of Cells Without Walls Tonicity: ability of a solution to cause a cell to gain or lose water Isotonic: Solute concentration is = to that inside the cell; no net water movement Hypertonic solution: Solute concentration is > that inside the cell; cell loses water Hypotonic solution: Solute concentration is < that inside the cell; cell gains water Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig Hypotonic solution (a ) Animal cell (b ) Plant cell H2OH2O Lysed H2OH2O Turgid (normal) H2OH2O H2OH2O H2OH2O H2OH2O Normal Isotonic solution Flaccid H2OH2O H2OH2O Shriveled Plasmolyzed Hypertonic solution

Osmoregulation, the control of water balance, is a necessary adaptation for life The protist Paramecium, hypertonic to its pond water environment, has a contractile vacuole that acts as a pump Video: Chlamydomonas Video: Chlamydomonas Video: Paramecium Vacuole Video: Paramecium Vacuole Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig Filling vacuole 50 µm (a) A contractile vacuole fills with fluid that enters from a system of canals radiating throughout the cytoplasm. Contracting vacuole (b) When full, the vacuole and canals contract, expelling fluid from the cell.

B. Water Balance of Cells with Walls A plant cell in hypotonic solution swells until the wall opposes uptake; the cell is now turgid (firm) Isotonic, no net movement of water into the cell; the cell becomes flaccid (limp), and the plant may wilt

Hypertonic, plant cells lose water; eventually, the membrane pulls away from the wall, a usually lethal effect called plasmolysis

X. Facilitated Diffusion Facilitated diffusion, transport proteins speed passive movement of molecules across plasma membrane Channel proteins provide corridors that allow a specific molecule or ion Channel proteins include: – Aquaporins, for facilitated diffusion of water – Ion channels that open or close in response to a stimulus (gated channels)

Fig EXTRACELLULAR FLUID Channel protein (a) A channel protein Solute CYTOPLASM Solute Carrier protein (b) A carrier protein

XI. Active Transport Moves substances against their concentration gradient Requires energy, usually in the form of ATP Performed by specific proteins embedded in the membranes

Cells can maintain concentration gradients that differ from their surroundings Ex. sodium-potassium pump

Fig EXTRACELLULAR FLUID [Na + ] high [K + ] low Na + [Na + ] low [K + ] high CYTOPLASM Cytoplasmic Na + binds to the sodium-potassium pump. 1

Na + binding stimulates phosphorylation by ATP. Fig Na + ATP P ADP 2

Fig Phosphorylation causes the protein to change its shape. Na + is expelled to the outside. Na + P 3

Fig K + binds on the extracellular side and triggers release of the phosphate group. P P K+K+ K+K+ 4

Fig Loss of the phosphate restores the protein’s original shape. K+K+ K+K+ 5

Fig K + is released, and the cycle repeats. K+K+ K+K+ 6

2 EXTRACELLULAR FLUID [Na + ] high [K + ] low [Na + ] low [K + ] high Na + CYTOPLASM ATP ADP P Na + P 3 K+K+ K+K+ 6 K+K+ K+K+ 5 4 K+K+ K+K+ P P 1 Fig

Fig Passive transport Diffusion Facilitated diffusion Active transport ATP

XII. Ion Pumps Maintain membrane potential = voltage difference across a membrane (+ and – ions) An electrogenic pump = transport protein that generates voltage across a membrane The sodium-potassium pump is the major electrogenic pump of animal cells The main electrogenic pump of plants, fungi, and bacteria is a proton pump

Fig EXTRACELLULAR FLUID H+H+ H+H+ H+H+ H+H+ Proton pump H+H+ H+H+ + + H+H+ – – – – ATP CYTOPLASM –

XIII. Cotransport Occurs when active transport of a solute indirectly drives transport of another solute Plants commonly use the gradient of hydrogen ions generated by proton pumps to drive active transport of nutrients into the cell

Fig Proton pump – – – – – – ATP H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ Diffusion of H + Sucrose-H + cotransporter Sucrose

XIV. Exocytosis Transport vesicles migrate to the membrane, fuse with it, and release their contents Many secretory cells use exocytosis to export their products

XV. Endocytosis Cell takes in macromolecules by forming vesicles from the plasma membrane Reversal of exocytosis, involving different proteins There are three types of endocytosis: – Phagocytosis (“cellular eating”) – Pinocytosis (“cellular drinking”) – Receptor-mediated endocytosis - binding of ligands (molecule that binds to a receptor site of another molecule) to receptors triggers vesicle formation

Fig. 7-20a PHAGOCYTOSIS CYTOPLASM EXTRACELLULAR FLUID Pseudopodium “Food” or other particle Food vacuole Food vacuole Bacterium An amoeba engulfing a bacterium via phagocytosis (TEM) Pseudopodium of amoeba 1 µm

Fig. 7-20b PINOCYTOSIS Plasma membrane Vesicle 0.5 µm Pinocytosis vesicles forming (arrows) in a cell lining a small blood vessel (TEM)

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