5. CELL MEMBRANE
CELL MEMBRANE - is a 2-dimensional fluid mosaic of lipids and proteins and also is in constant motion.
- It’s a thin, elastic structure, ~7.5 nanometers thick and envelops the cell.
- Composed mainly of proteins and lipids.
- Basic structure is a lipid bilayer - a thin film of ~ 2 molecules thick and continuous over the entire cell surface.

6. 2. CHOLESTEROL 3. GLYCOLIPID. CELL MEMBRANE
Lipid bilayer is made of 3 types of molecules ;
1. PHOSPHOLIPIDS
2. CHOLESTEROL
3. GLYCOLIPID.

7. Cholesterol

8. iii) GLYCOLIPIDs -are lipids with attached carbohydrate groups
~ 5% of membrane lipids
- Non-polar parts - are the fatty acid “tails”
Polar parts - are the attached carbohydrate groups
Only appear on membrane layer that faces extra-cellular fluid (one reason for the asymmetry of the two sides).

9. Asymetrical Distribution of Lipids
Extracellular
Intracellular
Modified from Figure 11-17, Page 355 from: Essential Cell Biology by Alberts et al. 1997, Garland Publishing Inc. New York, NY

10. Cholesterol can fill gaps between phospholipids
Saturated FA’s = increase in fluidity
Modified from Figure and Panel 2-4 from: Essential Cell Biology: An introduction to the Molecular Biology of the Cell by Alberts, Bray, Johnson, Lewis, Raff, Roberts and Walter 1997, Garland Publishing Inc. New York, NY

13. MEMBRANE PROTEINS 1. PERIPHERAL PROTEINS
Are membrane proteins that are located on the periphery of membranes and they are either on the cell surface or on the inside of cell
They associate with membrane lipids or integral proteins at inner or outer surface of the membrane.
They can be stripped away from membrane by methods that do not disrupt membrane integrity
FUNCTION
1. As enzymes on cell surfaces
2. As regulatory portions of ion channels and transmembrane receptors
3. Roles in cell signaling – by some reversible attachment of proteins on
cell surface

14. Membrane Proteins 14

15. The proteins in the plasma membrane may provide a variety of major cell functions.
Fig. 8.9
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings 15

16. 16

17. 17

18. 2. SELECTIVE PERMEABILITY
PROPERTIES OF CELL MEMBRANES
2 main properties are;
1. FLUIDITY
2. SELECTIVE PERMEABILITY

19. PROPERTIES OF CELL MEMBRANES
1. FLUIDITY – (2-dimensional fluid in constant motion)
SIGNIFICANCE :
Allows for fusion of membranes (e.g. fusion of vesicles with organelles)
Allows for diffusion of new lipids and new proteins laterally, so they are equally distribution
Allows for diffusion of proteins and other molecules laterally across the membrane in signaling/reactions
Allows for proper separation of membranes during cell division

20. MEMBRANE FLUIDlTY is determined by;
PROPERTIES OF CELL MEMBRANES - FLUIDITY
MEMBRANE FLUIDlTY is determined by;
1. LIPID COMPOSITION
2. TEMPERATURE

21. Phospholipids

22. Cholesterol 22

23. Cholesterol

24. PROPERTIES OF CELL MEMBRANES - FLUIDITY
2. TEMPERATURE
Organisms regulate lipid composition (thus membrane fluidity) in response to temperature.
Cold/ low temperatures – membranes “gel” and are not fluid.
Hot/ high temperature – membranes are too fluid and become “leaky” allowing ions to cross.
Hibernating animals incorporate more unsaturated fatty acids (fatty acids with double bonds) to prepare for drop in their body temperature .

25. PROPERTIES OF CELL MEMBRANES - SELECTIVE PERMEABILITY
SELECTIVE PERMEABILITY OF MEMBRANE – they are selective for the movement of molecules across the membrane.
MOVEMENT OF MOLECULE across membrane is limited by;
i) SIZE - small molecules cross membrane and large molecules do not
E.g. Water, O2, CO2, ethanol (46 MW) and glycerol (92 MW) can cross the membrane
- Glucose (180 MW) can NOT cross membrane
ii) POLARITY - Hydrophobic molecules can “dissolve” in the lipid bilayer, not polar molecules.
E.g. Ethanol is more hydrophobic than glycerol so crosses membrane faster
iii) IONIC CHARGE-– membranes are highly impermeable to ions
But ions and large molecules do pass across biological
membranes – through proteins that pass through membranes (integral proteins – channel/transport protein)

27. 15.1 A pure phospholipid bilayer acts as a selectively permeable barrier
Figure 15-1

30. Membrane Transport Simple Diffusion - 2 types of molecules
1. Small, nonpolar
Oxygen
2. Small, polar, noncharged
H2O (some - more later)
Ethanol

31. Water Channels- The aquaporins
Selectivity filter generated by hydrophobic residues that line the channel allowing only one molecule of water to pass at a time HO H
H2O
H2O
H2O
Aquaporins

34. Membrane Transport

36. Ion Channels -small aqueous holes
Properties
selective
fast
passive
gated - open or closed

37. Ion Channels -small aqueous holes
Properties
selective
fast
passive
gated - open or closed

38. CHANNELS The pore in some channels can be opened or closed.
Opening/ closing of channels are controlled/gated by a specific stimulus.
Example of a specific stimulus:
i) Voltage; - VOLATGE - GATED CHANNEL
ii) Ligand; - LIGAND -GATED CHANNELS
iii) Specific stress; - STRESS – ACTIVATED CHANNELS

39. VOLTAGE - GATED CHANNEL
Example;
Na+ voltage gated channels opens when the membrane potential
depolarizes (i.e. becomes more positive). It has activation and inactivation
gates.

40. LIGAND- GATED CHANNEL
Binding of a chemical (ligand) to a specific site on the receptor causes a change in membrane potential and causes it to allow a specific ion to pass through the channel in the membrane.

41. STRESS - ACTIVATED GATED ION CHANNEL
The channels open/close when a physical stress is applied to the channel protein
E.g. Auditory hair cells converts a physical stress to an electrical signal.

42. Selectivity Filter K+ O H - O O

43. Selectivity Filter K+ O H - O O

44. Selectivity Filter K+ O H - O O

45. Selectivity Filter O H - O H - K+ O O

46. Selectivity Filter O H - O H - O O K+

47. Selectivity Filter O H - O H - O O K+ O H - O H -

48. Selectivity Filter O H - O H - O O K+ O H - O H -

49. Selectivity Filter O H O H
Na+ O O

50. Selectivity Filter
Na+ O H O O

51. Selectivity Filter
Na+ O H O O

52. زندگي ، خواب گراني است به ارزاني عمر

53. 3 Types of Gating

54. Active Transport REQUIRES ENERGY!
Used to move molecules against a gradient
REQUIRES ENERGY!
Two types
. Pumps
. Coupled Transport (co-transport)

55. Na+/K+ ATPase

56. Primary Active Transport: Pumps Products
Figure 5-24: Mechanism of the Na+ - K+ -ATPase (75%)

59. Secondary Active Transport: Uses Kinetic Energy of [ion]
Cotransports
[Ion ] restored
using ATP
Figure 5-25: Sodium-glucose symporter

61. Endocytosis
Exocytosis

64. (extracellular fluid)
pinocytosis
(extracellular fluid) 1 3 3 2
vesicle containing
extracellular
fluid
(cytoplasm)
cell
(b)
Figure: 03-07
Title:
Two types of endocytosis.
Caption:
(a) To capture drops of liquid, a dimple in the plasma membrane deepens and eventually pinches off as a fluid-filled vesicle, which contains a random sampling of the extracellular fluid. (b) Pseudopodia encircle an extracellular particle. The ends of the pseudopodia fuse, forming a large vesicle that contains the engulfed particle.
phagocytosis
food particle
pseudopod 1 2 3
particle
enclosed in vesicle

65. Endocytosis and Exocytosis: VacuoleTransport
Figure 5-28: Receptor-mediated endocytosis and exocytosis

70. Transepithelial and Transcytosis
Cross two membranes
Apical
Basolatera
Absorption
Secretion

71. Transepithelial and Transcytosis
Figure 5-30: Transepithelial transport of glucose

72. Transepithelial and Transcytosis
Figure 5-31: Transcytosis across the capillary endothelium