1. Chapter 7: Membrane Structure and Function

2. Essential Knowledge
2.b.1 – Cell membranes are selectively permeable due to their structure (7.1 & 7.2).
2.b.2 – Growth and dynamic homeostasis are maintained by the constant movement of molecules across membranes ( ).

3. Plasma Membrane The membrane at the boundary of every cell
Functions as a selective barrier for the passage of materials in and out of cells.
Called a semi-permeable membrane (regulates crossing of materials)

4. Membrane Composition Lipids Proteins Carbohydrates Most numerous
Phospholipids and cholesterol
Proteins
Very large (in size)
Peripheral and integral
Carbohydrates

5. Historical Cell Membrane Models

6. Davson-Danielli Model 1935
Lipid bilayer
Proteins coat the surfaces
Sometimes called the “sandwich” model
Evidence:
Biochemical work
TEM pictures showed double line
Accepted until 1960s

7. Problems w/ Davson model
Not all membranes in a cell were the same
How could the proteins stay in place?
Protein placement was confusing
Result - the model was questioned and tested by scientific process

8. Fluid Mosaic Model 1972 Current/New model to fit the new evidence
Example of “Science as a Process”
Refers to the way the lipids and proteins behave in a membrane

9. “Fluid” Refers to the lipid bilayer
Molecules are not bonded together, so are free to shift.
Must remain "fluid" for membranes to function.
Fluid = dynamic, changing
Cell membrane will remain fluid until temperature drops to extreme levels

10. Ways to keep the membrane “fluid”
Lipid changes or shifts:
Plants:
Cold hardening (shift to unsaturated fatty acids – remember unsaturated fats are kinked in shape)
Animals:
Hibernating - Cholesterol amount increases – eat more fatty foods right BEFORE they hibernate

11. Sat = NO double bonds
Unsat = double bonds

12. “Mosaic” Proteins: float in a sea of lipids
Proteins form a collage or mosaic pattern that shifts over time

13. Evidence for fluid-mosaic
TEM pictures of fractured membranes
Cell fusion studies
Tagging of membrane proteins by antibodies

14. MEMORIZE THIS!!!

15. Protein Function in Membranes
Main function:
Determine cell’s specific function
Other functions:
Transport
Enzymatic activity
Receptor sites for signals (hormones)
Cell adhesion
Cell-cell recognition (immunity)
Attachment to the cytoskeleton (cilia and flagella)

16. 2 types of transport:
Channel
Carrier

17. Types of Membrane Proteins
1) Integral
Inserted into the lipid bilayer
Go through ENTIRE bilayer
2) Peripheral
Not embedded in the lipid bilayer
Attached to the membrane surface

19. Question? How do the integral proteins stick to the membrane?
By the solubility of their amino acids
Protein folding/structure type

20. Hydrophilic Amino Acids
Hydrophobic Amino Acids
Hydrophilic Amino Acids

21. Membranes are Bifacial
The lipid composition of the two layers is different
The proteins have specific orientations.
Carbohydrates are found only on the outer surface

23. Membrane Carbohydrates
Branched oligosaccharides form glycolipids and glycoproteins on external surface
Glyco = carbohydrate
Made from modification of existing cellular molecules
Glycolipids = Lipids+carbohydrates
Glycoproteins = Proteins + carbohydrates
Function: recognition of "self" vs "other”

24. 1 2 2 3 3 3 4
Carbohydrates

25. Questions How do materials get across a cell's membrane?
Do they use energy/no energy?
Do the molecules move against/with concentration gradient?
REMEMBER: Cell membrane is regulatory membrane (semi-permeable)

26. Problems with using cell membrane to move materials
1) Lipid bilayer is hydrophobic
Hydrophilic materials don't cross easily
Ex: ions, H2O, polar molecules
Hydrophobic materials will cross easily
Ex: CO2, O2, hydrocarbons
2) Large molecules don't cross easily
Too big to get through the membrane (without assistance or through the use of energy)
Proteins play HUGE role in getting certain molecules across (hydrophilic)

27. Two Mechanisms for Movement
1. Passive Transport
Active Transport
* Both involve concentration gradients (movement of molecules from areas of high/low concentrations)

28. Passive Transport
Movement across membranes that does NOT require cellular energy
Types:
Diffusion
Osmosis
Facilitated Diffusion

29. Diffusion
The net movement of atoms, ions or molecules down a concentration gradient
Ex: smells crossing room
Movement is from: High Low
Diffusion movie

31. Equilibrium When the concentration is equal on both sides
There is no net movement of materials
Molecules are constantly in motion (don’t stop moving!)
However, CONCENTRATION STAYS THE SAME!!!

32. Factors that Affect Diffusion
Concentration
a) Of solute or solvent/s
2. Temperature
3. Pressure
Particle size
Smaller size = quicker movement
Mixing
a) More = faster diffusion

33. Osmosis Diffusion of water
Water moving from an area of its high concentration to an area of its low concentration.
No cell energy is used
Passive transport
Relies upon tonicity of solutions (both internally and externally)

34. Tonicity The concentration of water relative to a cell.
1. Isotonic (same)
2. Hypotonic (below)
a) The hypotonic solution has a solute concentration BELOW that of the cell
3. Hypertonic (above) Tonicity Tutorial

36. Isotonic Isosmotic solution
Cell and water/solution are equal in solute concentration
No net movement of water in or out of the cell
Water still MOVES, but concentration of solutes stays the same!!!
RESULT: No change in cell size
Ex: Marine mammals

37. Hypotonic Hypoosmotic solution
Cell's water is lower than the outside water (more solutes)
Water moves into the cell
RESULT: Cell swells, may burst or the cell is turgid
Ex: place raisin in water (raisin will swell, because water rushes in to attempt to EQUAL out concentrations of solutes and water)
Ex: place egg in vinegar/water solution

38. Hypertonic Hyperosmotic solution
Cell's water is higher inside than the outside water (less solutes)
Water moves out of the cell
RESULT: Cell shrinks or plasmolysis occurs
Ex: Placing RBC in salt solution (RBC shrivels to attempt to push water out of cell to = solute concentrations)
Ex: Placing egg in corn syrup
Ex: onion cell w/ salt solution added
Osmosis movie

39. Facilitated Diffusion
Transport protein that helps materials through the cell membrane
Polar molecules and ions USE this!!!
Doesn't require energy (ATP)
Still passive transport
Works on a downhill concentration gradient
Facilitated Diff movie

40. Channel proteins
Carrier proteins

41. Active Transport
Movement across membranes that DOES require cellular energy
Uses ALL carrier proteins
Allows cells to differ in solute concentration
Why? Important in Ps, Rs and hormones
Types:
Carrier-mediated
Endocytosis
Exocytosis

42. Key terms for Active Transport
Membrane potential:
Voltage across membrane (opposite charges and concentration of them) – likelihood charges will pass across the membrane
Electrochemical gradient:
Electro – change in charge
Chemical – change in concentration

43. Carrier-Mediated Transport
General term for the active transport of materials into cells AGAINST the concentration gradient
Movement is: low high
Examples:
Na+ - K+ pump
Electrogenic/H+ pump
Cotransport

44. Na+- K+ pump Moves Na+ ions out of cells while moving K+ ions in
Occurs in animals
Sodium ions increase outside cell while potassium ions increase inside cell

46. Electrogenic or H+ pumps
Also called Proton pumps.
Create voltages (change of charge) across membranes for other cell processes
Ps and Rs
Used by plants, fungi and bacteria.
Transports H+ OUT of cell
This change in charge (or change in voltage) allows for storage of ATP
Used in later rxns

48. Cotransport
Movement of H+ that allows other materials to be transported into the cell as the H+ diffuses back across the cell membrane
Example - Sucrose transport
This is how plants transport food/sugar to non-photosynthetic organs (like the roots)

49. Outside cell
Inside cell

50. Exocytosis Moves bulk material out of cells
Uses Golgi vesicles to do this
Example:
Secretion of enzymes
Hormone movement
Secretion of insulin by pancreas

51. Endocytosis Moves bulk materials into cells Types of Endocytosis:
Cell forms new transport vesicles (from parts of cell membrane)
Types of Endocytosis:
1. Pinocytosis – move liquids
2. Phagocytosis – move solids
3. Receptor Mediated - uses receptors to "catch" specific kinds of molecules.

53. Summary Recognize the Davson-Danielli model of cell membranes.
Identify the components and structure of the fluid mosaic model of cell membranes.
Identify methods that keep cell membranes fluid.
Identify methods that cells use for transporting small molecules across membranes.
Recognize the conditions that regulate osmosis and tonicity in cells.
Identify methods that cells use for transporting large molecules across membranes.
Be able to solve problems in osmosis – pgs. 140 and 141 #1-6