MEMBRANE STRUCTURE & FUNCTION Chapter 8 KEY CONCEPTS Cellular s are fluid mosaics of lipids and proteins Membrane structure results in selective permeability Passive transport is diffusion of a substance across a with no energy investment Active transport uses energy to move solutes against their gradients Bulk transport across the plasma occurs by exocytosis and endocytosis CELLULAR MEMBRANES ARE FLUID MOSAICS OF LIPIDS AND PROTEINS Phospholipids are the most abundant lipid in the plasma Phospholipids are amphipathicmolecules, containing hydrophobic and hydrophilic regions The fluid mosaic model states that a is a fluid structure with a mosaic of various proteins embedded in it MEMBRANE MODELS: SCIENTIFIC INQUIRY Membranes have been chemically analyzed and found to be made of proteins and lipids Scientists studying the plasma reasoned that it must be a phospholipid Hydrophilic head Hydrophobic tail WATER WATER In 1935, Hugh Davson and James Danielli proposed a sandwich model in which the phospholipid lies between two layers of globular proteins Later studies found problems with this model, particularly the placement of proteins, which have hydrophilic and hydrophobic regions In 1972, S. J. Singer and G. Nicolson proposed that the is a mosaic of proteins dispersed within the, with only the hydrophilic regions exposed to water 1
Phospholipid Freeze-fracture studies of the plasma supported the fluid mosaic model Freeze-fracture is a specialized preparation technique that splits a along the middle of the phospholipid Hydrophobic regions of protein Hydrophilic regions of protein TECHNIQUE Knife Plasma RESULTS Extracellular layer Cytoplasmic layer Proteins PLASMA MEMBRANE Selectively permeable allows some substances to cross more easily than others Fluid mosaic model fluid structure w/ a mosiac of proteins in or attached to a double layer () of phospholipids Composed primarily of lipids, proteins & carbohydrates Inside of extracellular layer Inside of cytoplasmic layer FLUID MEMBRANES Not static held together by hydrophobic interactions ; weaker than covalent bonds Lipids and proteins can drift laterally (within the plane) Remain fluid as temperatures decrease Phospholipids then solidify Temp. at which this occurs depends on type of lipid present Unsaturated phospholipids lower temps due to kinks in tails Saturated phospholipids packed together more tightly Cholesterol (steroid) which is wedged between phospholipids also affected by temp. 2
Warm temp (37C) makes s less fluid ; restricts movement of Permeability of dependent on fluidity Solidified s less permeable & enzymes are inactive Cells alter lipid composition of their to temp conditions MEMBRANE PROTEINS Proteins determine the function of the Integral proteins Penetrate the hydrophobic core of lipid Some are trans completely span the Hydrophobic region consists of nonpolar amino acids Hydrophilic region exposed to aqueous solution on either side of Peripheral Proteins Not embedded in the lipid Appendages that are loosely bound to surface of PROTEI N STRUC TURE DETER MINES FUNCT ION 3
MEMBRANE PERMEABILITY Hydrophobic molecules (hydrocarbons, oxygen & carbon dioxide) dissolve in lipid with ease Ions & polar molecules (hydrophilic) blocked by the hydrophobic core of lipid Other polar molecules like glucose, other sugars & water only pass slowly TRANSPORT PROTEINS Proteins that allow ions & some polar molecules to avoid the lipid Ways to aid in moving (translocating) substances across the Channel proteins hydrophilic channels that tunnel molecules across Aquaporins channel water Carrier proteins hold on to molecules & change shape in ways that shuttle them across PASSIVE TRANSPORT Movement of molecules with no energy being used Diffusion movement of molecules from regions of high concentration to regions of low concentration Down the concentration gradient OSMOSIS Diffusion of water Moves from regions of high concentration to low concentration Tonicity ability of a solution to change the shape / size of a cell by gaining or losing water Directions of water diffusion depends upon the concentration of solutes in the cell s environment Isotonic solute concentration is the same on each side of the plasma No net movement of water Water flows in and out of cell at same rate 4
Hypertonic solute concentration is higher outside the plasma Water diffuses out of cell at a faster rate Causes the cell to shrivel Crenated shrunken cell Hypotonic solute concentration is lower outside the plasma Water diffuses into cell at a higher rate Cell swells and may burst (lysis) OSMOREGULATION CONTROL OF WATER BALANCE If no cell wall (animal, protists) contractile vacuole If cell wall (plant, prokaryotes, fungi & some protists) As water diffuses into cell (hypotonic), pressure is exerted on the cell wall turgid As water diffuses out of cell (hypertonic), no pressure of on cell wall flaccid Cell shrivels & plasma pulls away from cell wall - plasmolysis FACILITATE DIFFUSION Passive transport with aid of proteins Transport proteins are specific only transport particular substances 2 types: carrier proteins & channel proteins Channel proteins can be aquaporins (channel water) or gated channels (stimulus causes them to open or close) ; stimulus can be chemical or electrical Carrier proteins are triggered by binding & release of transporting molecule to change shape 5
ACTIVE TRANSPORT Moving molecules against the concentration gradient (low to high) Cell must expend energy Only involves carrier proteins Energy supplied by ATP NA-K PUMP More Na outside cell than inside More K inside cell than outside Pump cycles 3 Na+ out of cell and 2 K+ into the cell Carrier protein receives energy through phosphorylation high energy phosphate of ATP binds to protein ELECTROGENIC PUMP / PROTON PUMPS Membrane proteins that generate energy by producing a voltage across the Translocatespositive charge in form of H+ Energy is stored for other cellular activities COTRANSPORT A single ATP powered pump that transports a specific solute can indirectly drive the active transport of several other solutes A substance that has diffused across a can be used to do work as it moves back in Ex. In plants sucrose enters a cell in the company of a H+ ion 6
BULK TRANSPORT Movement of large molecules or particles across the via vesicles Exocytosis secretion of particles out of the cell ; removal of secretory proteins by the Golgi apparatus Endocytosis cell takes in macromolecules by forming new vesicles from the plasma Membrane forms a pocket that sinks inward then pinches off to form a vesicle 3 TYPES OF ENDOCYTOSIS Phagocytosis cellular eating ; cell engulfs large particle forming a vacuole which fusing to a lysosome to be digested Pinocytosis cell drinking ; cell gulps droplets of extracellular fluid into vesicles; not specific to substance it transports Receptor-mediated endocytosis coated pits form vesicles when specific molecules (ligands) bind to receptors on the cell surface 7