Membrane Structure and Transport through cell membranes

1. Membrane Structure and Transport through cell membranes

2. Plasma Membrane • Is the boundary that separates the living cell from its nonliving surroundings • Selectively Permeable (chooses what may cross the membrane) • Fluid mosaic of lipids and proteins • Lipid bilayer • Contains embedded proteins

4. Phospholipids • Are the most abundantlipid in the plasma membrane • Are amphipathic, containing both hydrophilic (head) and hydrophobic regions (tails) • Head composed of phosphate group attached to one carbon of glycerol is hydrophilic • Two fatty acidtailsare hydrophobic

5. WATER Hydrophilic head Hydrophobic tail WATER Phospholipid Bilayer

6. Hydrophilic region of protein Phospholipid bilayer Hydrophobic region of protein Singer and Nicolson • In 1972, Singer and Nicolson, Proposed that membrane proteins are dispersed and individually inserted into the phospholipidbilayerof the plasma membrane

7. Fluid Mosaic Model • A membrane is a fluid structure with a “mosaic” of various proteins embedded in it when viewed from the top • Phospholipids can move laterally a small amount and can “flex” their tails • Membrane proteins also move side to side or laterally making the membrane fluid

8. Freeze-fracture studies of the plasma membrane support the fluid mosaic model of membrane structure A cell is frozen and fractured with a knife. The fracture plane often follows the hydrophobic interior of a membrane, splitting the phospholipid bilayer into two separated layers. The membrane proteins go wholly with one of the layers.

9. Lateral movement (~107 times per second) Flip-flop (~ once per month) The Fluidity of Membranes • Phospholipids in the plasma membrane Can move within the bilayer two ways

10. Fluid Viscous Unsaturated hydrocarbon tails with kinks Saturated hydro- Carbon tails The Fluidity of Membranes • The type of hydrocarbon tails in phospholipids Affects the fluidity of the plasma membrane

11. Cholesterol Figure 7.5 The Fluidity of Membranes • The steroid cholesterol Has different effects on membrane fluidity at different temperatures

12. Membrane Proteins and Their Functions • A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer Fibers of extracellular matrix (ECM)

13. Types of Membrane Proteins • Integral proteins • Penetrate the hydrophobic core of the lipid bilayer • Are often transmembrane proteins, completely spanning the membrane EXTRACELLULAR SIDE

14. Transport Proteins • Transport proteins • Allow passage of hydrophilic substances across the membrane

15. Types of Membrane Proteins • Peripheral proteins • Are appendages loosely bound to the surface of the membrane

16. The Role of Membrane Carbohydrates in Cell-Cell Recognition • Cell-cell recognition • Is a cell’s ability to distinguish one type of neighboring cell from another • Membrane carbohydrates • Interact with the surface molecules of other cells, facilitating cell-cell recognition

17. Transport through cell membranes • The phospholipidbilayer is a good barrier around cells, especially to water soluble molecules. However, for the cell to survive some materials need to be able to enter and leave the cell. • There are 2 basic ways: • Passive Transport – uses no cellular energy and goes down the concentration gradient • Active Transport – uses cellular energy and goes against the concentration gradient

18. Diffusion of liquids

19. Diffusion is the net movement of molecules (or ions) from a region of their high concentration to a region of their lower concentration.The molecules move down a concentration gradient.Molecules have kinetic energy, which makes them move about randomly. As a result of diffusion molecules reach an equilibrium where they are evenly spread out.This is when there is no net movement of molecules from either side.

20. DIFFUSION Diffusion is a PASSIVE process which means no energy is used to make the molecules move, they have a natural kinetic energy. AS Biology, Cell membranes and Transport

21. Diffusion of Bromine

22. Diffusion of Bromine

23. Diffusion through a membrane Cell membrane Outside cell Inside cell

24. Diffusion through a membrane Cell membrane diffusion Outside cell Inside cell

25. Diffusion through a membrane Cell membrane Outside cell Inside cell EQUILIBRIUM

27. `` AS Biology, Cell membranes and Transport

28. Osmosis ‘The diffusion of water from an area of high concentration of water molecules to an area of low concentration of water across a partially permeable membrane.’

29. Molecules that diffuse through cell membranes • Oxygen – Non-polar so diffuses very quickly. • Carbon dioxide – Polar but very small so diffuses quickly. • Water – Polar but also very small so diffuses quickly.

30. Osmosis CONCENTRATED SOLUTION DILUTE SOLUTION Cell membrane partially permeable. Sugar molecule VERY Low conc. of water molecules. High water potential. VERY High conc. of water molecules. High water potential. Outside cell Inside cell

31. Osmosis Cell membrane partially permeable. Low conc. of water molecules. High water potential. OSMOSIS High conc. of water molecules. High water potential. Outside cell Inside cell

32. Osmosis Cell membrane partially permeable. OSMOSIS Outside cell Inside cell EQUILIBRIUM. Equal water concentration on each side. Equal water potential has been reached. There is no net movement of water AS Biology, Cell membranes and Transport

34. AS Biology, Cell membranes and Transport

35. 3 types of solutions • Isotonic – concentrations of solute and water are the same • Hypertonic – where there is a greater concentration of solute (sugar, salt) and less water • Hypotonic – where there is less solute and more water.

36. Two parts to a solution • Solute – What is dissolved • Solvent – what the solute is being dissolved in (usually water)

39. Which picture shows RBCs in a hypertonic, isotonic and hypotonic solution? A B C D E

40. What determines the rate of diffusion?There 4 factors: • The steepness of the concentration gradient. The bigger the difference between the two sides of the membrane the quicker the rate of diffusion. • Temperature. Higher temperatures give molecules or ions more kinetic energy. Molecules move around faster, so diffusion is faster. • The surface area. The greater the surface area the faster the diffusion can take place. This is because the more molecules or ions can cross the membrane at any one moment. • The type of molecule or ion diffusing. Large molecules need more energy to get them to move so they tend to diffuse more slowly. Non-polar molecules diffuse more easily than polar molecules because they are soluble in the non polar phospholipid tails.

41. Facilitated diffusion • Large polar molecules such as glucose and amino acids, cannot diffuse across the phospholipid bilayer. Also ions such as Na+or Cl-cannot pass. • These molecules pass through protein channels instead. Diffusion through these channels is called FACILITATED DIFFUSION. • Movement of molecules is still PASSIVE just like ordinary diffusion, the only difference is, the molecules go through a protein channel instead of passing between the phospholipids.

43. Facilitated Diffusion through a membrane Cell membrane Protein channel Outside cell Inside cell

44. Facilitated Diffusion through a membrane Cell membrane diffusion Protein channel Outside cell Inside cell

45. Facilitated Diffusion through a membrane Cell membrane diffusion Protein channel Outside cell Inside cell EQUILIBRIUM

46. Facilitated Diffusion:Molecules will randomly move through the opening like pore, by diffusion. This requires no energy, it is a PASSIVE process. Molecules move from an area of high concentration to an area of low conc.

47. Facilitated diffusion

48. Dialysis Dialysis is transport of a solute. This process is extremely important in the human kidneys for filtering out waste products.