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

Overview: Life at the Edge The plasma membrane separates the living cell from its surroundings It exhibits selective permeability, letting some, but not all, substances across it Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Concept 7.1: Cellular membranes are fluid mosaics of lipids and proteins Phospholipids- – Make up most plasma membrane lipids – amphipathic molecules- have hydrophobic and hydrophilic regions Fluid mosaic model- membranes are fluid structures with a “mosaic” of proteins embedded in them All membranes are a phospholipid bilayer of proteins and lipids Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 7-2 Hydrophilic head WATER Hydrophobic tail WATER

1935, Hugh Davson & James Danielli- the sandwich model; a phospholipid bilayer between two layers of globular proteins – Problem: doesn’t account for hydrophilic and hydrophobic regions on membrane proteins 1972, J. Singer & G. Nicolson- Fluid mosaic model; membrane is a mosaic of proteins dispersed in the bilayer with only hydrophilic regions exposed to water Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Early Membrane Models

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

TECHNIQUE Extracellular layer Knife Proteins Inside of extracellular layer RESULTS Inside of cytoplasmic layer Cytoplasmic layer Plasma membrane Fig. 7-4 Freeze-fracture- specialized preparation technique that splits a membrane along the middle of the phospholipid bilayer Freeze-fracture studies of the plasma membrane support the fluid mosaic model Freeze Fractioning of Cell Membranes

The Fluidity of Membranes Phospholipids can move within the lipid bilayer – Most lipids and some proteins drift laterally – Rarely, a molecule flip-flops transversely across the membrane Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings (a) Movement of phospholipids Lateral movement (  10 7 times per second) Flip-flop (  once per month) Fig. 7-5a

Do membrane proteins move? An experiment. RESULTS Membrane proteins Mouse cell Human cell Hybrid cell Mixed proteins after 1 hour Fig. 7-6 a) Two different markers were used to tag mouse & human membrane proteins b) The two cells were fused together c) When the cells were examined, the proteins had moved laterally

Membranes must be fluid to work properly; they are usually about as fluid as salad oil Temperature: – At cooler temperatures, membranes switch from fluid to solid – The temperature depends on the types of lipids Chemical composition: – Unsaturated fatty acids membranes are more fluid than saturated fatty acid membranes Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings What affects membrane protein movement?

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

Cholesterol effect: – At warm temperatures (like 37°C), cholesterol prevents phospholipid movement – At cool temperatures, it prevents tight packing to maintain fluidity Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 7-5c Cholesterol (c) Cholesterol within the animal cell membrane

Membrane Proteins and Their Functions Membranes are made of different proteins embedded in the fluid matrix of the lipid bilayer Proteins determine the membrane’s functions Peripheral proteins are bound to the surface Integral proteins are in the hydrophobic core Transmembrane proteins- integral proteins spanning the membrane – have one or more areas of nonpolar amino acids coiled into alpha helices – Nonpolar AA’s make hydrophobic regions in proteins Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

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

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 (passing a signal through) – Cell-cell recognition – Intercellular joining – Attachment to the cytoskeleton and extracellular matrix (ECM) Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Role of Membrane Proteins

Fig. 7-9 (a) Transport ATP (b) Enzymatic activity Enzymes (c) Signal transduction Signal transduction Signaling molecule Receptor (d) Cell-cell recognition Glyco- protein (e) Intercellular joining (f) Attachment to the cytoskeleton and extracellular matrix (ECM)

The Role of Membrane Carbohydrates in Cell-Cell Recognition Cells recognize each by binding to molecules on the cell membrane surface, like carbohydrates, Membrane carbs. may covalently bond to lipids (glycolipids) or to proteins (glycoproteins) Carbs. on the outer side of the cell membrane vary among species, individuals, and cell types in an individual Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Synthesis and Sidedness of Membranes Membranes have inside and outside faces The uneven distribution of proteins, lipids, and carbohydrates on the membrane is set when it is built by the ER and Golgi apparatus Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

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

Concept 7.2: Membrane structure results in selective permeability Cells must exchange materials with the surroundings – The plasma membranes control the process – Plasma membranes are selectively permeable, regulating the cell’s molecular traffic Hydrophobic molecules (nonpolar hydrocarbons) dissolve in lipid bilayer & pass through rapidly Polar molecules (sugars) don’t cross the easily Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Transport Proteins Transport proteins let hydrophilic substances cross the membrane – Channel proteins have a hydrophilic channel some molecules & ions can pass through – Aquaporins aid the passage of water – Carrier proteins, bind molecules and change shape to take them across membranes A transport protein is specific for the substance it moves 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 used Diffusion- tendency of molecules to spread evenly in available space Molecules move randomly, but a large group may show a net movement in one direction Dynamic equilibrium- as many molecules cross one way as the other direction Animation: Membrane Selectivity Animation: Membrane Selectivity Animation: Diffusion Animation: Diffusion 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

Substances diffuse down a concentration gradient Concentration gradient- difference in concentration of substances between two areas – No work is done to move things down the gradient Passive transport- diffusion of a substance across a membrane that requires no energy from the cell Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Effects of Osmosis on Water Balance Osmosis- diffusion of water across a selectively permeable membrane Water diffuses from the region of lower solute concentration (higher water concentration) to the region of higher solute concentration (lower water concentration) Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

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

Water Balance of Cells Without Walls Tonicity- ability of a solution to make a cell gain or lose water Isotonic solution: Solute concentration is the same in & out of the cell; no net water movement across the membrane Hypertonic solution: Solute concentration is greater outside than inside cell; cell loses water Hypotonic solution: Solute concentration is less outside than inside 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

Hypertonic or hypotonic environments create osmotic problems for organisms Osmoregulation, the control of water balance, is necessary for life in such environments – Ex. 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.

Water Balance of Cells with Walls Cell walls help maintain water balance Plant cell in hypotonic solution-swells until the wall opposes uptake; cell is turgid (firm) Plant cell in isotonic solution- no net movement of water into cell (water is used for metabolism); cell becomes flaccid (limp), plant may wilt Plant cells in hypertonic solution- lose water; membrane pulls away from the wall, usually lethal; called plasmolysis Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Video: Turgid Elodea Video: Turgid Elodea Animation: Osmosis Animation: Osmosis Video: Plasmolysis Video: Plasmolysis

Facilitated Diffusion: Passive Transport Aided by Proteins Facilitated diffusion- proteins passively carry molecules across membrane; no energy is used Channel proteins make corridors for specific molecules & ions to cross membrane including – Aquaporins- facilitated diffusion of water – Ion channels- open or close in response to a stimulus (gated channels) Carrier proteins change shape, translocating the solute-binding site across the membrane Some diseases are caused by malfunctions in transport systems; ex. cystinuria, a kidney disease Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

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

Concept 7.4: Active transport uses energy to move solutes against their gradients Some transport proteins can move solutes against their concentration gradients Active transport moves substances against their concentration gradient Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig Passive transport Diffusion Facilitated diffusion Active transport ATP

The Need for Energy in Active Transport Active transport: – requires energy, usually in the form of ATP – is performed by specific proteins embedded in the membranes – allows cells to maintain concentration gradients that differ from their surroundings The sodium-potassium pump is one type of active transport system 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 Fig Na + binding stimulates phosphorylation by ATP. Na + ATP P ADP 2 Active transport with the sodium potassium pump

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

How Ion Pumps Maintain Membrane Potential Membrane potential is the voltage difference across a membrane Voltage is created by differences in the distribution of positive and negative ions Two combined forces, the electrochemical gradient, drive ion diffusion across membranes: – A chemical force (ion’s concentration gradient) – An electrical force (effect of the membrane potential on the ion’s movement) Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Electrogenic pump- transport protein that generates voltage across a membrane – Sodium-potassium pump- the major electrogenic pump of animal cells – proton pump- the main electrogenic pump of plants, fungi, and bacteria Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Electrogenic pumps

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

Cotransport: Coupled Transport by a Membrane Protein Cotransport- active transport of one solute indirectly drives transport of another solute Plants use a hydrogen ion gradient generated by proton pumps to drive active transport of nutrients into the cell Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

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 Large molecules (polysaccharides, proteins) cross the membrane in bulk via vesicles Bulk transport requires energy Exocytosis- – transport vesicles migrate to the membrane, fuse with it, and release their contents – Secretory cells use exocytosis to export products Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Animation: Exocytosis Animation: Exocytosis

Endocytosis- macromolecules are taken in by forming vesicles from the plasma membrane – A reversal of exocytosis, it uses different proteins Three types of endocytosis: – Phagocytosis (“cellular eating”) – Pinocytosis (“cellular drinking”) – Receptor-mediated endocytosis Animation: Exocytosis and Endocytosis Introduction Animation: Exocytosis and Endocytosis Introduction Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Phagocytosis- a cell engulfs a particle in a vacuole – It fuses with a lysosome to digest the particle Pinocytosis- molecules are taken up when extracellular fluid is “gulped” into tiny vesicles Receptor-mediated endocytosis- binding ligands to receptors triggers vesicle formation Ligand- any molecule binding specifically to a receptor site of another molecule Animation: Phagocytosis Animation: Phagocytosis Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Animation: Pinocytosis Animation: Pinocytosis Endocytosis Animation: Receptor-Mediated Endocytosis Animation: Receptor-Mediated Endocytosis

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)