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

Fig. 7-2 Hydrophilic head WATER Hydrophobic tail WATER 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 Overview: Life at the Edge

Fig. 7-3 Phospholipid bilayer Hydrophobic regions of protein Hydrophilic regions of protein The most abundant lipids in most membranes are phospholipids. However, proteins determine most of the membranes specific functions.

Fig. 7-5 Lateral movement (~10 7 times per second) Flip-flop (~ once per month) (a) Movement of phospholipids – Lipids move laterally in a membrane, but flip-flopping across the membrane is quite rare. (b) Membrane fluidity – Unsaturated hydrocarbon tails of phospholipids have kinks that keep the molecules from packing together, enhancing membrane fluidity. Fluid Viscous Unsaturated hydrocarbon tails with kinks Saturated hydro- carbon tails ( c) Cholesterol within the animal cell membrane – Cholesterol reduces membrane fluidity at moderate temperatures by reducing phospholipid movement, but at low temperatures it hinders solidification by disrupting the regular packing of phospholipids. Cholesterol

Fig. 7-6 RESULTS Membrane proteins Mouse cell Human cell Hybrid cell Mixed proteins after 1 hour Do membrane proteins move? In this experiment, labeled proteins of a mouse cell and a human cell indicates that at least some membrane proteins move sideways within the plane of the plasma membrane. If, after many hours, the protein distribution still looked like that in the third image, would you be able to conclude that proteins don’t move within the membrane? What other explanation could there be? You couldn’t rule out movement of proteins within the cell membrane of the same species. You might speculate that the membrane lipids and proteins from one species weren’t able to mingle with those from the other species because of some incompatibility.

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

Six major functions of membrane proteins: – Transport – Enzymatic activity – Signal transduction – Cell-cell recognition – Intercellular joining – Attachment to the cytoskeleton and extracellular matrix (ECM) Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

(a) Transport – (Left) A protein that spans the membrane may provide a hydrophilic channel across the membrane that is selective for a particular solute. (Right) Other transport proteins shuttle a substance from one side to the other by changing shape. Some of these proteins hydrolyze ATP as an energy source to actively pump substances across the membrane. (b) 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 caries out sequential steps of a metabolic pathway. (Ex. – Cellular respiration & photosynthesis) (c) 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 a shape change in the protein that relays, the message to the inside of the cell, usually by binding to a cytoplasmic protein. ATP Enzymes Signal transduction Signaling molecule Receptor

(d) Cell-cell recognition – Some glycoproteins serve as identification tags that are specifically recognized by membrane proteins of other cells. (Ex. – embryonic induction, antibody/antigen) Glyco- protein (e) Intercellular joining – Membrane proteins of adjacent cells may hook together in various kinds of junction such as gap junctions or tight junctions. (f) Attachment to the cyto- Skeleton and extracellular matrix (ECM) – Microfilaments or other elements of the cytoskeleton may be noncovalently bound to membrane proteins, 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.

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 Diffusion is the movement of solute down it’s concentration gradient (no energy is required) Osmosis is the diffusion of water across a selectively permeable membrane (no energy is required) Water diffuses across a membrane from the region of lower solute concentration to the region of higher solute concentration Tonicity is the ability of a solution to cause a cell to gain or lose water. The tonicity of a solution depends in part on its concentration of solutes that cannot cross the membrane. Isotonic (iso means “same”) solution: Solute concentration is the same as that inside the cell; no net water movement across the plasma membrane Hypertonic (hyper means “more”) solution: Solute concentration is greater than that inside the cell; cell loses water Hypotonic (hypo means “less”) solution: Solute concentration is less than that inside the cell; cell gains water Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig Molecules of dye Membrane (cross section) WATER Net diffusion Equilibrium (a) Diffusion of one solute Net diffusion Equilibrium (b) Diffusion of two solutes

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

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

Fig Filling vacuole 50 µm (a) A contractile vacuole fills with fluid that enters from a system of canals radiating throughout the cytoplasm. The vacuole of this fresh water protist is an evolutionary adaptation for osmoregulation. Contracting vacuole (b) When full, the vacuole and canals contract, expelling fluid from the cell.

Fig EXTRACELLULAR FLUID Channel protein (a) A channel protein has a channel through which water molecules or a specific solute can pass Solute CYTOPLASM Solute Carrier protein (b) A carrier protein – alternates between two shapes, moving a solute acros the membrane during the shape change. Facilitated Diffusion: Passive Transport Aided by Proteins

The Need for Energy in 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 Animation: Active Transport Animation: Active Transport Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

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

Fig Passive transport Diffusion – Hydrophobic molecules and (at a slow rate) very small uncharged polar molecules (like water) can diffuse through the lipid bilayer. Facilitated diffusion – Many hydrophilic substances diffuse through membranes with the assistance of transport proteins, either channel or carrier proteins. Active transport ATP

How Ion Pumps Maintain Membrane Potential Membrane potential – the voltage across a membrane. The inside of the cell is negative compared with the outside, therefore the the membrane potential favors the passive transport of cations into the cell and anions out of the cell. Thus, two forces drive the diffusion of ions across a membrane: a chemical force (the ion’s concentration gradient) and the electrical force (the effect of the membrane potential on the ion’s movement). This combination of forces acting on an ion is called the electrochemical gradient. Electrogenic pump – a transport protein that generates voltage across a membrane. The major electrogenic pump in animals seems to be the sodium-potassium pump. The main electrogenic pump of plants, fungi, and bacteria is a proton pump, which actively transports hydrogen ions (protons) out of the cell. This creates a electric gradient that can be used as a source of potential energy. Cotransport – active transport driven by a concentration gradient

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

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

Concept 7.5 – Bulk transport across the plasma membrane occurs by exocytosis and endocytosis In exocytosis, transport vesicles migrate to the membrane, fuse with it, and release their contents Many secretory cells use exocytosis to export their products 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 Animation: Exocytosis Animation: Exocytosis Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

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 In phagocytosis a cell engulfs a particle in a vacuole The vacuole fuses with a lysosome to digest the particle

Fig. 7-20b PINOCYTOSIS Plasma membrane Vesicle 0.5 µm Pinocytosis vesicles forming (arrows) in a cell lining a small blood vessel (TEM) In pinocytosis, molecules are taken up when extracellular fluid is “gulped” into tiny vesicles

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) 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

Fig. 7-UN3 Environment: 0.01 M sucrose 0.01 M glucose 0.01 M fructose “Cell” 0.03 M sucrose 0.02 M glucose

Fig. 7-UN4