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CHAPTER 5 LECTURE SLIDES Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Membranes Chapter 5 Membrane Structure • Phospholipids arranged in a bilayer • Globular proteins inserted in the lipid bilayer • Fluid mosiac model – mosaic of proteins floats in or on the fluid lipid bilayer like boats on a pond 3 4 5 • Cellular membranes have 4 components 1. Phospholipid bilayer • Flexible matrix, barrier to permeability 2. Transmembrane proteins • Integral membrane proteins 3. Interior protein network • Peripheral membrane proteins 4. Cell surface markers • Glycoproteins and glycolipids 6 • Both transmission electron microscope (TEM) and scanning (SEM) used to study membranes • One method to embed specimen in resin – 1µm shavings – TEM shows layers 7 • Freeze-fracture visualizes inside of membrane 8 Phospholipids • Structure consists of – Glycerol – a 3-carbon polyalcohol – 2 fatty acids attached to the glycerol • Nonpolar and hydrophobic (“water-fearing”) – Phosphate group attached to the glycerol • Polar and hydrophilic (“water-loving”) • Spontaneously forms a bilayer – Fatty acids are on the inside – Phosphate groups are on both surfaces 9 10 • Bilayers are fluid • Hydrogen bonding of water holds the 2 layers together • Individual phospholipids and unanchored proteins can move through the membrane 11 • Environmental influences – Saturated fatty acids make the membrane less fluid than unsaturated fatty acids • “Kinks” introduced by the double bonds keep them from packing tightly • Most membranes also contain sterols such as cholesterol, which can either increase or decrease membrane fluidity, depending on the temperature – Warm temperatures make the membrane more fluid than cold temperatures • Cold tolerance in bacteria due to fatty acid desaturases 12 Membrane Proteins • Various functions: 1. 2. 3. 4. 5. 6. Transporters Enzymes Cell-surface receptors Cell-surface identity markers Cell-to-cell adhesion proteins Attachments to the cytoskeleton 13 14 Structure relates to function • Diverse functions arise from the diverse structures of membrane proteins • Have common structural features related to their role as membrane proteins • Peripheral proteins – Anchoring molecules attach membrane protein to surface 15 • Anchoring molecules are modified lipids with 1. Nonpolar regions that insert into the internal portion of the lipid bilayer 2. Chemical bonding domains that link directly to proteins 16 • Integral membrane proteins – Span the lipid bilayer (transmembrane proteins) • Nonpolar regions of the protein are embedded in the interior of the bilayer • Polar regions of the protein protrude from both sides of the bilayer – Transmembrane domain • Spans the lipid bilayer • Hydrophobic amino acids arranged in α helices 17 • Proteins need only a single transmembrane domain to be anchored in the membrane, but they often have more than one such domain 18 • Bacteriorhodopsin has 7 transmembrane domains forming a structure within the membrane through which protons pass during the light-driven pumping of protons 19 Membrane Proteins • Pores – Extensive nonpolar regions within a transmembrane protein can create a pore through the membrane – Cylinder of sheets in the protein secondary structure called a -barrel • Interior is polar and allows water and small polar molecules to pass through the membrane 20 21 Passive Transport • Passive transport is movement of molecules through the membrane in which – No energy is required – Molecules move in response to a concentration gradient • Diffusion is movement of molecules from high concentration to low concentration – Will continue until the concentration is the same in all regions 22 23 • Major barrier to crossing a biological membrane is the hydrophobic interior that repels polar molecules but not nonpolar molecules – Nonpolar molecules will move until the concentration is equal on both sides – Limited permeability to small polar molecules – Very limited permeability to larger polar molecules and ions 24 • Facilitated diffusion – Molecules that cannot cross membrane easily may move through proteins – Move from higher to lower concentration – Channel proteins • Hydrophilic channel when open – Carrier proteins • Bind specifically to molecules they assist • Membrane is selectively permeable 25 Channel proteins • Ion channels – Allow the passage of ions – Gated channels – open or close in response to stimulus (chemical or electrical) – 3 conditions determine direction • Relative concentration on either side of membrane • Voltage differences across membrane • Gated channels – channel open or closed 26 27 Carrier proteins • Can help transport both ions and other solutes, such as some sugars and amino acids • Requires a concentration difference across the membrane • Must bind to the molecule they transport – Saturation – rate of transport limited by number of transporters 28 29 Osmosis • Cytoplasm of the cell is an aqueous solution – Water is solvent – Dissolved substances are solutes • Osmosis – net diffusion of water across a membrane toward a higher solute concentration 30 31 Osmotic concentration • When 2 solutions have different osmotic concentrations – Hypertonic solution has a higher solute concentration – Hypotonic solution has a lower solute concentration • When two solutions have the same osmotic concentration, the solutions are isotonic • Aquaporins facilitate osmosis 32 Osmotic pressure • Force needed to stop osmotic flow • Cell in a hypotonic solution gains water causing cell to swell – creates pressure • If membrane strong enough, cell reaches counterbalance of osmotic pressure driving water in with hydrostatic pressure driving water out – Cell wall of prokaryotes, fungi, plants, protists • If membrane is not strong, may burst – Animal cells must be in isotonic environments 33 34 Maintaining osmotic balance • Some cells use extrusion in which water is ejected through contractile vacuoles • Isosmotic regulation involves keeping cells isotonic with their environment – Marine organisms adjust internal concentration to match sea water – Terrestrial animals circulate isotonic fluid • Plant cells use turgor pressure to push the cell membrane against the cell wall and keep the cell rigid 35 Active Transport • Requires energy – ATP is used directly or indirectly to fuel active transport • Moves substances from low to high concentration • Requires the use of highly selective carrier proteins 36 • Carrier proteins used in active transport include – Uniporters – move one molecule at a time – Symporters – move two molecules in the same direction – Antiporters – move two molecules in opposite directions – Terms can also be used to describe facilitated diffusion carriers 37 Sodium–potassium (Na+–K+) pump • Direct use of ATP for active transport • Uses an antiporter to move 3 Na+ out of the cell and 2 K+ into the cell – Against their concentration gradient • ATP energy is used to change the conformation of the carrier protein • Affinity of the carrier protein for either Na+ or K+ changes so the ions can be carried across the membrane 38 39 Coupled transport • Uses ATP indirectly • Uses the energy released when a molecule moves by diffusion to supply energy to active transport of a different molecule • Symporter is used • Glucose–Na+ symporter captures the energy from Na+ diffusion to move glucose against a concentration gradient 40 41 Bulk Transport • Endocytosis – – – – • Exocytosis – • Movement of substances into the cell Phagosytosis – cell takes in particulate matter Pinocytosis – cell takes in only fluid Receptor-mediated endocytosis – specific molecules are taken in after they bind to a receptor Movement of substances out of cell Requires energy 42 43 • In the human genetic disease familial hypercholesterolemia, the LDL receptors lack tails, so they are never fastened in the clathrin-coated pits and as a result, do not trigger vesicle formation. The cholesterol stays in the bloodstream of affected individuals, accumulating as plaques inside arteries and leading to heart attacks. 44 • Exocytosis – – – Movement of materials out of the cell Used in plants to export cell wall material Used in animals to secrete hormones, neurotransmitters, digestive enzymes 45