Chapter 7: Membrane Structure & Function

Transcription

1 Chapter 7: Membrane Structure & Function 1. Membrane Structure 2. Transport Across Membranes

2 1. Membrane Structure Chapter Reading pp

3 What are Biological Membranes? Hydrophilic head WATER They re basically a 2-layered sheet of phospholipids with some proteins & cholesterol. Hydrophobic tail WATER Why phospholipids? Because they are amphipathic i.e., part is hydrophilic and part is hydrophobic. They self-assemble spontaneously into a variety of organized structures, one of which is a lipid bilayer.

4 Phospholipids phospholipids have a variety of polar head groups Hydrophilic head WATER Hydrophobic tail WATER

5 Membrane Viscosity Fluid Viscous TEMPERATURE higher temp = lower viscosity Unsaturated hydrocarbon tails Saturated hydrocarbon tails (a) Unsaturated versus saturated hydrocarbon tails (b) Cholesterol within the animal cell membrane Cholesterol SATURATION more saturation of HC tails = more viscosity CHOLESTEROL increases viscosity at higher temp, prevents hardening at lower temp

6 Membrane Proteins Membrane proteins may penetrate the interior of the membrane (integral) or interact with it externally (peripheral). the portions of a membrane protein that interact with the hydrophobic interior contain non-polar R groups TECHNIQUE FREEZE FRACTURE Extracellular layer N-terminus C-terminus Helix RESULTS EXTRA- CELLULAR SIDE CYTOPLASM Knife Proteins Inside of extracellular layer Plasma membrane Cytoplasmic layer Inside of cytoplasmic layer

7 RESULTS The Fluid Mosaic Model This model (hypothesis) proposes that proteins are scattered within a membrane and can move freely within the plane of the membrane. supported by the experiment shown below Membrane proteins Mouse cell Human cell Hybrid cell Mixed proteins after 1 hour ***some proteins are fixed by attachment to cytoskeleton or ECM***

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

9 Main Roles of Membrane Proteins Glycoprotein (a) Cell-cell recognition (b) Intercellular joining (c) Attachment to the cytoskeleton & ECM (extracellular matrix) These are the most common, though there are many other functions for membrane proteins. Signaling molecule Enzymes Receptor ATP Signal transduction (d) Transport (e) Enzymatic activity (f) Signal transduction

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

11 Membrane Faces are Different While phospholipids move freely and rapidly within a given layer or face, they rarely switch layers. phospholipid composition of cytoplasmic vs extracellular face is different & is set up in the ER Lateral movement occurs 10 7 times per second. Flip-flopping across the membrane is rare ( once per month).

12 2. Transport Across Membranes Chapter Reading pp

13 Diffusion Diffusion is the net or overall movement of a substance from higher to lower concentration molecules dissolved in liquid move randomly over time the net effect is equal dispersion of the molecules (provided there is no barrier) aka moving down concentration gradient

14 Molecules of dye WATER Membrane (cross section) Diffusion across a permeable barrier Net diffusion Net diffusion (a) Diffusion of one solute Net diffusion Net diffusion Net diffusion Net diffusion (b) Diffusion of two solutes Equilibrium Equilibrium Equilibrium As long as there is nothing to block the passage of solutes, the 2 compartments will reach equilibrium

15 Lower concentration of solute (sugar) Higher concentration of sugar Same concentration of sugar Diffusion across a selectively permeable H 2 O barrier OSMOSIS is the diffusion of water across a selectively permeable membrane Selectively permeable membrane Osmosis The barrier allows water (solvent) to pass freely while the sugar (solute) cannot pass the net flow of water from [high] to [low] creates osmotic pressure on one side of the barrier

16 Osmosis and Cells Hypotonic solution Isotonic solution Hypertonic solution H 2 O H 2 O H 2 O H 2 O (a) Animal cell Lysed Normal Shriveled H 2 O H 2 O H 2 O H 2 O (b) Plant cell Turgid (normal) Flaccid Plasmolyzed

17 Combating Osmotic Pressure Unicellular freshwater organisms that lack cell walls (such as many protozoa) are vulnerable to osmotic lysis (osmolysis). Filling vacuole 50 µm Paramecium (a) A contractile vacuole fills with fluid that enters from a system of canals radiating throughout the cytoplasm. Contracting vacuole Contractile vacuoles provide protection by taking on excess water and releasing it externally by exocytosis. (b) When full, the vacuole and canals contract, expelling fluid from the cell.

18 Small-scale Transport Cells accomplish membrane transport on a small scale (molecule by molecule) in 3 basic ways: 1) passive transport (simple diffusion) diffusion directly through the membrane bilayer 2) facilitated diffusion diffusion with the help of specific membrane proteins Passive transport Active transport 3) active transport movement from low to high concentration Diffusion Facilitated diffusion ATP requires special membrane proteins and energy

19 EXTRACELLULAR FLUID Facilitated Diffusion Channel protein (a) A channel protein Solute CYTOPLASM CHANNEL PROTEINS, some of which are gated, allow the passive transport of small molecules such as ions Carrier protein Solute CARRIER PROTEINS bind to specific solutes and change conformation to release the solute on the opposite side (b) A carrier protein works both directions with overall movement from [high] to [low]

20 The Sodium-Potassium Pump EXTRACELLULAR FLUID [Na ] high [K ] low Na Na Na Na Na Na Na Na [Na ] low ATP CYTOPLASM Na P 1 [K ] high 2 ADP 3 P K K K K K 6 K 5 4 P P i

21 Ion Transport & Membrane Potential * *cells normally have a negative membrane potential Ion diffusion is driven by differences in concentration & charge across a membrane (electrochemical gradient) i.e., electrochemical gradient

22 Cotransport Active Transport can be fueled by ATP or other energyrich molecules, OR by the cotransport of another molecule down its concentration gradient ATP H + Proton pump H + H + H + this example shows how plants carry out the active transport of sucrose into vascular cells for distribution to the rest of the plant Sucrose H + Sucrose-H + cotransporter + + H + H + H + Diffusion of H + ATP is still required for this process since it is used to set up the proton gradient sucrose transport depends on + Sucrose

23 Large-scale Transport Cells accomplish membrane transport on a large scale (in bulk) in 2 basic ways: 1) exocytosis release of material packaged in membrane vesicles to the outside of a cell 2) endocytosis ingestion of large objects or large amounts of material by enclosing within a membrane vesicle: PINOCYTOSIS PHAGOCYTOSIS RECEPTOR-MEDIATED ENDOCYTOSIS

24 Exocytosis Fluid outside cell Vesicle Protein Cytoplasm A general process for releasing material from a cell. e.g., neurotransmitters into a synapse, water from a contractile vacuole, antibodies from a B cell

25 Phagocytosis ( cell eating ) Capture of large extracellular particles in vesicles. 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) how many single-celled organisms feed (e.g., amoeba) how cells of the immune system destroy invaders

26 Pinocytosis ( cell drinking ) PINOCYTOSIS Plasma membrane 0.5 µm Pinocytosis vesicles forming (arrows) in a cell lining a small blood vessel (TEM) Vesicle Capture of extracellular fluid in vesicles. a non-specific process of capturing solutes in the fluid immediately surrounding a cell

27 Receptor-mediated Endocytosis RECEPTOR-MEDIATED ENDOCYTOSIS Receptor Coat protein Coated vesicle A highly specific process of capturing substances in vesicles. Coat protein Plasma membrane Ligand Coated pit 0.25 µm A coated pit and a coated vesicle formed during receptormediated endocytosis (TEMs) receptors on the cell surface bind specific substances (receptor ligand) this triggers the formation of a coated pit which ultimately forms a vesicle transporting the receptor-ligand complex inside the cell

28 Key Terms for Chapter 7 integral vs peripheral proteins, freeze fracture amphipathic, fluid mosaic model cytoplasmic & exoplasmic faces of membranes diffusion, osmosis, isotonic, hypertonic, hypotonic osmotic pressure, osmolysis, contractile vacuole passive transport, active transport, cotransport facilitated diffusion, electrochemical gradient carrier proteins, protein channels, pumps exocytosis, endocytosis, receptor-mediated end. Relevant Chapter Questions 1-6 vesicle, pinocytosis, phagocytosis