Unit 2: Chapter 5

1. Unit 2: Chapter 5 Biological Membranes

2. Structure of the Cell Membrane 6 main components • Phospholipids • Proteins • Cholesterol • Carbohydrates • Glycoproteins • glycolipids

3. Phospholipids (review) • Membrane is a bilayer • One layer of polar heads – they are hydrophilic because they love water • One layer of nonpolar tails – they are hydrophobic because they hate water

5. Membranes have proteins! • 2 main types • Integral proteins • Inserted into the membrane • May be unilateral – reach only partway across the membrane • Or they may be transmembrane – completely span the membrane • Removal disrupts cell membrane • Peripheral proteins • Not embedded in membrane, but attached to the membrane surface by either integral proteins or filaments from the cytoskeleton • Removal has little effect on cell membrane

7. 7 functions of membrane proteins • Anchoring cell • Passive transport • Active transport • Enzyme activity • Signal transduction – transmitting info into cell • Cell recognition – like ID tags • Junction between cells – cell adhesion

8. Cholesterol • Controls the fluidity of the cell membrane

9. History of the Cell Membrane • 2 models • Sandwich Model • Fluid Mosaic Model

10. Davson and Danielli in 1935 Perceived the plasma membrane as a sandwich where the proteins were the bread and the phospholipid bilayer was the meat Singer and Nicholson in 1972 Said the membrane is fluid and must be If it solidifies, its permeability will change and enzymes will denature Said the membrane is mosaic – there are proteins embedded in it Sandwich Model Fluid Mosaic Model

11. Which model was correct?

13. Nature of protein and lipid mobility • Lateral movement of lipids/proteins is quick • Lipids and proteins rarely flip across the lipid bilayer

14. Basic Terms to Understand • Selectively permeable – prevents passage of most materials through the membrane • Solute – what is dissolving (salt, sugar, etc.) • Solvent – what it is dissolving in (water etc.) • Solution – mixture of solvent and solute

15. 7 ways substances can get into a cell • Diffusion • Bulk flow • Osmosis • Facilitated diffusion • Active transport • Vesicle mediated transport • Cell to cell junction

16. Diffusion • Moves materials from a high concentration to a low concentration • Requires no energy – type of passive transport • Easy passage through – oxygen, carbon dioxide, nitrogen, and small polar molecules • Slow passage through – large polar molecules like glucose and charged ions • Proteins allow movement of charged/polar molecules • Particles move until equilibrium

17. Bulk Flow • Molecules move all together in same direction due to hydrostatic pressure

18. Osmosis • ALL things undergo diffusion. • Water also diffuses, however, the water diffusion is not evident unless it crosses a membrane. • Osmosisis the diffusion of water across a membrane from high to a low concentration • No energy is required – type of passive transport • Since cells have membranes, osmosis is important to cells

19. Membranes and Osmosis • Tank w/ semipermeable membrane: water may pass, solute can’t • At first the concentration of solute is very high on the left. • But over time, the water moves across the semipermeable membrane, and dilutes the solute.

20. Water Moves Because It is Polar • Because water is polar, it binds to the solute by hydrogen bonds • The concentration of water is higher on the right • Water will then flow across the membrane, down its concentration gradient, to the left

21. Three osmotic environments • Hypertonic • Hypotonic • Isotonic • Same solute concentration inside and outside the cell • Water flows in and out of the cell equally in both directions • Most cells in our body are isotonic

22. Hypertonic • High concentration of solute outside the cell • Therefore there is more water inside the cell • Water will move out of the cell • If this process continues, the cell collapses and dies – this is called plasmolysis

23. Hypotonic • Low concentration of solute outside the cell • Therefore there is more water outside the cell • Water will move inside the cell • This causes the cell to expand, causing turgor pressure in plant cells Animal cells could burst – this is called cytolysis

25. Plant Cells and Osmotic Pressure • In plants, hypotonic solutions produce osmotic pressure that produces turgor pressure • Turgor means “tight or stiff owing to being very full” • Keeps plant upright; in hypertonic conditions plants wilt Vacuole fills Vacuole shrinks Hypertonic Hypotonic solution Hypertonic

26. Dialysis • Dialysis is the diffusion of solutes across a membrane • The selectively permeable membrane allows small sugar molecules to move across the membrane, but large proteins cannot

27. Facilitated Diffusion • Transport proteins move materials through membrane • 3 kinds of transport proteins • Uniport – carries a single molecule across the membrane • Symport – moves 2 different molecules at the same time in the same direction • Antiport – exhanges 2 molecules in opposite directions This is passive transport

29. Vesicle Mediated Transport • When vesicles or vacuoles fuse with the membrane to move substances in or out of the cell • 2 main types • Exocytosis – when vesicle expels contents outside the cell • Endocytosis – when vesicles bring substances into the cell • 3 types This is Active Transport

30. 3 types of Endocytosis • Phagocytosis – solid being taken into the cell • Pinocytosis – liquid being taken into the cell • Receptor Mediated Endocytosis – substance binds to a specific receptor on the cell before it is brought in

31. Exocytosis

32. phagocytosis

33. water Pinocytosis

34. Receptor Mediated Endocytosis

35. Active Transport • Energy is needed • Moves materials against the concentration gradient • Main example: sodium potassium pump

36. Sodium Potassium Pump • Occurs in animal cells • Required ATP (active transport) • Exchanges 3 Na+ ions on inside for 2 K+ ions on outside • This exchange is uneven so an electric potential is generated and so the membrane is now considered to be polarized • Let’s see this in action

38. How do cells stick together and let materials? • Plasmodesmata • Gap junctions • Desmosomes • Tight junctions

39. Plasmodesmata • They are channels that allow movement of certain molecules and ions between plant cells

40. Gap Junctions • Cytoplasmic channels between neighboring animal cells • Let adjacent cells communicate • Small dissolved molecules and electrical signals may pass from one cell to the other • Very similar to plasmodesmata

42. Desmosomes • Function as rivets and join animal cells together • They are reinforced by intermediate filaments made of keratin • Still permits materials to move around them in the intercellular space

43. Tight Junctions • Continuous belt around animal cells that fuse membranes of neighboring cells • It is leak proof • Contains no intercellular space

44. Water Potential • Chemical potential of water • Measure of the energy available for reaction or movement • Measures the ability of water to move and water always moves from areas of higher potential to areas of low potential • Has the symbol Ψ (psi)

45. Water Potential Continued… • Is measured in the unit bars • The formula for calculated water potential is osmotic potential (solute) + pressure potential So, the equation is Ψ = Ψs + Ψp

46. Equation Components Ψp = pressure on the system = 0, if the system or container is open ** all of our problems will be open so Ψp will always equal 0. Ψs = change in water potential due to solute molecules The more solute, the lower the water potential

47. Ψs = -iCRT Where i = ionization constant = # of ions in the solute = 1 when there are no ions present C = Molar Concentration usually given in problem equal to Molarity (M), or moles/volume R = pressure constant = 0.0831 Liters X Bars/moles x Kelvin this number never changes T = temperature needs to be in Kelvin Conversion of Celsius to Kelvin is K = 273 + Celsius

48. Suppose we have a beaker of distilled water at room temperature. (0M) What is the water potential? Ψ= Ψs + Ψp Ψ = -iCRT + 0 Ψ = -(1)( 0) (0.0831)(23 + 273) + 0 Ψ = 0 + 0 Ψ = 0

49. Suppose we have an open beaker A that contains Sucrose (2.5M) at 25 C. What is the water potential? Ψ = Ψ s + Ψ p Ψ = -iCRT + 0 Ψ = -(1)(2.5)(0.0831)(25 + 273) Ψ = -61.91 bars