Chapter 7~ Membrane Structure & Function

What You Must Know: Why membranes are selectively permeable. The role of phospholipids, proteins, and carbohydrates in membranes. How water will move if a cell is placed in an isotonic, hypertonic, or hypotonic solution. How electrochemical gradients are formed.

Early membrane model (1935) Davson/Danielli – Sandwich model phospholipid bilayer between 2 protein layers Problems: varying chemical composition of membrane, hydrophobic protein parts

Membrane structure, I Selective permeability Amphipathic~ hydrophobic & hydrophilic regions Singer-Nicolson: fluid mosaic model

The freeze-fracture method: revealed the structure of membrane’s interior

Fluid Mosaic Model

Membrane structure, II Phospholipids~ membrane fluidity Cholesterol~ membrane stabilization “Mosaic” Structure~ Membrane carbohydrates ~ cell to cell recognition; oligosaccharides (cell markers); glycolipids; glycoproteins Integral proteins~ transmembrane proteins Peripheral proteins~ surface of membrane Eg. blood transfusions are type-specific

Synthesis and sidedness of membranes

Membrane Proteins Integral Proteins Peripheral Proteins Embedded in membrane Determined by freeze fracture Transmembrane with hydrophilic heads/tails and hydrophobic middles Extracellular or cytoplasmic sides of membrane NOT embedded Held in place by the cytoskeleton or ECM Provides stronger framework

Transmembrane protein structure Hydrophobic interior Hydrophilic ends

Integral & Peripheral proteins

Membrane fluidity Low temps: phospholipids w/unsaturated tails (kinks prevent close packing) Cholesterol resists changes by: limit fluidity at high temps hinder close packing at low temps Adaptations: bacteria in hot springs (unusual lipids); winter wheat ( unsaturated phospholipids) Stays fluid even in cold temperatures.

Membrane structure, III Membrane protein function: •transport •enzymatic activity •signal transduction •intercellular joining •cell-cell recognition •ECM attachment

Membrane traffic Diffusion~ tendency of any molecule to spread out into available space Concentration gradient Passive transport~ NO ENERGY (ATP) needed! Diffusion down concentration gradient (high  low concentration) Osmosis~ the diffusion of water across a selectively permeable membrane

Water balance Isotonic~ equal concentrations of solutes Osmoregulation~ control of water balance Isotonic~ equal concentrations of solutes Hypertonic~ higher concentration of solutes Hypotonic~ lower concentration of solutes Cells with Walls: Turgid (very firm) Flaccid (limp) Plasmolysis~ plasma membrane pulls away from cell wall

Osmoregulation Control solute & water balance Contractile vacuole: “bilge pump” forces out fresh water as it enters by osmosis Eg. paramecium caudatum – freshwater protist

Tonicity is a relative term Isotonic Solution - both solutions have same concentrations of solute. 10% ions in water If the outside solution is isotonic: Concentration of ions and water is equal on both sides of the cell membrane. 10% ions in cell H2O moves in both directions K. White

Tonicity is a relative term Hypertonic Solution - one solution has a higher concentration of solute than another. 20% ions in water If the outside solution is hypertonic: Concentration of salt is lower and water concentration is higher inside the cell than the solution outside the cell. Water will move out of the cell. 10% ions in cell H2O H2O moves out of the cell K.White

Tonicity is a relative term Hypotonic Solution - One solution has a lower concentration of solute than another. 10% ions in water If the outside solution is hypotonic: concentration of salt is higher and water concentration is lower inside the cell than outside. Water will move into the cell. H2O H2O H2O H2O 20% ions in cell H2O moves into the cell K. White

Specialized Transport Facilitated diffusion~ passage of molecules and ions with transport proteins across a membrane down the concentration gradient Transport proteins (channel or carrier proteins) help hydrophilic substances cross Two ways: Provide hydrophilic channel Loosely bind/carry molecule across Eg. ions, polar molecules (H2O, glucose) Active transport~ movement of a substance against its concentration gradient with the help of cellular energy

Aquaporin: channel protein that allows passage of H2O

Glucose Transport Protein (carrier protein)

Specialized Transport Active transport~ Proteins transport substances against concentration gradient (low  high conc) Requires ENERGY (ATP) Eg. Na+/K+ pump, proton pump

Membrane potential cytoplasm is negative compared to ECM Unequal distribution of cations and anions -50 to -200 mV Membrane potential acts like a battery Helps determine movement of cations/anions in and out of cell electrochemical gradient – movement of cations/anions affected by their concentration as well as charge

Electrogenic Pumps: generate voltage across membrane Na+/K+ Pump Proton Pump Pump Na+ out, K+ into cell Nerve transmission Push protons (H+) across membrane Eg. mitochondria (ATP production)

Eg. sucrose-H+ cotransporter (sugar-loading in plants) Cotransport: membrane protein enables “downhill” diffusion of one solute to drive “uphill” transport of other Eg. sucrose-H+ cotransporter (sugar-loading in plants)

Bulk Transport Transport of proteins, polysaccharides, large molecules Endocytosis: take in macromolecules, form new vesicles Exocytosis: vesicles fuse with cell membrane, expel contents

BulkTransport Endocytosis~ take in macromolecules, form new vesicles Bulk transport -Transport of proteins, polysaccharides, large molecules Endocytosis~ take in macromolecules, form new vesicles Exocytosis~ vesicles fuse with cell membrane, expel contents

Types of Endocytosis Phagocytosis: “cellular eating” - solids Pinocytosis: “cellular drinking” - fluids Receptor-Mediated Endocytosis: Ligands bind to specific receptors on cell surface

Passive vs. Active Transport Little or no Energy High  low concentrations DOWN the concentration gradient eg. diffusion, osmosis, facilitated diffusion (w/transport protein) Requires Energy (ATP) Low  high concentrations AGAINST the concentration gradient eg. pumps, exo/endocytosis