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 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Phospholipids are the most abundant lipid in the plasma membrane Phospholipids are amphipathic molecules, containing hydrophobic and hydrophilic regions Fluid mosaic model states that a membrane is a fluid structure with a “mosaic” of various proteins embedded in it Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 7-2 Hydrophilic head WATER Hydrophobic tail WATER

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

The Fluidity of Membranes Phospholipids in the plasma membrane can move within the bilayer Most of the lipids, and some proteins, drift laterally Rarely does a molecule flip-flop transversely across the membrane Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

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

As temperatures cool, membranes switch from a fluid state to a solid state Membranes rich in unsaturated fatty acids (double bonds) are more fluid that those rich in saturated fatty acids (single bonds) Membranes must be fluid to work properly (salad oil) Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

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

Quick think How would you expect the saturation levels of membrane phospholipid fatty acids to differ in plants adapted to cold environments and plants adapted to hot environments?

Fig. 7-5c Cholesterol (c) Cholesterol within the animal cell membrane Cholesterol regulates fluidity - Warm temperatures (body temp), cholesterol restrains movement of phospholipids - At cool temperatures, it maintains fluidity by preventing tight packing

Different proteins embedded in the fluid matrix of lipid bilayer Peripheral proteins = bound to surface of membrane Integral proteins penetrate the hydrophobic core – transmembrane proteins (span membrane) Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 7-8 N-terminus C-terminus  Helix CYTOPLASMIC SIDE EXTRACELLULAR 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) Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

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)

Cell-Cell Recognition Cells recognize each other by binding to surface molecules, often carbohydrates, on the plasma membrane – Carbohydrates can form glycolipids or glycoproteins 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 Membranes have distinct inside and outside faces distribution of proteins, lipids, etc. in the plasma membrane is determined when the membrane is built by the ER and Golgi apparatus

Selective permeability Plasma membranes are selectively permeable (since cells must exchange material with surroundings) Hydrophobic (nonpolar) molecules, like hydrocarbons, can dissolve in lipid bilayer and pass through the membrane rapidly Polar molecules, such as sugars, do not cross the membrane easily Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Transport Proteins Transport proteins allow passage of hydrophilic substances across the membrane – channel proteins, have a hydrophilic channel that certain molecules or ions can use as a tunnel aquaporins facilitate the passage of water – carrier proteins bind to molecules and change shape to shuttle them across the membrane specific for the substance it moves Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Quick think Why would water molecules need a transport protein to move rapidly and in large quantities across a membrane?

Passive transport Diffusion is the tendency for molecules to spread out evenly into the available space – Move from region of higher solute concentration to lower solute concentration 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 their concentration gradient The diffusion of a substance across a biological membrane (down concentration gradient) is passive transport (requires no energy from the cell to make it happen) Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Osmosis 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 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

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

Water Balance of Cells Without Walls 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 plasma membrane Hypertonic solution: Solute concentration outside the cell is greater than that inside the cell; cell loses water Hypotonic solution: Solute concentration outside cell is less than that inside the cell; cell gains water Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Osmoregulation, the control of water balance, is a necessary adaptation for life in such environments 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 a hypotonic solution swells until the wall opposes uptake (turgid) In isotonic solution the cell becomes flaccid (limp), and the plant may wilt Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Video: Plasmolysis Video: Plasmolysis Video: Turgid Elodea Video: Turgid Elodea Animation: Osmosis Animation: Osmosis In a hypertonic environment, plant cells lose water; eventually, the membrane pulls away from the wall, a usually lethal effect called plasmolysis Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Quick think In the supermarket, produce is often sprayed with water. Explain why this makes vegetables look crisp.

Facilitated diffusion Diffusion of solutes across membrane with protein channels Proteins are specific; solute binds to protein, it changes conformation and solute is released to other side of membrane

Active Transport Moves molecules from low concentration to high concentration AGAINST a concentration gradient Requires ATP, so cell must use energy

Sodium-Potassium Pump Na+ binds to protein pump ATP then adds a phosphate group to the protein: phosphorylation

Sodium-Potassium Pump Phosphorylation causes protein to change shape

Sodium-Potassium Pump Conformational change in protein releases Na+ ions to outside; K+ ions bind to protein

Sodium-Potassium Pump K+ binding triggers release of phosphate group

Sodium-Potassium Pump Protein returns to original shape due to loss of phosphate; K+ is released

Sodium-Potassium Pump Cycle starts all over again as Na+ binds to protein

Cotransport ATP-powered pump drives the transport of other materials As H+ is pumped out, sucrose is brought in

Transport of Large Molecules Bulk transport brings in large molecules such as food, solids #1-4: endocytosis #5-8: exocytosis

Exocytosis Substance is contained within vesicle Vesicle meets up with inner face of cell membrane, fuses Contents are expelled to outside

Phagocytosis Engulfing large particles of food Cell surrounds food Food is enclosed in a vesicle and taken into cell

Pinocytosis Like phagocytosis, except water is being taken in

Receptor-mediated endocytosis Molecule binds to receptor protein Protein-molecule migrates to coated pit Pit pinches off, forms a vacuole Receptor protein migrates back to membrane