Membrane Structure and Function. Chapter 7

Transcription

1 Membrane Structure and Function Chapter 7

2 Overview: Life at the Edge The plasma membrane selectively permeable

3 Phospholipids most abundant lipids in plasma membrane amphipathic = hydrophobic and hydrophilic regions Polar head Hydrocarbon tails Cell membranes

4 Phospholipid

5 Membrane Models: Scientific Inquiry Hydrophilic head WATER Hydrophobic tail WATER

6 1935Davson and Danielli - bilayer model 1972 Singer and Nicolson - fluid mosaic model membrane mosaic of proteins dispersed within bilayer Current model: mosaicism

7 TECHNIQUE Extracellular layer RESULTS Knife Proteins Inside of extracellular layer Plasma membrane Cytoplasmic layer Inside of cytoplasmic layer

8 Proteins embedded in bilayer Phospholipid bilayer Hydrophobic regions of protein Hydrophilic regions of protein

9 The Fluidity of Membranes Phospholipids in membrane move laterally within bilayer rarely flip flop Lipids, proteins, may move laterally Lateral movement ( 10 7 times per second) Flip-flop ( once per month) (a) Movement of phospholipids

10 RESULTS Membrane proteins Mouse cell Human cell Hybrid cell Mixed proteins after 1 hour

11 Membrane fluidity affected by: 1. Type of phospholipid Fluid Viscous Unsaturated hydrocarbon tails with kinks Saturated hydrocarbon tails

12 2. Temperature cool gel Tightly packed tails warm fluid

13 3. Cholesterol Stabilizes membrane fluidity with changing temperature FYI Cholesterol can compose ½ of the membrane Bacterial cell membranes do not contain cholesterol Plant cells do not contain much

14 Cholesterol (c) Cholesterol within the animal cell membrane

15 Membrane Proteins and Their Functions Mosaic of proteins embedded in lipid bilayer Proteins determine most of membrane s functions

16 Membrane Proteins Peripheral proteins bound to of membrane

17 Integral proteins penetrate hydrophobic region Transmembrane proteins N- terminus C-terminus

18 1. Receptor proteins for signal transduction ex. Insulin receptor

19 2. Channel proteins for passage of molecules hydrophilic core Hydrophilic core Ex. Aquaporins

20 3. Transport proteins Ex. Glucose transporter shuttles glucose across membrane

21 Signaling molecule Enzymes Receptor ATP Signal transduction (a) Transport (b) Enzymatic activity (c) Signal transduction

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

23 4. Cell-cell recognition Glycoproteins Carbohydrates attached to proteins mucin

24 5. Intercellular joining proteins Ex. gap junctions allow passage of ions and small molecules from cell to cell

25 6. Extracellular matrix proteins 7. Membrane enzymes

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

27 Selective permeability Plasma membrane regulates cell molecular traffic

28 Permeability of Lipid Bilayer Hydrophobic molecules dissolve in bilayer and pass through membrane rapidly O2, CO2, NO, steroids, nanoparticles O2 CO2 Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

29 Hydrophilic (polar) molecules do not cross easily sugar, water, ions

30 Passive transport = no energy used molecules move randomly A. Diffusion Molecules diffuse down their concentration gradient from high to lower concentration until equilibrium

31 What molecules can diffuse across the cell membrane? Answer: O2, CO2, urea Net diffusion Net diffusion Equilibrium Net diffusion (b) Diffusion of two solutes Net diffusion Equilibrium

32 B. Osmosis is diffusion of water across a selectively permeable membrane Water diffuses across membrane from region of higher water concentration to the region of lower water concentration until equilbrium

33 Lower concentration of sugar) Higher Concentration of sugar Same concentration of sugar H 2 O Selectively permeable membrane Osmosis

34 Water Balance of Cells Without Walls Tonicity =ability of solution to cause cell to gain or lose water Isotonic solution: Solute concentration same as in cell no net water movement across membrane Hypertonic solution: Solute concentration greater than inside cell cell loses water Hypotonic solution: Solute concentration is less than inside cell cell gains water

35 Solution type? Isotonic, hypotonic, hypertonic? Hypotonic solution Isotonic solution Hypertonic solution H 2 O H 2 O H 2 O H 2 O (a) Animal cell Lysed Normal Shriveled

36 How do cells deal with changing external water concentrations? Osmoregulation= control of water balance Ex. Paramecium video pond water is to the protista contractile vacuole

37 Filling vacuole 50 µm (a) A contractile vacuole fills with fluid that enters from a system of canals radiating throughout the cytoplasm. Contracting vacuole (b) When full, the vacuole and canals contract, expelling fluid from the cell.

38 Water Balance in plants (cell wall) Isotonic solution no net movement of water into cell flaccid (limp)

39 Hypotonic solution cell (vacuole) swells cell wall opposes uptake turgid (firm)

40 Hypertonic lose water; membrane pulls away from wall plasmolysis (lethal)

41 Hypotonic solution H 2 O Isotonic solution H 2 O H 2 O Hypertonic solution H 2 O (b) Plant cell Turgid (normal) Flaccid Plasmolyzed

42 C. Facilitated Diffusion: Passive Transport aided by proteins Channel proteins Carrier proteins What molecules use facilitated diffusion to cross membrane? Answer: glucose, sodium ions, chloride ions, water

43 EXTRACELLULAR FLUID Channel protein (a) A channel protein Solute CYTOPLASM Carrier protein Solute (b) A carrier protein

44 Active transport Energy (ATP) required to move solutes against their gradients (from lower to higher conc.!) Pumps are membrane proteins Ex. sodium-potassium pump (an enzyme) FYI: All animals Nobel prize 1997 (Jens Skou) Uses 1/3 of cells total energy production Provides driving force for other cell processes (secondary transport, volume, gradients)

45 Examine the figure: EXTRACELLULAR FLUID [Na + ] high [K + ] low Na+ high outside cell K+ low Na + Na + Na+ low inside cell K+ high CYTOPLASM Na + [Na + ] low [K + ] high According to diffusion? 1 Cytoplasmic Na + binds to the sodium-potassium pump.

46 1. Cytoplasmic Na+ binds to pump

47 2. ADP phosphorylated to ATP What is ATP? Na + Na + Na + ATP P ADP 2 Na + binding stimulates phosphorylation by ATP.

48 3. Na+ out of cell Na Na + + Na Na + + Na Na + + shape change of pump Against conc. grad. P 3 Phosphorylation causes the protein to to change its its shape. Na + is is expelled to to the outside.

49 4. Extracellular K+ binds to pump ATP used P 4 K + binds on the extracellular side and triggers release of the phosphate group. P

50 5 + 6 K+ inside cell Pump animation Step by step Shape change

51 Passive transport Active transport Review the difference passive vs active transport Diffusion Facilitated diffusion ATP NO ATP IS USED WHICH ONE REPRESENTS THE CHANNEL PROTEIN?

52 Why do cells need pumps? a. Maintain membrane potential = voltage difference across membrane Inside of cell more electronegative than out = negative membrane potential

53 + EXTRACELLULAR FLUID ATP + H + H + Proton pump H + + H + CYTOPLASM + H + + H +

54 b. Maintain electrochemical gradients chemical = concentration gradient electrical = membrane potential

55 Bulk transport Exocytosis To secrete products from cell Vesicles fuse with membrane

56 2. Endocytosis cell takes in macromolecules by forming vesicles from membrane a. Phagocytosis for large particle Vesicle fuses with lysosome to digest particle

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

58 b. Pinocytosis for fluids/small molecules PINOCYTOSIS Plasma membrane 0.5 µm Pinocytosis vesicles forming (arrows) in a cell lining a small blood vessel (TEM) Vesicle

59 c. receptor-mediated endocytosis, ligand binds to receptor vesicle RECEPTOR-MEDIATED ENDOCYTOSIS Receptor Coat protein Coated vesicle Ligand Coated pit