Membrane Structure and Function

Transcription

1 Chapter 7 Membrane Structure and Function PowerPoint lectures are originally from Campbell / Reece Media Manager and Instructor Resources for BIOLOGY, 7 th & 8 th Edition by N. A. Campbell & J. B. Reece Copyright 2005 & 2008 Pearson Education, Inc. Publishing as Benjamin Cummings

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

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4 Concept 7.1: Cellular membranes are fluid mosaics of lipids and proteins Phospholipids are the most abundant lipid in the plasma membrane Hydrophilic head Phospholipids are amphipathic molecules, containing hydrophobic and hydrophilic regions Hydrophobic tails

5 Membrane Model Membranes have been chemically analyzed and found to be made of proteins and lipids In 1972, Singer and Nicolson proposed that the membrane is a mosaic of proteins dispersed and individually inserted into the phospholipid bilayer

6 The fluid mosaic model states that a membrane is a fluid structure with a mosaic of various amphipatic proteins embedded in it Hydrophilic region of protein Phospholipid bilayer Hydrophobic region of protein

7 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

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

9 The Fluidity of Cell Membranes is affected by: 1- Temperature. 2- Fatty acids composition. 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

10 LE 7-5b Fluid Viscous Unsaturated hydrocarbon tails with kinks Membrane fluidity Saturated hydrocarbon tails

11 Cholesterol and Membranes Fluidity 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 Cholesterol acts as a temperature buffer for Membrane fluidity

12 LE 7-5c Cholesterol Cholesterol within the animal cell membrane

13 Membrane Proteins Some proteins in the plasma membrane can drift within the bilayer Proteins are much larger than lipids and move more slowly To investigate whether membrane proteins move, researchers fused a mouse cell and a human cell

14 LE 7-6 Membrane proteins Mouse cell Human cell Hybrid cell Mixed proteins after 1 hour

15 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. According to their location, membrane proteins are classified into: Peripheral proteins are bound to the surface of the membrane Integral proteins penetrate the hydrophobic core and often span the membrane

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

17 Membrane Proteins 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 LE 7-8 N-terminus EXTRACELLULAR SIDE C-terminus a Helix CYTOPLASMIC SIDE

19 Functions of Membrane Proteins Six major functions of membrane proteins: 1) Transport proteins -form small openings for molecules to difuse through 2) Enzymatic Proteins - carry out metabolic reactions 3) Signal transduction proteins - molecular triggers that set off cell responses (such as release of hormones or opening of channel proteins) 4) Cell to cell Recognition Proteins - ID tags, to identify cells to the body's immune system 5) Intercellular joining- link adjacent cells together 6) Attachment to the cytoskeleton and extracellular matrix (ECM)

20 LE 7-9a Enzymes Signal ATP Receptor Transport Enzymatic activity Signal transduction

21 LE 7-9b Glycoprotein Cell-cell recognition Intercellular joining Attachment to the cytoskeleton and extracellular matrix (ECM)

22 Transport

23 Enzymatic Activity

24 Receptors for Signal Transduction

25 Cell-Cell Recognition

26 The Role of Membrane Carbohydrates in Cell-Cell Recognition Cells recognize each other by binding to surface molecules, often carbohydrates, on the plasma membrane 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

27 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

28 ER 1 Secretory protein Glycolipid Golgi 2 apparatus Transmembrane glycoproteins Vesicle Secreted protein 3 4 Plasma membrane: Cytoplasmic face Extracellular face Transmembrane glycoprotein Membrane glycolipid

29 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

30 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

31 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

32 Channel Transport Proteins EXTRACELLULAR FLUID Channel protein Solute CYTOPLASM

33 Channel Transport Proteins

34 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

35 Carrier Transport Proteins Carrier protein Solute

36 Carrier Transport Proteins

37 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.

38 Diffusion 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 The diffusion of a substance across a biological membrane is passive transport because it requires no energy from the cell to make it happen

39 Types of Membrane Transport Passive processes No cellular energy (ATP) required Substance moves down its concentration gradient (i.e from high to low concentration) Active processes Energy (ATP) required Occurs only in living cell membranes Substance moves against its concentration gradient (i.e from low to high concentration)

40 Types of Passive Processes Simple diffusion Facilitated diffusion: Carrier-mediated facilitated diffusion Channel-mediated facilitated diffusion Osmosis

41 Passive Processes: Simple Diffusion Nonpolar lipid-soluble (hydrophobic, such as O 2, CO 2 ) substances diffuse directly through the phospholipid bilayer

42 At dynamic equilibrium, as many molecules cross one way as cross in the other direction Molecules of dye Membrane (cross section) WATER Net diffusion Net diffusion Equilibrium Simple Diffusion of one solute

43 LE 7-11b Net diffusion Net diffusion Equilibrium Net diffusion Net diffusion Equilibrium Simple Diffusion of two solutes

44 Passive Processes: Facilitated Diffusion Certain lipophobic molecules (e.g., glucose, amino acids, and ions) use carrier proteins OR channel proteins, both of which: Exhibit specificity (selectivity) Are saturable; rate is determined by number of carriers or channels

45 Facilitated Diffusion Using Carrier Proteins Transmembrane integral proteins transport specific polar molecules (e.g., sugars and amino acids) Binding of substrate causes shape change in carrier

46 Lipid-insoluble solutes (such as sugars or amino acids) (b) Carrier-mediated facilitated diffusion via a protein carrier specific for one chemical; binding of substrate causes shape change in transport protein Figure 3.7b

47 Facilitated Diffusion Using Channel Proteins Aqueous channels formed by transmembrane proteins. Two types: Leakage channels Always open Gated channels Controlled by chemical or electrical signals

48 Small lipidinsoluble solutes (c) Channel-mediated facilitated diffusion through a channel protein; mostly ions selected on basis of size and charge Figure 3.7c

49 Channel-mediated facilitated diffusion

50 Effects of Osmosis on Water Balance Osmosis is the diffusion of water across a selectively permeable membrane The direction of osmosis is determined only by a difference in total solute concentration Water diffuses across a membrane from the region of lower solute concentration to the region of higher solute concentration

51 LE 7-12 Lower concentration of solute (sugar) Higher concentration of sugar Same concentration of sugar H 2 O Selectively permeable membrane: sugar molecules cannot pass through pores, but water molecules can Osmosis

52 Water Balance of Cells Tonicity is the ability of a 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

53 Water Balance of Cells 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

54 Water Balance of Cells Without Cell Walls 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 The protist Paramecium, which is hypertonic to its pond water environment, has a contractile vacuole that acts as a pump to get rid of excess water.

55 (a) Isotonic solutions (b) Hypertonic solutions (c) Hypotonic solutions Cells retain their normal size and shape in isotonic solutions (same solute/water concentration as inside cells; water moves in and out). Cells lose water by osmosis and shrink in a hypertonic solution (contains a higher concentration of solutes than are present inside the cells). Cells take on water by osmosis until they become bloated and burst (lyse) in a hypotonic solution (contains a lower concentration of solutes than are present in cells). Figure 3.9

56 LE 7-14 Filling vacuole 50 µm Contracting vacuole 50 µm

57 Water Balance of Cells with Cell 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 In a hypertonic environment, plant cells lose water; eventually, the membrane pulls away from the wall, a usually lethal effect called plasmolysis

58 LE 7-13 Animal cell Hypotonic solution Isotonic solution Hypertonic solution H 2 O H 2 O H 2 O H 2 O Lysed Normal Shriveled Plant cell H 2 O H 2 O H 2 O H 2 O Turgid (normal) Flaccid Plasmolyzed

59 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

60 Summary of Passive Processes Process Energy Source Simple diffusion Kinetic energy Facilitated diffusion Kinetic energy Example Movement of O 2 through phospholipid bilayer Movement of glucose into cells Osmosis Kinetic energy Movement of H 2 O through phospholipid bilayer or AQPs

61 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 embedded in the membranes.

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

63 LE 7-16 EXTRACELLULAR FLUID [Na + ] high [K + ] low Na + Na + Na + Na + CYTOPLASM Na+ [Na + ] low [K + ] high Cytoplasmic Na + bonds to the sodium-potassium pump Na + Na + Na + P ADP ATP Na + binding stimulates phosphorylation by ATP. Na + Phosphorylation causes the protein to change its conformation, expelling Na + to the outside. P P P Extracellular K + binds to the protein, triggering release of the phosphate group. Loss of the phosphate restores the protein s original conformation. K + is released and Na + sites are receptive again; the cycle repeats.

64 Active Transport / The sodium-potassium pump

65 LE 7-17 Passive transport Active transport Diffusion Facilitated diffusion ATP

66 Membrane Potential Membrane potential is the voltage difference across a membrane. Voltage is created by differences in the distribution of positive and negative ions

67 Diffusion of Ions (charged atoms or molecules) Across a Membrane Two forces, collectively called the electrochemical gradient, drive the diffusion of ions (charged atoms or molecules) across a membrane: A chemical force (the ion s concentration gradient) An electrical force (the effect of the membrane potential, differences in charge distribution across the membrane, on the ion s movement)

68 An electrogenic pump is a transport protein that generates the charage differential (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

69 LE EXTRACELLULAR FLUID ATP + H + H + Proton pump H + + H + CYTOPLASM + H + + H +

70 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

71 LE 7-19 ATP + + H + H + H + Proton pump H H + Sucrose-H + cotransporter H + Diffusion of H + H Sucrose

72 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 in bulk via vesicles. Bulk transport requires energy

73 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

74 Endocytosis In endocytosis, the cell takes in macromolecules by forming vesicles from the plasma membrane Endocytosis is a reversal of exocytosis, involving different proteins

75 Three types of endocytosis: Phagocytosis ( cellular eating ): Cell engulfs particle in a vacuole. The vacuole fuses with a lysosome to digest the particle Pinocytosis ( cellular drinking ): Cell creates vesicle around fluid. Molecules are taken up when extracellular fluid is gulped into tiny vesicles. 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

76 Phagocytosis PHAGOCYTOSIS EXTRACELLULAR FLUID Pseudopodium CYTOPLASM 1 µm Pseudopodium of amoeba Food or other particle Food vacuole Bacterium Food vacuole An amoeba engulfing a bacterium via phagocytosis (TEM)

77 Pinocytosis PINOCYTOSIS Plasma membrane 0.5 µm Pinocytosis vesicles forming (arrows) in a cell lining a small blood vessel (TEM) Vesicle

78 LE 7-20c Receptor Coat protein Coated vesicle Ligand Coated pit Substances taken up by this method include enzymes, insulin and some other hormones, LDL cholesterol, and iron. Flu viruses, diphtheria, and cholera toxins use this route to enter and attack our cells. Receptor-mediated endocytosis Coat protein A coated pit and a coated vesicle formed during receptor-mediated endocytosis (TEMs).

79 Fig. 7-UN3 Cell 0.03 M sucrose 0.02 M glucose Environment: 0.01 M sucrose 0.01 M glucose 0.01 M fructose

80 You should now be able to: 1. Define the following terms: amphipathic molecules, aquaporins, diffusion 2. Explain how membrane fluidity is influenced by temperature and membrane composition 3. Distinguish between the following pairs or sets of terms: peripheral and integral membrane proteins; channel and carrier proteins; osmosis, facilitated diffusion, and active transport; hypertonic, hypotonic, and isotonic solutions

81 You should now be able to: 4. Explain how transport proteins facilitate diffusion 5. Explain how an electrogenic pump creates voltage across a membrane, and name two electrogenic pumps 6. Explain how large molecules are transported across a cell membrane