Unformatted text preview: Lecture #5: Biological membranes • Reading: Chapter 7 • Lecture outline: Membrane structure and func:on – Fluid mosaic model – Membrane proteins – Osmosis – Ac:ve transport Overview: Life at the Edge • The plasma membrane is the boundary that separates the living cell from its surroundings • The plasma membrane exhibits selec5ve permeability, allowing some substances to cross it more easily than others Channel for K+ Potassium Channel Protein Important for nerve transmission Cellular membranes are ﬂuid mosaics of lipids and proteins • Phospholipids are the most abundant lipid in the plasma membrane • Phospholipids are amphipathic molecules, containing hydrophobic and hydrophilic regions • The ﬂuid mosaic model states that a membrane is a ﬂuid structure with a “mosaic” of various proteins embedded in it current model for membranes glcyerol backbone with phosphate group on one end (hydrophillic) Two Fatty Acids esterified to oxygens (hydrophobic) Hydrophilic head Hydrophobic tail WATER WATER Fluid mosiac model of membrane structure aqueous environment Phospholipid bilayer Before the 1960s, scientists believed that the membrane followed the Sandwich model, that a layer of lipids was sandwiched between two layers of proteins. In the late 1960s, scientists saw that this model was wrong because some proteins were hydrophobic and how were they outside the membrane? Microscopy improvements saw that organelle membranes did not have three layers. It looks like 3 layers because of the lipid bilayer and ECM.
hydrophobic environment Hydrophobic regions of protein Hydrophilic regions of protein The Fluid-Mosaic Model The lipid bilayer and an extracellular matrix makes up the three layers. Freeze‐fracture experiments • Freeze‐fracture studies of the plasma membrane supported the ﬂuid mosaic model • Freeze‐fracture is a specialized prepara:on technique that splits a membrane along the middle of the phospholipid bilayer freeze the membrane and then using a thin knife to peel it apart TECHNIQUE Extracellular layer RESULTS Knife Proteins Inside of extracellular layer Plasma membrane Cytoplasmic layer Inside of cytoplasmic layer mostly horizontal movement The Fluidity of Membranes moves at the length of a bacterial cell
Lateral movement (~107 times per second) (a) Movement of phospholipids Fluid Viscous Flip-flop (~ once per month) • Phospholipids in the plasma membrane can move within the bilayer • Most of the lipids, and some proteins, driL laterally • Rarely does a molecule ﬂip‐ﬂop transversely across the membrane Have to be able to move around and interact with substrates
adds fluidity to membranes by stopping phospholipids from being too compact Unsaturated hydrocarbon tails with kinks (b) Membrane fluidity Saturated hydrocarbon tails Cholesterol (c) Cholesterol within the animal cell membrane 1970s JHU experiment Experiment: are membranes ﬂuid? RESULTS Membrane proteins Mixed proteins after 1 hour Human cell Hybrid cell proteins intermixed = proteins move around in the membrane Mouse cell dyed protein Membrane Proteins and Their Func5ons • Peripheral proteins are bound to the surface of the membrane • Integral proteins penetrate the hydrophobic core • Integral proteins that span the membrane are called transmembrane proteins • The hydrophobic regions of an integral protein consist of one or more stretches of nonpolar amino acids, oLen coiled into alpha helices interacting with the surface of the membrane proteins that are embedded in the membrane; possesses hydrophobic regions Six major func:ons of membrane proteins: molecules into/out of cell Transport Enzyma:c ac:vity Signal transduc:on Cell‐cell recogni:on – Intercellular joining – AQachment to the cytoskeleton and extracellular matrix (ECM) – – – –
location of cell and recognition of foreign invaders N-terminus helping cell to respond to environment thru chem rxns EXTRACELLULAR SIDE gap and tight junctions C-terminus α Helix CYTOPLASMIC SIDE Fig. 7-9 Signaling molecule Enzymes passive transport Receptor active transport = hydrolysis of ATP for energy ATP (a) Transport (b) Enzymatic activity Signal transduction (c) Signal transduction proteins with carbohydrates on their surfaces Glycoprotein (d) Cell-cell recognition (e) Intercellular joining (f) Attachment to the cytoskeleton and extracellular matrix (ECM) Synthesis and Sidedness of Membranes • Membranes have dis:nct inside and outside faces • The asymmetrical distribu:on of proteins, lipids, and associated carbohydrates in the plasma membrane is determined when the membrane is built by the ER and Golgi apparatus Builds the plasma membrane Synthesis and Sidedness of Membranes • Membranes have dis:nct inside and outside faces • The asymmetrical distribu:on of proteins, lipids, and associated carbohydrates in the plasma membrane is determined when the membrane is built by the ER and Golgi apparatus ER 1 Transmembrane glycoproteins Secretory protein Glycolipid Golgi 2 apparatus Vesicle 3 Plasma membrane: Cytoplasmic face Extracellular face Transmembrane glycoprotein 4 Secreted protein Membrane glycolipid Membrane structure results in selec5ve permeability • A cell must exchange materials with its surroundings, a process controlled by the plasma membrane • Plasma membranes are selec:vely permeable, regula:ng the cell’s molecular traﬃc • Hydrophobic (nonpolar) molecules, such as hydrocarbons, can dissolve in the lipid bilayer and pass through the membrane rapidly • Polar molecules, such as sugars, do not cross the membrane easily CO2, O2 Passive transport: diﬀusion across a membrane with no energy investment higher concentration to lower concentration no use of ATP • Diﬀusion is the tendency for molecules to spread out evenly into the available space • Although each molecule moves randomly, diﬀusion of a popula:on of molecules may exhibit a net movement in one direc:on • At dynamic equilibrium, as many molecules cross one way as cross in the other direc:on Fig. 7-11 Molecules of dye Membrane (cross section) WATER Net diffusion (a) Diffusion of one solute Net diffusion Equilibrium Net diffusion Net diffusion (b) Diffusion of two solutes Net diffusion Net diffusion Equilibrium Equilibrium Eﬀects of Osmosis on Water Balance • Osmosis is the diﬀusion of water across a selec:vely permeable membrane • Water diﬀuses across a membrane from the region of lower solute concentra:on to the region of higher solute concentra:on If this wasn't possible, cells could burst or shrivel up, depending on the environment Fig. 7-12 Lower concentration of solute (sugar) Higher concentration of sugar Same concentration of sugar H2O Selectively permeable membrane Osmosis Water Balance of Cells Without Walls • Tonicity is the ability of a solu:on to cause a cell to gain or lose water refers to concentrations of salts and solutes • Isotonic solu:on: Solute concentra:on is the same as that inside the cell; no net water movement across the plasma membrane • Hypertonic solu:on: Solute concentra:on is greater than that inside the cell; cell loses water • Hypotonic solu:on: Solute concentra:on is less than that inside the cell; cell gains water Fig. 7-13 Hypotonic solution H2O (a) Animal cell Isotonic solution H2O H2O Hypertonic solution H2O Lysed H2O Normal H2O H2O Shriveled H2O (b) Plant cell cell wall helps keep cell stiffer and buffers changes in water Turgid (normal) Flaccid Plasmolyzed Transport Proteins • Transport proteins allow passage of hydrophilic substances across the membrane • Some transport proteins, called channel proteins, have a hydrophilic channel that certain molecules or ions can use as a tunnel • Channel proteins called aquaporins facilitate the passage of water Facilitated Diﬀusion: Passive Transport Aided by Proteins • In facilitated diﬀusion, transport proteins speed the passive movement of molecules across the plasma membrane • Channel proteins provide corridors that allow a speciﬁc molecule or ion to cross the membrane • Channel proteins include – Aquaporins, for facilitated diﬀusion of water – Ion channels that open or close in response to a s:mulus (gated channels) Fig. 7-15 EXTRACELLULAR FLUID fixed channel that allows particles to follow diffusion gradient Channel protein (a) A channel protein Solute CYTOPLASM binds to the solutes outside the cell and change confirmation to release them inside the cell Carrier protein (b) A carrier protein Solute ATP Ac5ve transport uses energy to move solutes against their gradients • Facilitated diﬀusion is s:ll passive because the solute moves down its concentra:on gradient • Some transport proteins, however, can move solutes against their concentra:on gradients • Ac5ve transport moves substances against their concentra:on gradient • Ac:ve transport requires energy, usually in the form of ATP • Ac:ve transport is performed by speciﬁc proteins embedded in the membranes Na+ is usually in low concentration in cell while K+ is usually in high concentrations in the cell SODIUM POTASSIUM PUMP EXTRACELLULAR FLUID
3 Na+ molecules Na+ Na+ [Na+] high [K+] low Na+ Na+ Na+ Na+ Na+ Na+ [Na+] low [K+] high CYTOPLASM Na+ 1 2 P ADP ATP 3 P Triggers a structural change that increases affinity for K+ K+ K+ + K+ K+ 2 K+ molecules K K+ P P 6 5 for every 3 changes we pump out, we pump in 2 changes, setting up an membrane potential 4 Ion Pumps Maintain Membrane Poten5al • Membrane poten5al is the voltage diﬀerence across a membrane • Voltage is created by diﬀerences in the distribu:on of posi:ve and nega:ve ions anion • Two combined forces, collec:vely called the electrochemical gradient, drive the diﬀusion of ions across a membrane: – A chemical force (the ion’s concentra:on gradient) – An electrical force (the eﬀect of the membrane poten:al on the ion’s movement) cation • An electrogenic pump is a transport protein that generates voltage across a membrane • The sodium‐potassium pump is the major electrogenic pump of animal cells • The main electrogenic pump of plants, fungi, and bacteria is a proton pump H+ are actively moved across membrane Fig. 7-19 – ATP – H+ triggers pumping of sucrose into cell + + H+ H+ H+ Proton pump – H+ – Sucrose-H+ cotransporter + + H+ H+ Diffusion of H+ H+ Sucrose – – to import sugar
molecules coupling H+ pump + + Sucrose Bulk transport by exocytosis and endocytosis • Small molecules and water enter or leave the cell through the lipid bilayer or by transport proteins • Large molecules, such as polysaccharides and proteins, cross the membrane in bulk via vesicles • Bulk transport requires energy • In exocytosis, transport vesicles migrate to the membrane, fuse with it, and release their contents • In endocytosis, the cell takes in macromolecules by forming vesicles from the plasma membrane Fig. 7-20
EXTRACELLULAR FLUID Pseudopodium PHAGOCYTOSIS CYTOPLASM 1 µm Pseudopodium of amoeba “Food”or other particle Food vacuole Bacterium Food vacuole An amoeba engulfing a bacterium via phagocytosis (TEM) PINOCYTOSIS Plasma membrane 0.5 µm Pinocytosis vesicles forming (arrows) in a cell lining a small blood vessel (TEM) Vesicle RECEPTOR-MEDIATED ENDOCYTOSIS Coat protein Receptor Coated vesicle Coated pit Ligand Coat protein A coated pit and a coated vesicle formed during receptormediated endocytosis (TEMs) Plasma membrane 0.25 µm SUMMARY Passive transport Active transport ATP Diffusion Facilitated diffusion You should now be able to: 1. Deﬁne the following terms: amphipathic molecules, aquaporins, diﬀusion 2. Explain how membrane ﬂuidity is inﬂuenced by temperature and membrane composi:on 3. Dis:nguish between the following pairs or sets of terms: peripheral and integral membrane proteins; channel and carrier proteins; osmosis, facilitated diﬀusion, and ac:ve transport; hypertonic, hypotonic, and isotonic solu:ons 4. Explain how transport proteins facilitate diﬀusion 5. Explain how an electrogenic pump creates voltage across a membrane, and name two electrogenic pumps 6. Explain how large molecules are transported across a cell membrane ...
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