Membrane Structure and Function

Transcription

1 LECTURE PRESENTATIONS For CAMPBELL BIOLOGY, NINTH EDITION Jane B. Reece, Lisa A. Urry, Michael L. Cain, Steven A. Wasserman, Peter V. Minorsky, Robert B. Jackson Chapter 7 Membrane Structure and Function Lectures by Erin Barley Kathleen Fitzpatrick

2 Why do cells need a plasma membrane?

3 Overview: Life at the Edge The plasma membrane is the boundary that separates the living cell from its surroundings The plasma membrane exhibits selective permeability, allowing some substances to cross it more easily than others

4 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

5 Figure 7.2 Hydrophilic head WATER Hydrophobic tail WATER

6 In 1935, Hugh Davson and James 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 In 1972, S. J. Singer and G. Nicolson proposed that the membrane is a mosaic of proteins dispersed within the bilayer, with only the hydrophilic regions exposed to water

7 Figure 7.3 Phospholipid bilayer Hydrophobic regions of protein Hydrophilic regions of protein Why are there proteins embedded in the phospholipid bilayer?

8 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 Figure 7.5 Fibers of extracellular matrix (ECM) Glycoprotein Carbohydrate Glycolipid EXTRACELLULAR SIDE OF MEMBRANE Cholesterol Microfilaments of cytoskeleton Peripheral proteins Integral protein CYTOPLASMIC SIDE OF MEMBRANE

10 Figure 7.6 Lateral movement occurs 10 7 times per second. Flip-flopping across the membrane is rare ( once per month). If the phospholipids move, does that mean membrane proteins move too?

11 Figure 7.7 RESULTS Membrane proteins Mouse cell Human cell Hybrid cell Mixed proteins after 1 hour

12 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 than those rich in saturated fatty acids Membranes must be fluid to work properly; they are usually about as fluid as salad oil Compare and Contrast Saturated and Unsaturated Fats.

13 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

14 Figure 7.8 Fluid Viscous Unsaturated hydrocarbon tails Saturated hydrocarbon tails (a) Unsaturated versus saturated hydrocarbon tails (b) Cholesterol within the animal cell membrane helps maintain fluidity by preventing tight packing Cholesterol

15 Evolution of Differences in Membrane Lipid Composition Variations in lipid composition of cell membranes of many species appear to be adaptations to specific environmental conditions Ability to change the lipid compositions in response to temperature changes has evolved in organisms that live where temperatures vary

16 Membrane Proteins and Their Functions A membrane is a collage of different proteins, often grouped together, embedded in the fluid matrix of the lipid bilayer Proteins determine most of the membrane s specific functions

17 Peripheral proteins are bound to the surface of the membrane Integral proteins penetrate the hydrophobic core 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

18 Figure 7.9 EXTRACELLULAR SIDE N-terminus helix C-terminus CYTOPLASMIC SIDE

19 Six major functions of membrane proteins Transport Enzymatic activity Signal transduction Cell-cell recognition Intercellular joining Attachment to the cytoskeleton and extracellular matrix (ECM)

20 Figure 7.10a Signaling molecule Enzymes Receptor ATP Signal transduction (a) Transport (b) Enzymatic activity (c) Signal transduction

21 Figure 7.10b Glycoprotein (d) Cell-cell recognition (e) Intercellular joining (f) Attachment to the cytoskeleton and extracellular matrix (ECM)

22 The Role of Membrane Carbohydrates in Cell-Cell Recognition Cells recognize each other by binding to surface molecules, often containing carbohydrates, on the extracellular surface of the plasma membrane Sugar Label Membrane carbohydrates may be covalently bonded to lipids (forming glycolipids) or more commonly to proteins (forming glycoproteins) Carbohydrates on the external side of the plasma membrane vary among species, individuals, and even cell types in an individual

23 Figure 7.11 HIV Receptor (CD4) Co-receptor (CCR5) Receptor (CD4) but no CCR5 Plasma membrane HIV can infect a cell that has CCR5 on its surface, as in most people. HIV cannot infect a cell lacking CCR5 on its surface, as in resistant individuals.

24 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

25 Figure 7.12 Transmembrane glycoproteins Secretory protein Golgi apparatus ER ER lumen Vesicle Glycolipid Plasma membrane: Cytoplasmic face Extracellular face Transmembrane glycoprotein Membrane glycolipid Secreted protein

26 Concept 7.2: Membrane structure results in selective permeability A cell must exchange materials with its surroundings, a process controlled by the plasma membrane Plasma membranes are selectively permeable, regulating the cell s molecular traffic Gate

27 The Permeability of the Lipid Bilayer Hydrophobic (nonpolar) molecules, such as hydrocarbons, can dissolve in the lipid bilayer and pass through the membrane rapidly Polar molecules, such as sugars, do not cross the membrane easily

28 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 Channel proteins called aquaporins facilitate the passage of water

29 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

30 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 be directional At dynamic equilibrium, as many molecules cross the membrane in one direction as in the other Think of putting a few drops of dye into a cup of water. What happens over time?

31 Animation: Membrane Selectivity Right-click slide / select Play

32 Animation: Diffusion Right-click slide / select Play

33 Figure 7.13 Molecules of dye Membrane (cross section) WATER Net diffusion Net diffusion Equilibrium (a) Diffusion of one solute Net diffusion Net diffusion Equilibrium Net diffusion (b) Diffusion of two solutes Net diffusion Equilibrium

34 Substances diffuse down their concentration gradient, the region along which the density of a chemical substance increases or decreases No work must be done to move substances down the concentration gradient The diffusion of a substance across a biological membrane is passive transport because no energy is expended by the cell to make it happen If you are a passive person, you let things slide by

35 Effects of Osmosis on Water Balance Osmosis is the diffusion of water across a selectively permeable membrane Water diffuses across a membrane from the region of lower solute concentration to the region of higher solute concentration until the solute concentration is equal on both sides LOW solute HIGH solute Water follows the highly concentrated solute.

36 Figure 7.14 Lower concentration of solute (sugar) Higher concentration of solute Same concentration of solute Sugar molecule H 2 O Selectively permeable membrane Sugar (green dots) cannot cross the selectively permeable membrane. Osmosis

37 Water Balance of Cells Without Walls Tonicity is the ability of a surrounding solution to cause a cell to gain or lose water 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 (hyper = high) The water follows the solute, which is higher outside the cell, so the water leaves the cell, so the cell shrinks. Hypotonic solution: Solute concentration is less than that inside the cell; cell gains water (hypo = low) The water follows the solute, which is higher inside the cell, so the cell gets bigger.

38 Figure 7.15 (a) Animal cell Hypotonic solution Isotonic solution Hypertonic solution H 2 O H 2 O H 2 O H 2 O Lysed Normal Shriveled (b) Plant cell H 2 O Cell wall H 2 O H 2 O H 2 O Turgid (normal) Flaccid Plasmolyzed Osmosis

39 Hypertonic or hypotonic environments create osmotic problems for organisms Osmoregulation, the control of solute concentrations and water balance, is a necessary adaptation for life in such environments The protist Paramecium, which is hypertonic to its pond water environment, has a contractile vacuole that acts as a pump

40 Water Balance of Cells with Walls Cell walls help maintain water balance A plant cell in a hypotonic solution swells until the wall opposes uptake; 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

41 In a hypertonic environment, plant cells lose water; eventually, the membrane pulls away from the wall, a usually lethal effect called plasmolysis

42 Video: Plasmolysis

43 Video: Turgid Elodea

44 Animation: Osmosis Right-click slide / select Play

45 Facilitated Diffusion: Passive Transport Aided by Proteins In facilitated diffusion, transport proteins speed the passive movement of molecules across the plasma membrane Facilitate = proteins help move molecules across Channel proteins provide corridors that allow a specific molecule or ion to cross the membrane Channel proteins include Aquaporins, for facilitated diffusion of water Ion channels that open or close in response to a stimulus (gated channels)

46 Figure 7.17 EXTRACELLULAR FLUID (a) A channel protein Channel protein CYTOPLASM Solute Carrier protein Solute (b) A carrier protein

47 Carrier proteins undergo a subtle change in shape that translocates the solute-binding site across the membrane There are numerous diseases associated with ion channel dysfunction. Search for a disease caused by ion channel dysfunction. How does it lead directly to a disease? Extra Credit!

48 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, and the transport requires no energy Some transport proteins, however, can move solutes against their concentration gradients

49 The Need for Energy in Active Transport Active transport moves substances against their concentration gradients Active transport requires energy, usually in the form of ATP Active transport is performed by specific proteins embedded in the membranes You need energy to be active person that exercises.

50 Active transport allows cells to maintain concentration gradients that differ from their surroundings The sodium-potassium pump is one type of active transport system

51 Figure EXTRACELLULAR FLUID [ ] high [K ] low CYTOPLASM 1 [ ] low [K ] high Cytoplasmic sodium binds to the sodium-potassium pump. The protein likes to bind sodium when it has this particular shape. *Notice that the sodium is being pumped against its concentration gradient. From a low cytoplasmic side to a high extracellular side.

52 Figure EXTRACELLULAR FLUID [ ] high [K ] low CYTOPLASM [ ] low 1 [K ] high 2 P ADP ATP Sodium binding then stimulates the phosphorylation by ATP. ATP gives up one of its phosphates to the protein, so ATP becomes ADP (tri to di phosphate)

53 Figure EXTRACELLULAR FLUID [ ] high [K ] low [ ] low ATP CYTOPLASM P 1 [K ] high 2 ADP 3 P Phosphorylation causes the protein to change its shape, causing it to no longer like to bind to sodium. So it is released to the extracellular side.

54 Figure EXTRACELLULAR FLUID [ ] high [K ] low [ ] low ATP CYTOPLASM P 1 [K ] high 2 ADP 3 P The new shape now likes to bind potassium. It binds on the extracellular side and triggers the release of the phosphate. K K 4 P P i

55 Figure EXTRACELLULAR FLUID [ ] high [K ] low [ ] low ATP CYTOPLASM P 1 [K ] high 2 ADP 3 P Loss of the phosphate restores the proteins original shape, which now no longer likes to bind to potassium. K K K K 5 4 P P i

56 Figure EXTRACELLULAR FLUID [ ] high [K ] low [ ] low ATP CYTOPLASM P 1 [K ] high 2 3 ADP Potassium is released, and now the protein s shape likes to bind sodium again. The cycle repeats. P K K K K K 6 K 5 4 P P i

57 Animation: Active Transport Right-click slide / select Play

58 Figure 7.19 Passive transport Active transport Diffusion Facilitated diffusion ATP

59 How Ion Pumps Maintain Membrane Potential Membrane potential is the voltage difference across a membrane Voltage is created by differences in the distribution of positive and negative ions across a membrane

60 Two combined forces, collectively called the electrochemical gradient, drive the diffusion of ions across a membrane A chemical force (the ion s concentration gradient) An electrical force (the effect of the membrane potential on the ion s movement)

61 An electrogenic pump is a transport protein that generates voltage across a membrane The sodium-potassium pump is the major electrogenic pump of animal cells The main electrogenic pump of plants, fungi, and bacteria is a proton pump Electrogenic pumps help store energy that can be used for cellular work

62 Figure 7.20 ATP H Proton pump EXTRACELLULAR FLUID H H H H CYTOPLASM H Proton pumps are electrogenic pums that store energy by generating a voltage across the membrane. It moves positive charges in the form of hydrogens. The voltage and the hydrogen concentration gradient represent a dual energy source that can drive other processes, such as the uptake of nutrients. Most are powered by ATP!

63 Cotransport: Coupled Transport by a Membrane Protein Cotransport occurs when active transport of a solute indirectly drives transport of other solutes Plants commonly use the gradient of hydrogen ions generated by proton pumps to drive active transport of nutrients into the cell

64 ATP H H H Proton pump H H H H H Sucrose-H cotransporter Diffusion of H Sucrose Sucrose A carrier protein such as the sucrose-proton cotransporter in a plant cell is able to use the diffusion of protons down its electrochemical gradient into the cell to drive the uptake of sucrose. The proton gradient is maintained by an ATP-driven proton pump that concentrates protons outside the cell, thus storing energy that can be used for active transport of the sucrose. So, ATP indirectly provides the energy necessary for cotransport.

65 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 via transport proteins Large molecules, such as polysaccharides and proteins, cross the membrane in bulk via vesicles Bulk transport requires energy

66 Exocytosis In exocytosis, transport vesicles migrate to the membrane, fuse with it, and release their contents Many secretory cells use exocytosis to export their products Exo = exit

67 Animation: Exocytosis Right-click slide / select Play

68 Endocytosis In endocytosis, the cell takes in macromolecules by forming vesicles from the plasma membrane Endocytosis is a reversal of exocytosis, involving different proteins There are three types of endocytosis Phagocytosis ( cellular eating ) Pinocytosis ( cellular drinking ) Receptor-mediated endocytosis

69 Animation: Exocytosis and Endocytosis Introduction Right-click slide / select Play

70 In phagocytosis a cell engulfs a particle in a vacuole The vacuole fuses with a lysosome to digest the particle

71 1 m Figure 7.22a Phagocytosis Pseudopodium of amoeba EXTRACELLULAR FLUID Solutes Pseudopodium Bacterium Food vacuole An amoeba engulfing a bacterium via phagocytosis (TEM). Food or other particle Food vacuole CYTOPLASM

72 Animation: Phagocytosis Right-click slide / select Play

73 In pinocytosis, molecules are taken up when extracellular fluid is gulped into tiny vesicles

74 0.5 m Figure 7.22b Pinocytosis Plasma membrane Pinocytosis vesicles forming in a cell lining a small blood vessel (TEM). Vesicle

75 Animation: Pinocytosis Right-click slide / select Play

76 In receptor-mediated endocytosis, binding of ligands to receptors triggers vesicle formation A ligand is any molecule that binds specifically to a receptor site of another molecule

77 0.25 m Figure 7.22c Receptor-Mediated Endocytosis Plasma membrane Coat proteins Ligand Receptor Coat proteins Top: A coated pit. Bottom: A coated vesicle forming during receptor-mediated endocytosis (TEMs). Coated pit Coated vesicle

78 Animation: Receptor-Mediated Endocytosis Right-click slide / select Play

79 Figure 7.UN01 Passive transport: Facilitated diffusion Channel protein Carrier protein

80 Figure 7.UN02 Active transport ATP