Membranes

1. Membranes Chapter 5

2. 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

5. Cellular membranes have 4 components • Phospholipid bilayer • Flexible matrix, barrier to permeability • Transmembrane proteins • Integral membrane proteins • Interior protein network • Peripheral membrane proteins • 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

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: • Transporters • Enzymes • Cell-surface receptors • Cell-surface identity markers • Cell-to-cell adhesion proteins • Attachments to the cytoskeleton

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 • Nonpolar regions that insert into the internal portion of the lipid bilayer • 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 b-barrel • Interior is polar and allows water and small polar molecules to pass through the membrane

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

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

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

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

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32. 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

33. 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

35. 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

36. 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

37. 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

38. 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

40. 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

42. 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

44. 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.

45. 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