1. The Plasma Membrane
Gateway to the Cell

2. Functions of Plasma Membrane
Protective barrier
Regulate transport in & out of cell (selectively permeable)
Allow cell recognition
Provide anchoring sites for filaments of cytoskeleton

3. Functions of Plasma Membrane
Provide a binding site for enzymes
Interlocking surfaces bind cells together (junctions)
Contains the cytoplasm (fluid in cell)

4. Membrane Components Phospholipids Proteins (peripheral and integral)
The Plasma Membrane
4/20/2017
Phospholipids
Proteins (peripheral and integral)
Cholesterol
Carbohydrates
G. Podgorski, Biol. 1010

5. Composition of Membranes
Water
Lipid
many different types
Carbohydrate
on lipids and proteins
Simple to very complex
Protein
Many different types

6. Membrane lipids Glycerophospholipids (phosphoglycerides)
Sphingolipids:
sphingophospholipids
sphingoglycolipids
Cholesterol

7. Phosphoglycerides Simplest glycerophos-pholipids is phosphatidic acid
Different molecules are added onto the phosphate group to make the other phosphoglycerides
Phosphate
Hydrophillic region
Glycerol
Ester linkage
Fatty acids (saturated and unsaturated)
Hydrophobic region

8. Phosphoglycerides (continued)
Polar groups attached to phosphatidic acid
Inositol Serine
Glycerol Choline
ethanolamine

9. Phosphoglycerides (continued)
Hydrophilic region
Hydrophobic region

10. Sphingolipids Based on Sphingosine
Fatty acyl group
Amide linkage
Based on Sphingosine
Sphingosine convertrd to cermide with the condensation with a long chain fatty acid
Sphingomyelin derived from cerimides

11. Glycolipids
Carbohydrate attached to lipid by way of beta glycosidic bond instead of phosphate ester
Usually sphingosine derivative (by way of ceramide)
Cerebroside
Galactocerebroside
Glucocerobroside
Ganglioside
More complex carbohydrate structure
Contains sialic acid (an acidic carbohydrate)

12. Cholesterol
Hydrophillic region
Hydrophobic region

13. Membrane proteins Varies from membrane to membrane
Amount
Diversity
Classes of membrane proteins
Integral
Peripheral
Lipid anchored

14. Proteins Are Critical to Membrane Function
The Plasma Membrane
4/20/2017
Proteins Are Critical to Membrane Function
G. Podgorski, Biol. 1010

15. Membrane protein function
•transport
•enzymatic activity
•signal transduction
•intercellular joining
•cell-cell recognition
•ECM attachment

16. Fluid mosaic model FLUID MOSAIC MODEL
The Plasma Membrane
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FLUID MOSAIC MODEL
Fluid mosaic model
FLUID- because individual phospholipids and proteins can move around freely within the layer, like it’s a liquid.
MOSAIC- because of the pattern produced by the scattered protein molecules when the membrane is viewed from above.
G. Podgorski, Biol. 1010

17. Movement of lipids in membrane
Rotation
Flexing of fatty acyl chain
Lateral diffusion
Transverse (flip-flop) from one side to other
Frequently
Infrequently

18. RBC plasma membrane

19. Semipermeable Membrane
Small molecules and larger hydrophobic molecules move through easily.
e.g. O2, CO2, H2O
Ions, hydrophilic molecules larger than water, and large molecules such as proteins do not move through the membrane on their own.

20. Types of Transport Across Cell Membranes

21. Simple Diffusion
Diffusion is a PASSIVE process which means no energy is used to make the molecules move
Molecules move from area of HIGH to LOW concentration

22. Diffusion of H2O Across A Membrane
High H2O potential Low solute concentration
Low H2O potential High solute concentration

23. NO NET MOVEMENT OF H2O (equal amounts entering & leaving)
Isotonic Solution
Hypotonic Solution
Hypertonic Solution
NO NET MOVEMENT OF H2O (equal amounts entering & leaving)
CYTOLYSIS
PLASMOLYSIS

24. Three Forms of Transport Across the Membrane
The Plasma Membrane
Three Forms of Transport Across the Membrane
4/20/2017
G. Podgorski, Biol. 1010

25. Passive Transport Simple Diffusion Doesn’t require energy
The Plasma Membrane
4/20/2017
Passive Transport
Simple Diffusion
Doesn’t require energy
Moves high to low concentration
Example: Oxygen or water diffusing into a cell and carbon dioxide diffusing out.
G. Podgorski, Biol. 1010

26. Facilitated diffusion
The Plasma Membrane
4/20/2017
Passive Transport
Facilitated diffusion
Doesn’t require energy
Uses transport proteins to move high to low concentration
Examples: Glucose or amino acids moving from blood into a cell.
G. Podgorski, Biol. 1010

27. Types of Transport Proteins
Channel proteins are embedded in the cell membrane & have a pore for materials to cross
Carrier proteins can change shape to move material from one side of the membrane to the other

28. Channels that are proteins
Cellular channels usually consist of large protein complexes with multiple transmembrane a-helices.
Control of channel gating is a form of allosteric regulation. Conformational changes associated with channel opening may be regulated by:
Voltage
Binding of a ligand (a regulatory molecule)
Membrane stretch (e.g., via link to cytoskeleton)

29. Ion Channels Channels cycle between open & closed conformations.
When open, a channel provides a continuous pathway through the bilayer, allowing flux of many ions.
Gramicidin is an example of a channel.

30. ionophores
Antibiotics of bacterial origin facilitate the movement of ions across membranes called ionophores
Ionophores :
carriers and channels.
valinomycin (a carrier)
gramicidin (a channel).

31. Valinomycin is a carrier for K+.
It is a circular molecule, made up of 3 repeats of the sequence shown above.

32. Valinomycin is highly selective for K+ relative to Na+.
The ionophore valinomycin is a uniport carrier.

33. Gramicidin A

34. Gating (opening & closing) of a gramicidin channel is thought to involve reversible dimerization.
An open channel forms when two gramicidin molecules join end to end to span the membrane.

35. Classes of carrier proteins
Uniport (facilitated diffusion) carriers mediate transport of a single solute.
An example is the GLUT1 glucose carrier.

36. Symport (cotransport) carriers bind two dissimilar solutes & transport them together across a membrane.
Transport of the two solutes is obligatorily coupled.
A gradient of one substrate, usually an ion, may drive uphill (against the gradient) transport of a co-substrate.
It is sometimes referred to as secondary active transport.
E.g:  glucose-Na+ symport, in plasma membranes
of some epithelial cells

37. Facilitated Diffusion
Some Carrier proteins do not extend through the membrane.
They bond and drag molecules through the lipid bilayer and release them on the opposite side

38. Carrier Proteins
Other carrier proteins change shape to move materials across the cell membrane

39. Active Transport Requires energy or ATP
The Plasma Membrane
4/20/2017
Active Transport
Requires energy or ATP
Moves materials from LOW to HIGH concentration
AGAINST concentration gradient
G. Podgorski, Biol. 1010

40. Active transport
Examples: Pumping Na+ (sodium ions) out and K+ (potassium ions) in against strong concentration gradients.
Called Na+-K+ Pump

41. Sodium-Potassium Pump
3 Na+ pumped in for every 2 K+ pumped out; creates a membrane potential

43. Transport of macromolecules
Exocytosis~ secretion of macromolecules by the fusion of vesicles with the plasma membrane
Endocytosis~ import of macromolecules by forming new vesicles with the plasma membrane
phagocytosis
pinocytosis
receptor-mediated endocytosis (ligands)

44. Exocytosis - moving things out.
Molecules are moved out of the cell by vesicles that fuse with the plasma membrane.
This is how many hormones are secreted and how nerve cells communicate with one another.

45. Pinocytosis Most common form of endocytosis.
The Plasma Membrane
Pinocytosis
4/20/2017
Most common form of endocytosis.
Takes in dissolved molecules as a vesicle.
G. Podgorski, Biol. 1010

46. Pinocytosis Cell forms an invagination
Materials dissolve in water to be brought into cell
Called “Cell Drinking”

47. Receptor-Mediated Endocytosis
The Plasma Membrane
4/20/2017
Receptor-Mediated Endocytosis
Some integral proteins have receptors on their surface to recognize & take in hormones, cholesterol, etc.
G. Podgorski, Biol. 1010

48. Endocytosis – Phagocytosis
The Plasma Membrane
4/20/2017
Endocytosis – Phagocytosis
Used to engulf large particles such as food, bacteria, etc. into vesicles
Called “Cell Eating”
G. Podgorski, Biol. 1010

50. Exocytosis The opposite of endocytosis is exocytosis
Exocytosis The opposite of endocytosis is exocytosis. Large molecules that are manufactured in the cell are released through the cell membrane.
Inside Cell
Cell environment

51. Specialized Transport
Transport proteins
Facilitated diffusion~ passage of molecules and ions with transport proteins across a membrane down the concentration gradient
Active transport~ movement of a substance against its concentration gradient with the help of cellular energy

52. Movement of compounds across membranes
Passive diffusion
Affected by
Size of molecule
Composition of molecule
Temperature
Lipid composition
Not mediated by proteins
Molecules move down a concentration gradient
Unimportant for most polar molecules S

53. Pore (a.k.a. Channel) Proteins
Actively regulated to control flow across membranes
Very specific for molecules able to pass through channel
Rate of movement of solutes is usually greater than for transporters
Fig. 9.30

54. Barriers, Pores, pumps and gates.mht