Chapter 6 Membrane Structure and Function

1. Chapter 6 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. Membrane Composition • phospholipids • Proteins • Question: How are the materials arranged?

4. Phospholipids Hydrophilic heads Hydrophobic tails Phosphophospholipid Bilayer

5. Membrane Models

6. Davson-Danielli Model 1935 • phospholipid bilayer. • Proteins coat the surfaces. • Sometimes called the “sandwich” model.

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

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 phospholipids and proteins behave in a membrane.

12. “Fluid” • Refers to the phospholipid 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” • phospholipid changes or shifts: • Cold hardening of plants (shift to unsaturated fatty acids). • Hibernating animals (Cholesterol increase).

15. “Mosaic” • Proteins: float in a sea of phospholipids. • 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.

20. Protein Function in Membranes • Transport. • Enzymatic activity. • Receptor sites for signals. • Cell adhesion. • Cell-cell recognition. • Attachment to the cytoskeleton.

22. Types of Membrane Proteins • Integral - inserted into the phospholipid bilayer. • Peripheral - not embedded in the phospholipid bilayer, but are attached to the membrane surface.

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 phospholipid 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 glycophospholipids and glycoproteins on external surface. • Function - recognition of "self" vs "other”.

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

30. Problems • phospholipid 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

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

37. Factors that Effect Diffusion 1. Concentration 2. Temperature 3. Pressure 4. Particle size 5. Mixing

38. 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.

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

41. 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.

42. 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.

43. 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.

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

46. Aquaporins • Newly found channels for osmosis. GFP labeled Aquaporins

47. Active Transport • Movement across membranes that DOES require cellular energy.

48. Types of Active Transport 1. Carrier-Mediated 2. Endocytosis 3. Exocytosis

49. Carrier-Mediated Transport • General term for the active transport of materials into cells AGAINST the concentration gradient. • Movement is: low high

50. Examples 1. Na+- K+ pump 2. Electrogenic or H+ pumps 3. Cotransport