1. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Overview: Life at the Edge The plasma membrane is the boundary that separates the living cell from its nonliving environment The plasma membrane exhibits selective permeability, allowing some substances to cross it more easily than others

2. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

3. 2. Concept 7.1: Cellular membranes are fluid mosaics of lipids and proteins Phospholipids are the most abundant lipid in the plasma membrane Phospholipids are amphipathic molecules, containing hydrophobic and hydrophilic regions The fluid mosaic model states that a membrane is a fluid structure with a “mosaic” of various proteins embedded in it

4. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Membrane Models: Scientific Inquiry Membranes have been chemically analyzed and found to be made of proteins and lipids Scientists studying the plasma membrane reasoned that it must be a phospholipid bilayer

5. LE 7-2 Hydrophilic head Hydrophobic tail WATER

6. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings In 1935, H. Davson and J. Danielli proposed a sandwich model in which the phospholipid bilayer lies between two layers of globular proteins Later studies found problems with this model, particularly the placement of membrane proteins, which have hydrophilic and hydrophobic regions and the generalization that all membranes are identical (remember structure = __________) 3.

7. LE 7-3 Hydrophilic region of protein Hydrophobic region of protein Phospholipid bilayer In 1972, Singer and Nicolson proposed that the membrane is a mosaic of proteins dispersed within the bilayer, with only the hydrophilic regions exposed to water 4.

8. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 5. The Fluidity of Membranes Phospholipids in the plasma membrane can move within the bilayer Most of the lipids, and some proteins, drift laterally Rarely does a molecule flip-flop transversely across the membrane

9. LE 7-5a Lateral movement (~10 7 times per second) Flip-flop (~ once per month) Movement of phospholipids

10. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings As temperatures cool, membranes switch from a fluid state to a solid state The temperature at which a membrane solidifies depends on the types of lipids Membranes rich in unsaturated fatty acids are more fluid that those rich in saturated fatty acids Membranes must be fluid to work properly; they are usually about as fluid as salad oil 6.a-b

11. LE 7-5b Viscous Fluid Unsaturated hydrocarbon tails with kinks Membrane fluidity Saturated hydro- carbon tails

12. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The steroid cholesterol has different effects on membrane fluidity at different temperatures At warm temperatures (such as 37°C), cholesterol restrains movement of phospholipids At cool temperatures, it maintains fluidity by preventing tight packing 6.c.

13. LE 7-5c Cholesterol Cholesterol within the animal cell membrane

14. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 7. Membrane Proteins and Their Functions A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer Proteins determine most of the membrane’s specific functions Peripheral proteins are not embedded Integral proteins penetrate the hydrophobic core and often span the membrane

15. LE 7-7 Fibers of extracellular matrix (ECM) Glycoprotein Carbohydrate Microfilaments of cytoskeleton Cholesterol Integral protein Peripheral proteins CYTOPLASMIC SIDE OF MEMBRANE EXTRACELLULAR SIDE OF MEMBRANE Glycolipid

16. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Integral proteins that span the membrane are called transmembrane proteins The hydrophobic regions of an integral protein consist of one or more stretches of nonpolar amino acids, often coiled into alpha helices

17. LE 7-8 EXTRACELLULAR SIDE N-terminus C-terminus CYTOPLASMIC SIDE  Helix

18. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Six major functions of membrane proteins: – Transport – Enzymatic activity – Signal transduction – Cell-cell recognition – Intercellular joining – Attachment to the cytoskeleton and extracellular matrix (ECM) 8.

19. LE 7-9a Enzymes Signal Receptor ATP Transport Enzymatic activity Signal transduction 8.

20. LE 7-9b Glyco- protein Cell-cell recognition Intercellular joining Attachment to the cytoskeleton and extra- cellular matrix (ECM) 8.

21. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 9.&10. The Role of Membrane Carbohydrates in Cell-Cell Recognition Cells recognize each other by binding to surface molecules, often carbohydrates, on the plasma membrane Examples: sorting of cells into tissue and organs in animal embryos and rejection of foreign cells Membrane carbohydrates may be glycolipids or glycoproteins (glyco = carbohydrate) Membrane carbohydrates vary among species, individuals, and even cell types in an individual –Ex. Blood Type

22. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings PowerPoint Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece Lectures by Chris Romero 11 Fibers of extracellular matrix (ECM) Glycoprotein Carbohydrate Microfilaments of cytoskeleton Cholesterol Integral protein Peripheral proteins CYTOPLASMIC SIDE OF MEMBRANE EXTRACELLULAR SIDE OF MEMBRANE Glycolipid

23. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 7.2: Membrane structure results in selective permeability The function of the plasma membrane is to control the exchange of materials between a cell and its surroundings. Plasma membranes are selectively permeable, regulating the cell’s molecular traffic

24. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Permeability of the Lipid Bilayer Small, hydrophobic (nonpolar) molecules, such as CO 2 and O 2, can dissolve in the lipid bilayer and pass through the membrane rapidly Polar molecules, such as sugars, do not cross the membrane easily

25. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 12. Transport Proteins Transport proteins allow passage of hydrophilic substances across the membrane Some transport proteins, called channel proteins, have a hydrophilic channel that certain molecules or ions can use as a tunnel. 14. Channel proteins called aquaporins facilitate the passage of water

26. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Other transport proteins, called carrier proteins, bind to molecules and change shape to shuttle them across the membrane A transport protein is specific for the substance it moves Example: Glucose 13.

27. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 15. Let’s fill in the chart….

28. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 16. Concept 7.3: Passive transport is diffusion of a substance across a membrane with no energy investment Diffusion is the tendency for molecules to spread out evenly into the available space Although each molecule moves randomly, diffusion of a population of molecules may exhibit a net movement in one direction At dynamic equilibrium, as many molecules cross one way as cross in the other direction Animation: Membrane Selectivity Animation: Membrane Selectivity Animation: Diffusion Animation: Diffusion

29. LE 7-11a Molecules of dyeMembrane (cross section) WATER Net diffusion Equilibrium Diffusion of one solute

30. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Substances diffuse “down their concentration gradient,” the difference in concentration of a substance from one area to another No work must be done to move substances down the concentration gradient So this is called passive transport because it requires no energy from the cell to make it happen 16.

31. LE 7-11b Net diffusion Equilibrium Diffusion of two solutes Net diffusion Equilibrium

32. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Effects of Osmosis on Water Balance Osmosis is the diffusion of water across a selectively permeable membrane There are several ways to think about OSMOSIS: – Water always flows (diffuses) from a region of higher water concentration to lower water concentration – Water always flows (diffuses) from a region of lower solute concentration to higher solute concentration – Water always flows (diffuses) from a higher water potential to a lower water potential – Water always flows (diffuses) from a hypotonic solution to a hypertonic solution.

33. LE 7-12 Lower concentration of solute (sugar) Higher concentration of sugar Same concentration of sugar Selectively permeable mem- brane: sugar mole- cules cannot pass through pores, but water molecules can H2OH2O Osmosis

34. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 16. Water Balance of Cells Without Walls The tonicity of a solution can help you predict which way water will move. If the cell is in a(n)… Isotonic solution: solute concentration is the same as that inside the cell; no net water movement across the plasma membrane Hypertonic solution: solute concentration is greater than that inside the cell; cell loses water Hypotonic solution: solute concentration is less than that inside the cell; cell gains water

35. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Animals and other organisms without rigid cell walls have osmotic problems in either a hypertonic or hypotonic environment To maintain their internal environment, such organisms must have adaptations for osmoregulation, the control of water balance Example: The protist Paramecium, which is hypertonic to its pond water environment, has a contractile vacuole that acts as a pump Video: Chlamydomonas Video: Chlamydomonas Video: Paramecium Vacuole Video: Paramecium Vacuole

36. LE 7-14 Filling vacuole 50 µm Contracting vacuole

37. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 16. Water Balance of Cells with Walls Cell walls help maintain water balance A plant cell in a hypotonic solution swells until the wall pushes back; the cell is now turgid (firm) If a plant cell and its surroundings are isotonic, there is no net movement of water into the cell; the cell becomes flaccid (limp), and the plant may wilt : / A plant cell in a hypertonic solution loses water; eventually, the membrane pulls away from the wall, a usually lethal effect called plasmolysis 

38. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 17. Use as many words from the list above to describe why a carrot left on the counter overnight would become limp. Underline each word you use.

39. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 18. Facilitated Diffusion: Passive Transport Aided by Proteins In facilitated diffusion, transport proteins speed movement of molecules across the plasma membrane Review: – Channel proteins provide corridors that allow a specific molecule or ion to cross the membrane – Carrier proteins undergo a subtle change in shape that translocates the solute-binding site across the membrane

40. LE 7-15a EXTRACELLULAR FLUID Channel protein Solute CYTOPLASM

41. LE 7-15b Carrier protein Solute

42. 19. Review Animal cell Lysed H2OH2O H2OH2O H2OH2O Normal Hypotonic solution Isotonic solutionHypertonic solution H2OH2O Shriveled H2OH2O H2OH2O H2OH2O H2OH2O Plant cell Turgid (normal) FlaccidPlasmolyzed 20. Why doesn’t the plant cell burst?

43. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 7.4: Active transport uses energy to move solutes against their gradients Facilitated diffusion is still passive because the solute moves down its concentration gradient Some transport proteins, however, can move solutes against their concentration gradients

44. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 21. The Need for Energy in Active Transport Active transport moves substances against their concentration gradient Active transport requires energy, usually in the form of ATP Active transport is performed by specific proteins (often called “pumps”) embedded in the membranes The sodium-potassium pump is one type of active transport system Animation: Active Transport Animation: Active Transport

45. 22. Cytoplasmic Na + bonds to the sodium-potassium pump CYTOPLASM Na + [Na + ] low [K + ] high Na + EXTRACELLULAR FLUID [Na + ] high [K + ] low Na + ATP ADP P Na + binding stimulates phosphorylation by ATP. Na + K+K+ Phosphorylation causes the protein to change its conformation, expelling Na + to the outside. P Extracellular K + binds to the protein, triggering release of the phosphate group. P P Loss of the phosphate restores the protein’s original conformation. K + is released and Na + sites are receptive again; the cycle repeats. K+K+ K+K+ K+K+ K+K+ K+K+

46. 23. Diffusion Facilitated diffusion Passive transport ATP Active transport

47. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 24. Membrane Potential All cells have voltages across plasma membrane. Voltage is electrical potential energy- a separation of opposite charges. Cytoplasm is negative because of unequal distribution of anions and cation. Voltage is called membrane potential Acts like a battery, an energy source that affects traffic of charged substances. Membrane potential favors passive transport of cations into cell and anions out

48. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 25. Two forces drive diffusion of ions – Chemical force- the ion’s concentration gradient – Electrical force – membrane potential Ions diffuse down the electrochemical gradient

49. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings An electrogenic pump is a transport protein that generates the voltage across a membrane Ex. Sodium Potassium Pump The main electrogenic pump of plants, fungi, and bacteria is a proton pump (H + )

50. LE 7-18 H+H+ ATP CYTOPLASM EXTRACELLULAR FLUID Proton pump H+H+ H+H+ H+H+ H+H+ H+H+ + + + + + – – – – –

51. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 26. Cotransport: Coupled Transport by a Membrane Protein Cotransport occurs when active transport of a solute indirectly drives transport of another solute Plants commonly use the gradient of hydrogen ions generated by proton pumps to drive active transport of nutrients into the cell

52. LE 7-19 H+H+ ATP Proton pump Sucrose-H + cotransporter Diffusion of H + Sucrose H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ + + + + + + – – – – – –

53. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 7.5: Bulk transport across the plasma membrane occurs by exocytosis and endocytosis Small molecules and water enter or leave the cell through the lipid bilayer or by transport proteins Large molecules, such as polysaccharides and proteins, cross the membrane via vesicles

54. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 27. Endocytosis In endocytosis, the cell takes in macromolecules by forming vesicles from the plasma membrane Endocytosis is the opposite of exocytosis

55. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Three types of endocytosis: – Phagocytosis (“cellular eating”): Cell engulfs particle in a vacuole – Pinocytosis (“cellular drinking”): Cell creates vesicle around fluid – Receptor-mediated endocytosis: Binding of ligands to receptors triggers vesicle formation [Animations listed on slide following the figure] 27.

56. LE 7-20c Receptor RECEPTOR-MEDIATED ENDOCYTOSIS Ligand Coated pit Coated vesicle Coat protein Coat protein Plasma membrane 0.25 µm A coated pit and a coated vesicle formed during receptor- mediated endocytosis (TEMs).

57. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 27. Exocytosis In exocytosis, transport vesicles migrate to the membrane, fuse with it, and release their contents Many secretory cells (cells that secrete stuff) use exocytosis to export their products

58. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings PowerPoint Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece Lectures by Chris Romero LE 7-10 Plasma membrane: Cytoplasmic face Extracellular face Transmembrane glycoprotein Plasma membrane: Secreted protein Vesicle Golgi apparatus Glycolipid Secretory protein Transmembrane glycoproteins ER

59. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 28. Ligands Human cells use receptor-mediated endocytosis to take in cholesterol for the synthesis of membranes and steroids. Cholesterol travels as LDLs that act as ligands. Ligands are molecules that bind specifically to a receptor site of another molecule. Hypercholesterolemia- missing or defective LDL receptor. LDL accumulates in blood

60. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Synthesis and Sidedness of Membranes Membranes have distinct inside and outside faces The asymmetrical distribution of proteins, lipids and associated carbohydrates in the plasma membrane is determined when the membrane is built by the ER and Golgi apparatus