2. Biological membranes Complex, dynamic structures made of lipid and protein molecules Perform many functions Define cell as a compartment Regulate passage of materials (helps to maintain homeostasis) Participate in chemical reactions Transmit signals between cell interior and the environment Communicates with other cells Act as part of energy transfer and storage

3. Cell Membrane flexible The cell membrane is flexible and allows a unicellular organism to move copyright cmassengale 3

4. All cells are separated from the external environment by a Plasma Membrane Key Feature: Semipermiable (only allowing certain materials in/out of cells.) It is composed of many components. – Phospholipids – Very similar to triglycerides however, one of the fatty acid chains has been replaced by a very polar Phosphate group – Cholesterol – Provides stability – Proteins – Helps to transport materials across the membrane

5. Phospholipids & Water The polar end is attracted to water, non polar tails are repelled by it. This forces the phospholipids to take an arrangement such that their tails face each other. This accomplishes 2 things. The polar heads can be adjacent to polar water molecules The non-polar tails can be adjacent to each other.

6. 6 Phospholipids Contains 2 fatty acid chains that are nonpolar Head is polar & contains a –PO 4 group & glycerol copyright cmassengale

7. Phospholipids form bilayers in water Phospholipids in waterDetergent in water

8. Fluid mosaic model Membranes consist of fluid phospholipid bilayer with a mosaic pattern of associated proteins Phospholipid molecules are amphipathic and contain Hydrophobic regions Hydrophilic regions

9. 9 FLUID- because individual phospholipids and proteins can move side-to-side within the layer, like it’s a liquid. MOSAIC- because of the pattern produced by the scattered protein molecules when the membrane is viewed from above. FLUID MOSAIC MODEL copyright cmassengale

10. 10 Cell Membrane Hydrophobic molecules pass easily; hydrophilic DO NOT copyright cmassengale

11. 11 Small molecules and larger hydrophobic molecules move through easily. e.g. O 2, CO 2, H 2 O Semipermeable Membrane copyright cmassengale

12. 12 Ions, hydrophilic molecules larger than water, and large molecules, such as proteins, do not move through the membrane on their own. Semipermeable Membrane copyright cmassengale

13. Plasma membrane of mammalian red blood cell

14. Membrane properties Orderly arrangement of phospholipid molecules make the cell membrane a liquid crystal Allow molecules to move rapidly Proteins move within membrane Lipid bilayers are Flexible Self-sealing Can fuse with other membranes

15. Membrane Components The membrane requires a few more components to make it complete: Cholesterol Functions to hold the membrane together like glue. Adds strength to the membrane. Prevents tails from bunching together Proteins (2 types) defined by how tightly they are associated with the membrane Integral Proteins Extend into or through lipid bilayer Often are channel proteins allowing large molecules in/out Peripheral membrane Proteins Exist only on surfaces of the bilayer Often serve as receptor molecules Glycoproteins serve as cellular markers (nametags)

16. Detailed structure of the plasma membrane

17. Integral membrane proteins Embedded in the bilayer Transmembrane proteins Integral proteins that extend completely through the membrane Peripheral member proteins Associated with the surface of the bilayer

18. 18 Proteins Are Critical to Membrane Function copyright cmassengale

19. Membrane proteins, lipids, and carbohydrates Asymmetrically positioned to bilayer Sides have different composition and structure Function of member proteins Transport of materials Acting as enzymes or receptors Cell recognition Structurally linking cells

20. Asymmetry of the plasma membrane Freeze-fracture method: the membrane splits along the hydrophobic interior of the lipid bilayer. Enables scientist to observe numerous particles on the fracture faces. Influenced the development of the Fluid Mosaic Model

21. Membranes are selectively permeable Physical processes Osmosis Diffusion Carrier-mediated processes Channel proteins Carrier proteins

22. 22 Types of Transport Across Cell Membranes copyright cmassengale

23. Concentration Gradient The difference in concentration of a substance from one point to another *transport is often referred to as moving either against or down the concentration gradient

24. 24 Simple Diffusion NO Requires NO energy HIGH to LOW Molecules move from area of HIGH to LOW concentration copyright cmassengale

25. 25 DIFFUSION PASSIVE Diffusion is a PASSIVE process which means no energy is used to make the molecules move, they have a natural KINETIC ENERGY copyright cmassengale

26. Diffusion What influences the rate of diffusion? Temperature Pressure Electrical currents Molecular size

27. Diffusion: net movement of a substance from a region of greater to lower concentration

28. 28 Diffusion of Liquids copyright cmassengale

29. 29 Diffusion through a Membrane Cell membrane Solute moves DOWN concentration gradient (HIGH to LOW)

30. Osmosis: water passes through selectively permeable membrane from region of higher concentration to lower

31. 31 Osmosis Diffusion of water across a membrane Diffusion of water across a membrane Moves from HIGH water potential (low solute) to LOW water potential (high solute) Moves from HIGH water potential (low solute) to LOW water potential (high solute) Diffusion across a membrane Semipermeable membrane copyright cmassengale

32. 32 Diffusion of H 2 O Across A Membrane High H 2 O potential Low solute concentration Low H 2 O potential High solute concentration copyright cmassengale

33. Osmotic pressure The pressure that must be exerted on the side of a selectively permeable membrane containing the higher solute concentration to prevent the diffusion of water from the side containing the lower solute concentration Influenced by the concentration of dissolved substances in a solution Turgor pressure Internal hydrostatic pressure in walled cells (plants, fungi, algae and bacteria)

34. 34 Cell in Isotonic Solution CELL 10% NaCL 90% H 2 O 10% NaCL 90% H 2 O What is the direction of water movement? **The cell is at dynamic equilibrium ENVIRONMENT NO NET MOVEMENT

35. 35 Cell in Hypotonic Solution CELL 10% NaCL 90% H 2 O 20% NaCL 80% H 2 O What is the direction of water movement?

36. 36 Cell in Hypertonic Solution CELL 15% NaCL 85% H 2 O 5% NaCL 95% H 2 O What is the direction of water movement? ENVIRONMENT

37. Response of animal cells to osmotic pressure

38. 38 Isotonic Solution NO NET MOVEMENT OF H 2 O (equal amounts entering & leaving) Hypotonic Solution CYTOLYSIS Hypertonic Solution PLASMOLYSIS copyright cmassengale

39. What Happens to Blood Cells? copyright cmassengale39

40. Turgor pressure and plasmolysis

41. 41 Three Forms of Transport Across the Membrane copyright cmassengale

42. 42 Passive Transport Simple Diffusion  Doesn’t require energy  Moves high to low concentration  Example: Oxygen or water diffusing into a cell and carbon dioxide diffusing out. copyright cmassengale

43. 43 Passive Transport Facilitated diffusion  Doesn’t require energy  Uses transport proteins to move high to low concentration  Occurs down a concentration gradient Examples: Glucose or amino acids m oving from blood into a cell. copyright cmassengale

44. 44 Types of Transport Proteins Channel proteins are embedded in the cell membrane & have a pore for materials to cross Carrier proteins can change shape to move material from one side of the membrane to the other copyright cmassengale

45. 45 Facilitated Diffusion Molecules will randomly move through the pores in Channel Proteins. copyright cmassengale

46. 46 Facilitated Diffusion Some Carrier proteins do not extend through the membrane. They bond and drag molecules through the lipid bilayer and release them on the opposite side. copyright cmassengale

47. 47 Carrier Proteins Other carrier proteins change shape to move materials across the cell membrane Other carrier proteins change shape to move materials across the cell membrane copyright cmassengale

48. Active transport Moves ions or molecules against a concentration gradient Cotransport ATP-powered pump maintains a concentration gradient

49. 49 Sodium-Potassium Pump 3 Na+ pumped out for every 2 K+ pumped in; creates a membrane potential

50. Sodium-potassium pump  Requires energy or ATP  Moves materials from LOW to HIGH concentration  AGAINST concentration gradient

51. Functions of membrane proteins

52. Cells expend metabolic energy to carry on physiological processes Exocytosis Endocytosis Phagocytosis Pinocytosis Receptor-mediated endocytosis

53. 53 Moving the “Big Stuff” Molecules are moved out of the cell by vesicles that fuse with the plasma membrane. Exocytosis - moving things out. This is how many hormones are secreted and how nerve cells communicate with one another.

54. 54 Exocytosis animation Inside CellCell environment copyright cmassengale

55. Exocytosis

56. 56 Endocytosis – Phagocytosis Used to engulf large particles such as food, bacteria, etc. into vesicles Called “Cell Eating” copyright cmassengale

57. 57

58. 58 Phagocytosis - Capture of a Yeast Cell (yellow) by Membrane Extensions of an Immune System Cell (blue) copyright cmassengale

59. Phagocytosis

60. Pinocytosis

61. 61 Pinocytosis Cell forms an invagination Materials dissolve in water to be brought into cell Called “Cell Drinking” copyright cmassengale

62. Receptor-mediated endocytosis

63. 63 Receptor-Mediated Endocytosis Some integral proteins have receptors on their surface to recognize & take in hormones, cholesterol, etc. copyright cmassengale

64. Cells communicate by cell signaling Signaling molecules include Neurotransmitters Hormones Regulatory molecules

65. Cell signaling involves Synthesis and release of signaling molecule Transport to target cells Reception by target cells Signal transduction Response by the cell Termination of signal

66. Cells in close contact often develop intercellular junctions Anchoring junctions Desmosomes Adhering junctions Tight junctions Gap junctions Plasmodesmata

67. Anchoring Junctions Desmosomes Points of attachment Anchored to system of intermediate filaments inside the cells Adhering junctions Cement cells together Ex: Epithelial cells

68. Tight junctions Literally tight connections between membranes of adjacent cells Help to seal off body cavities Ex: Form the blood- brain barrier to prevent substances in the blood from entering the brain

69. Gap junctions Bridge the space between cells Act as communicating junctions Contain channels connecting the cytoplasm of adjacent cells Provide for rapid chemical and electrical communication ex: Pancreatic cells, cardiac cells, nerve cells

70. Plasmodesmata Allow materials and ions to pass between plant cells Connect the cytoplasm of neighboring cells Most contain desmotubles that run through the channel and connects the smooth ER of two adjacent cells