chapt05_lecture.ppt

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 pr...

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. Transporters 2. Enzymes 3. Cell-surface receptors 4. Cell-surface identity markers 5. Cell-to-cell adhesion proteins 6. 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 – 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 Exocytosis – 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