1. Membranes Chapter 5 Adapted by G. Cornwall, Ph.D.
From Raven’s Biology, McGraw Hill Publishing

2. Membrane Structure Phospholipids arranged in a bilayer
Globular proteins inserted in the lipid bilayer
Integral membrane proteins
Peripheral membrane proteins
Fluid mosaic model – mosaic of proteins floats in or on the fluid lipid bilayer like boats on a pond

3. 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

4. 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

5. Freeze-fracture visualizes inside of membrane

6. 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”)

7. Spontaneously forms a bilayer
Fatty acids are on the inside
Phosphate groups are on both surfaces

8. Bilayers are fluid
Hydrogen bonding of water holds the 2 layers together
Individual phospholipids and unanchored proteins can move through the membrane

9. 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

10. Membrane Proteins Various functions: Transporters Enzymes
Cell-surface receptors
Cell-surface identity markers
Cell-to-cell adhesion proteins
Attachments to the cytoskeleton

11. 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

12. 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

13. 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

14. Transmembrane domain
Spans the lipid bilayer
Hydrophobic amino acids arranged in α helice
Proteins need only a single transmembrane domain to be anchored in the membrane, but they often have more than one such domain

15. Bacteriorhodopsin has 7 transmembrane domains forming a structure within the membrane through which protons pass during the light-driven pumping of protons

16. 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

17. Card Quiz A
Which part of the phospholipid is responsible for H bonding with water?
Fatty acid chains
Phosphate head
Glycerol molecule
Non polar tails
Bloom’s level: Knowledge/Understanding

18. Card Quiz A
What type of amino acids would you expect to find in the transmembrane portion of a membrane protein?
Charged amino acids
Polar amino acids
Non-polar amino acids
Hydrophilic amino acids
Bloom’s level: Application

19. Card Quiz A
How are transmembrane proteins held in the correct position in the membrane?
Covalent bonding to the phosphate group
Hydrophobic domain is held in place by hydrophobic exclusion
Cytoskeleton filaments keep them in place
The cell wall keeps the proteins in place
Bloom’s level: Knowledge/Understanding

20. Card Quiz Answers
Yellow
Red
Blue
Bloom’s level: Application

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
Simple Diffusion is movement of molecules from high concentration to low concentration across a semipermeable membrane
A form of Passive Transport
Will continue until the concentration is the same in all regions
Annimation

22. 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

23. 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

24. 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

25. 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

26. 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

27. 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

28. Osmotic pressure
The amount of water that enters a cell depends on the difference in solute concentration between the cell and the extracellular fluid
Cell in a hypotonic solution gains water causing cell to swell – creates pressure

29. If membrane is not strong, may burst
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

30. Maintaining osmotic balance
Some cells use extrusion in which water is ejected through contractile vacuoles

31. 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

32. Card Quiz B Permeability refers to –
the movement of molecules from an area of greater concentration to an area of lower concentration
the amount of solute in a solution
the extent a membrane allows a substance to pass through
the state of being permanent
Bloom’s level: Knowledge/Understanding

33. Card Quiz B
The movement of molecules from an area of high concentration to an area of low concentration is –
Osmosis
Active transport
Solubility
Diffusion
Bloom’s level: Knowledge/Understanding

34. Card Quiz B
A cell is placed in an hypotonic solution. Which way will the water move?
Into the cell
Out of the cell
No net movement
Bloom’s level: Knowledge/Understanding

35. Card Quiz B
If a blood cell (0.9% NaCl) shrivels, what type of solution was it placed in?
0.8%
0.1%
0.9% 4%
Bloom’s level: Application

36. Card Quiz Answers
Blue
Yellow
Bloom’s level: Application

37. Active Transport
Some molecules need to be moved against their concentration gradient
Requires energy – ATP is used directly or indirectly to fuel active transport
Moves substances from low to high concentration (against the gradient)
Requires the use of highly selective carrier proteins

38. Carrier proteins used in active transport
Uniporters – move one molecule at a time
Symporters – move two molecules in the same direction
Antiporters – move two molecules in opposite directions

39. 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

40. 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

41. Coupled transport Uses ATP indirectly (2° Active Transport)
Uses the energy released when a molecule moves by diffusion to supply energy to active transport of a different molecule

42. Symporter is used
Glucose–Na+ symporter captures the energy from Na+ diffusion to move glucose against a concentration gradient

43. Bulk Transport Endocytosis Movement of substances into the cell
Phagocytosis – cell takes in particulate matter
Pinocytosis – cell takes in only fluid

44. 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.

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

46. Card Quiz C A macrophage engulfing a bacterium is an example of –
Exocytosis
Pinocytosis
Chemocytosis
Phagocytosis
Bloom’s level: Knowledge/Understanding

47. Card Quiz C
What type of transport protein can move 2 different molecules in the same direction?
Uniporter
Antiporter
Symporter
Multiporter
Bloom’s level: Knowledge/Understanding

48. Card Quiz C
How is the sodium-potassium pump able to move ions against their concentration gradients?
Facilitated diffusion
Expending ATP
Ion channels
Bulk transport
Bloom’s level: Application

49. Card Quiz Answers
Green
Blue
Red
Bloom’s level: Application