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. One method to embed specimen in resin
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 Spontaneously forms a bilayer
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 Exocytosis 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