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Membrane Structure Membranes Chapter 5 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 2 Membrane Components Cellular Transmission electron microscope (TEM) of the cell membrane 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 3 4 Phospholipids Structure consists of Bilayers are fluid bonding of water holds the 2 layers together Individual phospholipids and unanchored proteins can move through the membrane ◦ Glycerol – a 3-carbon polyalcohol ◦ 2 fatty acids attached to the glycerol ◦ Phosphate group attached to the glycerol Spontaneously Hydrogen forms a bilayer ◦ Fatty acids are on the inside ◦ Phosphate groups are on both surfaces 5 6 1 Structure relates to function Membrane Proteins Various 1. 2. 3. 4. 5. 6. Diverse functions arise from the diverse structures of membrane proteins Have common structural features related to their role as membrane proteins Peripheral proteins functions: Transporters Enzymes Cell-surface receptors Cell-surface identity markers Cell-to-cell adhesion proteins Attachments to the cytoskeleton ◦ Anchoring molecules attach membrane protein to surface 7 Integral membrane proteins 8 Membrane Proteins - Pores Extensive nonpolar regions within a transmembrane protein can create a pore through the membrane Span the lipid bilayer (transmembrane proteins) ◦ Cylinder of β sheets in the protein secondary structure called a β-barrel Interior is polar and allows water and small polar molecules to pass through the membrane ◦ 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 9 Diffusion Rates 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 10 Diffusion is movement of molecules from high concentration to low concentration down their concentration gradient Factors affecting diffusion rate through a membrane ◦ temperature - ↑ temp., ↑ motion of particles ◦ molecular weight - larger molecules move slower ◦ steepness of concentrated gradient - ↑difference, ↑ rate ◦ membrane surface area - ↑ area, ↑ rate ◦ membrane permeability - ↑ permeability, ↑ rate ◦ Will continue until the concentration is the same in all regions . Animation 11 12 2 The Nature of the Membrane and Molecule Transport Transport of Polar Molecules Facilitated 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 diffusion ◦ Molecules that cannot cross membrane easily may move through proteins ◦ Move from higher to lower concentration- down their concentration gradient animation ◦ Channel proteins Hydrophilic channel when open ◦ Carrier proteins Bind specifically to molecules they assist Membrane is selectively permeable 13 Channel proteins Ion 14 Carrier proteins channels 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 ◦ 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 ◦ Saturation – rate of transport limited by number of transporters 15 Osmosis Cytoplasm 16 Osmotic concentration of the cell is an aqueous When 2 solutions have different osmotic concentrations solution ◦ Water is solvent ◦ Dissolved substances are solutes ◦ Hypertonic solution has a higher solute concentration ◦ Hypotonic solution has a lower solute concentration Osmosis – net diffusion of water across a membrane toward a higher solute concentration When two solutions have the same osmotic concentration, the solutions are isotonic Aquaporins facilitate osmosis 17 18 3 Osmotic pressure Force needed to stop osmotic flow 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 ◦ Cell wall of prokaryotes, fungi, plants, protists If membrane is not strong, may burst ◦ Animal cells must be in isotonic environments 19 Maintaining osmotic balance 20 Active Transport Requires 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 energy – ATP is used directly or indirectly to fuel active transport Moves substances from low to high concentration-up their concentration gradient. Requires the use of highly selective carrier proteins Plant cells use turgor pressure to push the cell membrane against the cell wall and keep the cell rigid 21 Sodium–potassium (Na+–K+) pump Carrier Protein and Active Transport Carrier 22 Direct use of ATP for active transport an antiporter to move 3 Na+ out of the cell and 2 K+ into the cell proteins used in active transport Uses 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 ◦ 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 23 24 4 Coupled transport- Secondary Active Transport Uses ATP indirectly 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 Uses 25 26 Bulk Transport Endocytosis ◦ ◦ ◦ ◦ Exocytosis ◦ Animation 27 Movement of substances out of cell Requires energy 28 29 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 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. 30 5 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 31 6