Chapter 7 Membrane Structure and Function

1
Chapter 7 Membrane Structure and Function
2
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.

3
Movie
4
Membrane Composition

• Lipids
• Proteins
• Question How are the
  materials arranged?

5
Membrane Models
6
Davson-Danielli Model 1935

• Lipid bilayer.
• Proteins coat the surfaces.
• Sometimes called the sandwich model.

7
Evidence

• Biochemical work.
• TEM pictures show the membrane as a double line.

8
(No Transcript)
9
Problems

• Not all membranes in a cell were the same.
• How could the proteins stay in place?
• Result - the model was questioned and tested by
  scientific process.

10
Fluid Mosaic Model 1972

• New model to fit the new evidence with membranes.
• Example of Science as a
  Process.

11
Fluid Mosaic Model

• Refers to the way the lipids and proteins behave
  in a membrane.

12
Fluid

• Refers to the lipid bilayer.
• Molecules are not bonded together, so are free to
  shift.
• Must remain "fluid" for membranes to function.

13
Ways to keep the membrane fluid

• Lipid changes or shifts
• Cold hardening of plants (shift to
  unsaturated fatty acids).
• Hibernating animals (Cholesterol increase).

14
(No Transcript)
15
Mosaic

• Proteins float in a sea of lipids.
• Proteins form a collage or mosaic pattern that
  shifts over time.

16
Evidence

• TEM pictures of fractured membranes.
• Cell fusion studies.
• Tagging of membrane proteins by antibodies.

17
(No Transcript)
18
(No Transcript)
19
(No Transcript)
20
Protein Function in Membranes

• Transport.
• Enzymatic activity.
• Receptor sites for signals.
• Cell adhesion.
• Cell-cell recognition.
• Attachment to the cytoskeleton.

21
(No Transcript)
22
Types of Membrane Proteins

• Integral - inserted into the lipid bilayer.
• Peripheral - not embedded in the lipid bilayer,
  but are attached to the membrane surface.

23
(No Transcript)
24
Question?

• How do the integral proteins stick to the
  membrane?
• By the solubility of their amino acids.

25
Hydrophilic Amino Acids
Hydrophobic Amino Acids
Hydrophilic Amino Acids
26
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.

27
Carbohydrates
28
Membrane Carbohydrates

• Branched oligosaccharides form glycolipids and
  glycoproteins on external surface.
• Function - recognition of "self" vs "other.

29
Question

• How do materials get across a cell's membrane?

30
Problems

• Lipid bilayer is hydrophobic. Hydrophilic
  materials don't cross easily.
• Large molecules don't cross easily. Too big to
  get through the membrane.

31
Mechanisms

• 1. Passive Transport
• 2. Active Transport

32
Passive Transport

• Movement across membranes that does NOT require
  cellular energy.

33
Types of Passive Transport

• 1. Diffusion
• 2. Osmosis
• 3. Facilitated Diffusion

34
Diffusion

• The net movement of atoms, ions or molecules down
  a concentration gradient.
• Movement is from High Low

35
Movie
36
(No Transcript)
37
Equilibrium

• When the concentration is equal on both sides.
• There is no net movement of materials.

38
Factors that Effect Diffusion

• 1. Concentration
• 2. Temperature
• 3. Pressure
• 4. Particle size
• 5. Mixing

39
Osmosis

• Diffusion of water.
• Water moving from an area if its high
  concentration to an area of its low
  concentration.
• No cell energy is used.

40
Tonicity

• The concentration of water relative to a cell.
• 1. Isotonic (same)
• 2. Hypotonic (below)
• 3. Hypertonic (above)

41
(No Transcript)
42
Isotonic

• Isosmotic solution.
• Cell and water are equal in solute concentration.
• No net movement of water in or out of the cell.
• No change in cell size.

43
Hypotonic

• Hypoosmotic solution
• Cell's water is lower than the outside water
  (more solutes).
• Water moves into the cell.
• Cell swells, may burst or the cell is turgid.

44
Hypertonic

• Hyperosmotic solution
• Cell's water is higher than the outside water
  (less solutes)
• Water moves out of the cell.
• Cell shrinks or plasmolysis occurs.

45
Movie
46
Facilitated Diffusion

• Transport protein that helps materials through
  the cell membrane.
• Doesn't require energy (ATP).
• Works on a downhill concentration gradient.

47
(No Transcript)
48
Active Transport

• Movement across membranes that DOES require
  cellular energy.

49
Movie
50
Types of Active Transport

• 1. Carrier-Mediated
• 2. Endocytosis
• 3. Exocytosis

51
Carrier-Mediated Transport

• General term for the active transport of
  materials into cells AGAINST the concentration
  gradient.
• Movement is low high

52
Examples

• 1. Na- K pump
• 2. Electrogenic or H pumps
• 3. Cotransport

53
(No Transcript)
54
Na- K pump

• Moves Na ions out of cells while moving K ions
  in.

55
Electrogenic or H pumps

• Also called Proton pumps.
• Create voltages across membranes for other cell
  processes.
• Used by plants, fungi and bacteria.

56
(No Transcript)
57
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

58
(No Transcript)
59
Exocytosis

• Moves bulk material out of cells.
• Example - secretion of enzymes.

60
Endocytosis

• Moves bulk materials into cells.
• Several types known.

61
Movie
62
Types

• 1. Pinocytosis - liquids
• 2. Phagocytosis - solids
• 3. Receptor Mediated - uses receptors to "catch"
  specific kinds of molecules.

63
Movie
64
Forming vesicles
65
Movie
66
(No Transcript)
67
Summary

• Know membrane structure.
• Be able to discuss the various methods by which
  cells move materials through membranes.
• Be able to solve problems in osmosis.