1. The Cell Membrane

2. Cell Environment Plasma membrane is the boundary that separates cells from their environment. Its function is to regulate what enters and leaves the cell.

3. Cell Environment The plasma membrane displays a property known as selective permeability.

4. Cell Environment Selective permeability means that only certain substances are allowed to pass through the membrane. It is selective for what that particular cell needs and its permeability may change.

5. Model of the Plasma Membrane When viewed under a microscope the plasma membrane looks like two thin lines, and is the Lipid Bilayer.

6. Model of the Plasma Membrane The Lipid Bilayer is composed of two layers of phospholipids.

7. Model of the Plasma Membrane In a Phospholipid one fatty acid chain is replaced with a phosphate group.

8. Model of the Plasma Membrane Phosphate groups are soluble in water. This determines the arrangement of phospholipids.

9. Model of the Plasma Membrane The phosphate heads are hydrophillic (“love water”) and point toward the outside of the cell and the fatty acid tails are hydrophobic (“fear water”) and point towards the center of the membrane.

10. Model of the Plasma Membrane Phospholipids are motile and move along the membrane (float around).

11. Model of the Plasma Membrane In many cells rigid molecules of cholesterol are embedded in the bilayer. Cholesterol restricts much of the movement of phospholipids, and adds strength and flexibility to the membrane.

12. Model of the Plasma Membrane Proteins are scattered throughout the lipid bilayer.

13. Model of the Plasma Membrane Some are embedded, called Integral proteins and others are on the surface, called Peripheral proteins.

14. Model of the Plasma Membrane Proteins have polar and nonpolar regions that determine their position in the membrane, and are capable of motility in the membrane.

15. Model of the Plasma Membrane The pattern of proteins in the membrane and the ability of proteins and phospholipids to move in the membrane give rise to what is called the Fluid Mosaic Model.

16. Model of the Plasma Membrane The proteins regulate which particles pass across the membrane, act as enzymes (chemical reactions) and serve as markers (possibly for fighting disease or so the cell can be recognized).

17. Model of the Plasma Membrane Other Organelles: enclosed in membranes display the fluid mosaic patterns. Their membranes are also selectively permeable to the substances that that organelle needs for its specific reactions.

18. Membrane Function Remember that the cell membrane is selectively permeable and regulates what enters and leaves the cell.

19. Membrane Function Random movement/Brownian Movement is unorganized (no patterns) movement of substances due to the collisions of particles which results in the even distribution of molecules.

20. Membrane Function Diffusion of Particles: moving from an area of greater (higher) concentration to an area of lesser (lower) concentration. Example: drop of food coloring in water.

21. Membrane Function The diffusion of particles continues to occur until the particles of become evenly distributed, at this point the Dynamic Equilibrium is reached.

22. Membrane Function Dynamic Equilibrium is the continuous movement of particles but no overall change. The particles are still evenly distributed.

23. Membrane Function Particles may diffuse through air, water, other liquids and across membranes.

24. Membrane Function Diffusion Through Membranes explains the physical process by which some particles enter and leave the membrane.

25. Membrane Function Lipids and materials dissolved in lipids can cross through the membrane. Oxygen (O 2 ) and Carbon Dioxide (CO 2 ) can pass right through the cell membrane because they are dissolved. Water (H 2 O), food and other molecules must pass through openings in the cell membrane.

26. Membrane Function

27. If there is a greater concentration of particles outside the cell than inside the cell, the particles will pass through the membrane and enter the cell.

28. Membrane Function The particles will move out of the cell if there is a greater concentration on the inside of the cell than on the outside of the cell.

29. Membrane Function Concentration Gradient is the difference in concentration of molecules from highest to lowest number of molecules.

30. Membrane Function The concentration gradient is also the difference in concentrations from inside the cell as compared to outside the cell.

31. Membrane Function Osmosis is the diffusion of water into and out of the cell.

32. Membrane Function For any cell to survive it must be in osmotic balance with its water environment.

33. Membrane Function Osmotic balance occurs when the movement of water into and out of a cell is equal (dynamic equilibrium is reached).

34. Membrane Function Osmotic Balance in solution and the responses of Plant and Animal Cells. Three types of solutions: ¬ Isotonic ­ Hypotonic ®Hypertonic

35. Membrane Function Isotonic (iso=same) equal concentrations of pure water inside and out side the cell, therefore no net movement of water.

36. Membrane Function Hypotonic (hypo=under) greater concentrations of pure water outside the cell than inside, thus water will flow into the cell from a greater concentration to a lesser concentration to reach equilibrium.

37. Membrane Function If the water is not removed, the animal cell may burst, while the plant cell swells to the limits of the cell wall (turgor pressure). This is why a plant that has been watered “stands” up and one that hasn’t been watered wilts.

38. Membrane Function Hypertonic (hyper=above) greater concentrations of pure water inside the cell than outside, thus the cells shrink as water flows out of the cell to reach equilibrium. In plants this is called plasmolysis.

39. Membrane Function The cell plays no active role and does no work in moving water, lipids, and lipid-soluble particles across the membrane.

40. Membrane Function The passage of water, lipids and lipid- soluble particles across the membrane is due to their net movement from greater to lesser concentrations, this type of movement is called Passive Transport.

41. Membrane Function In Passive transport the cell uses NO energy to move particles across the membrane.

42. Membrane Function A form of passive transport is Facilitated Diffusion. Facilitated Diffusion is when transport proteins (proteins embedded in the plasma membrane) are used to help particles cross the membrane from greater to lesser concentrations (with the concentration gradient).

43. Membrane Function Active Transport is the movement of substances into and out of cells, that requires the cell to expend/use energy.

44. Membrane Function Transport proteins known as pumps are used in active transport. Particular particles bind to each pump (like a “lock and key”). Energy is then used to alter the shape of the pump so that the particle is moved across the membrane.

45. Membrane Functions When the particle is released the pump returns to its original shape. For example: salt water fish getting excess salt out of their cells.

46. Membrane Function Really large molecules can be transported into or out of the cells by one of the two following ways:

47. Membrane Function Exocytosis (exo=out) is the movement of substances out of the cell. Endocytosis (endo=inside) the process by which the plasma membrane engulfs and takes in substances from a cell’s environment.

48. Membrane Function

49. Three Types of Endocytosis: ¬ Phagocytosis ­ Pinocytosis ® Receptor-Aided

50. Membrane Function Phagocytosis (phag=to eat) is the movement of solids into the cell. The membrane closes around the particle and forms a vesicle. Pinocytosis is the movement of liquids with solute and small particles into cells.

51. Membrane Function Receptor-Aided receptor proteins on the membrane where a vesicle is formed, in other words a group of particles gathered to be brought in all at once.