Ch. 7 Cell Membrane BIOL 222 Overview: Plasma Membrane Plasma boundary that separates the living cell from its surroundings Selec4ve permeability Allowance of some substances to cross more easily than others Cell Membrane Model Fluid mosaic model Membrane is a fluid structure with a mosaic of various proteins embedded in it Phospholipids most abundant lipid in the plasma amphipathic molecules contain hydrophobic and hydrophilic regions 1
Fig. 7-2 Hydrophilic head WATER Hydrophobic tail WATER The Fluidity of Membranes Phospholipids Can move within the bilayer Most of the lipids, and some proteins, drig laterally Rarely does a molecule flip-flop transversely across the Lateral movement ( 10 7 times per second) Flip-flop ( once per month) Fig. 7-7 Fibers of extracellular matrix (ECM) PLAY Glycoprotein Carbohydrate Glycolipid EXTRACELLULAR SIDE OF MEMBRANE Cholesterol Microfilaments of cytoskeleton Peripheral proteins Integral protein CYTOPLASMIC SIDE OF MEMBRANE 2
Membrane Proteins Peripheral proteins bound to the surface of the Integral proteins Penetrate the hydrophobic core trans proteins Integral proteins that span the Hydrophobic regions of an integral protein consist of one or more stretches of nonpolar amino acids ogen coiled into alpha helices N-terminus C-terminus α Helix EXTRACELLULAR SIDE CYTOPLASMIC SIDE Membrane Proteins Six major funcmons of proteins: Transport Enzymes Signaling molecule Receptor EnzymaMc acmvity ATP Signal transduction Signal transducmon (a) Transport (b) Enzymatic activity (c) Signal transduction Cell-cell recognimon Intercellular joining ARachment to the Glycoprotein (d) Cell-cell recognition (e) Intercellular joining (f) Attachment to the cytoskeleton and extracellular matrix (ECM) cytoskeleton and extracellular matrix (ECM) Membrane Carbohydrates Membrane carbohydrates Used by cells to recognize each other By binding recognimon proteins to surface carbohydrates Glycolipids Carbohydrates covalently bonded to lipids Glycoproteins Carbohydrates bonded to proteins vary among species, individuals, and even cell types in an individual 3
Synthesis and Sidedness of Membranes Membranes have dismnct inside and outside faces ER 1 Asymmetrical distribumon Trans glycoproteins Proteins Secretory protein Lipids Associated carbohydrates Glycolipid Golgi 2 apparatus Determined when the is built by the ER and Golgi apparatus Vesicle 3 Components on the inside of ER or Golgi vesicle Secreted protein 4 Plasma : Cytoplasmic face Extracellular face Trans glycoprotein end up on exterior of cell Membrane glycolipid The Permeability of the Lipid Bilayer Hydrophobic (nonpolar) molecules Dissolve in the lipid bilayer and pass through the rapidly Such as hydrocarbons But only very small ones Polar molecules Do not cross the easily Repelled by inner hydrophobic region Such as sugars Transport Proteins Transport proteins Allow passage of hydrophilic substances across the channel proteins hydrophilic channel that certain molecules or ions can use as a tunnel Aquaporins Facilitate the passage of water 4
Transport Proteins Carrier proteins Bind to molecules and change shape to shurle them across the Specific for the substance it moves Transport Diffusion Tendency for molecules to spread out evenly into the available space Due to Brownian movement Dynamic equilibrium As many molecules cross one way as cross in the other direcmon Fig. 7-11 Molecules of dye Membrane (cross section) WATER Net diffusion Net diffusion Equilibrium (a) Diffusion of one solute Net diffusion Net diffusion (b) Diffusion of two solutes Net diffusion Net diffusion Equilibrium Equilibrium 5
Types of Transport: Passive Simple Diffusion Simple Diffusion Substances move (diffuse) down their concentra4on gradient difference in concentramon of a substance from one area to another Passive transport No energy input required Types of Transport: Passive - Osmosis Osmosis diffusion of water across a selecmvely permeable from the region of lower solute concentramon to the region of higher solute concentramon Water will follow the solutes Fig. 7-12 Lower concentration of solute (sugar) Higher concentration of sugar Same concentration of sugar H 2 O Selectively permeable Osmosis 6
Tonicity Water Balance of Cells Without Walls ability of a solumon to cause a cell to gain or lose water Isotonic solumon Solute concentramon outside is the same as that inside the cell No net water movement across the plasma Hypertonic solumon SoluMon with higher solute concentramon Hypotonic solumon SoluMon with lower solute concentramon (a) Animal cell (b) Plant cell Hypotonic solution Lysed Turgid (normal) Isotonic solution Normal Flaccid Hypertonic solution Shriveled Plasmolyzed Water Balance of Cells Without Walls Hypertonic or hypotonic environments create osmomc problems for organisms Osmoregula4on Filling vacuole 50 µm Control of water balance Necessary adaptamon for life in such environments The promst Paramecium (a) A contractile vacuole fills with fluid that enters from a system of canals radiating throughout the cytoplasm. Contracting vacuole Hypertonic to its pond water environment (b) When full, the vacuole and canals contract, expelling fluid from the cell. Has a contracmle vacuole that acts as a pump Water Balance of Cells with Walls Plant cell in a hypotonic solumon Swells unml the wall opposes uptake Cell is now turgid (firm) If isotonic No net movement of water into the cell the cell becomes flaccid (limp) Plant may wilt 7
Water Balance of Cells with Walls Plasmolysis Hypertonic environment Plant cells lose water pulls away from the wall usually lethal Types of Transport: Passive - Facilitated Diffusion Facilitated diffusion EXTRACELLULA R FLUID transport proteins facilitate (enable) passive movement of molecules across the plasma Channel protein (a) A channel protein Solute CYTOPLASM Channel proteins corridors that allow a specific molecule or ion to cross the Carrier protein Solute Aquaporins for facilitated diffusion of water (b) A carrier protein Ion channels that open or close in response to a smmulus (gated channels) Types of Transport: Ac4ve Transport Ac4ve transport moves substances against their concentramon gradient Requires energy in the form of ATP Performed by specific proteins embedded in the s 8
AcMve transport Types of Transport: Ac4ve Transport allows cells to maintain concentramon gradients that differ from their surroundings Sodium-potassium pump Fig. 7-16-7 EXTRACELLULAR [ ] high FLUID [K + ] low CYTOPLASM 1 [ ] low [K + ] high 2 P ADP ATP 3 P K + K + K + K + K + 6 K + 5 4 P P Ion Pumps Maintain Membrane Poten4al Membrane poten4al Voltage difference across a Voltage is created by differences in the distribumon of posimve and negamve ions 9
Membrane Poten4al Electrogenic pump transport protein that generates voltage across a sodium-potassium pump major electrogenic pump of animal cells + EXTRACELLULAR FLUID proton pump ATP Proton pump + H + H + Main electrogenic pump H + + H + of plants, fungi, and bacteria CYTOPLASM + H + + H + Types of Transport: Ac4ve Transport - Exocytosis Exocytosis Transport vesicles migrate to the, fuse with it, and release their contents Used by secretory cells to export their products Types of Transport: Ac4ve Transport - Endocytosis Endocytosis cell takes in macromolecules by forming vesicles from the plasma reversal of exocytosis, involving different proteins Three types of endocytosis: 1. Phagocytosis ( cellular eamng ) 2. Pinocytosis ( cellular drinking ) 3. Receptor-mediated endocytosis 10
Ac4ve Transport: Endocytosis - Phagocytosis Fig. 7-20a PHAGOCYTOSIS EXTRACELLULAR CYTOPLASM FLUID Pseudopodium 1 µm Pseudopodium of amoeba Food or other particle Food vacuole Bacterium Food vacuole An amoeba engulfing a bacterium via phagocytosis (TEM) Fig. 7-20b Ac4ve Transport: Endocytosis - Pinocytosis PINOCYTOSIS Plasma 0.5 µm Pinocytosis vesicles forming (arrows) in a cell lining a small blood vessel (TEM) Vesicle Fig. 7-20c Ac4ve Transport: Receptor Mediated Endocytosis RECEPTOR-MEDIATED ENDOCYTOSIS Receptor Coat protein Coated vesicle Coated pit Ligand Coat protein A coated pit and a coated vesicle formed during receptormediated endocytosis (TEMs) Plasma 0.25 µm 11
You should now be able to: 1. Define the following terms: amphipathic molecules, aquaporins, diffusion 2. Explain how fluidity is influenced by temperature and composimon 3. DisMnguish between the following pairs or sets of terms: peripheral and integral proteins; channel and carrier proteins; osmosis, facilitated diffusion, and acmve transport; hypertonic, hypotonic, and isotonic solumons 4. Explain how transport proteins facilitate diffusion 5. Explain how an electrogenic pump creates voltage across a, and name two electrogenic pumps 6. Explain how large molecules are transported across a cell 12