Membrane Structure and Function Chapter 7 notes Membrane Structure and Function

Concept 7.1 Most abundant lipids in membranes are phospholipids. - phospholipids are amphipathic (head is hydrophilic, tail is hydrophobic ) Phospholipids and proteins are arranged in the “fluid mosaic model”: membrane is fluid w/ proteins embedded in or attached to the bilayer

Concept 7.1 Hydrophilic head WATER Hydrophobic tail

Concept 7.1 Phospholipids and proteins are arranged in the “fluid mosaic model”: membrane is fluid w/ proteins embedded in or attached to the bilayer - disproved the Davson-Danielli “sandwich” model

Concept 7.1 Phospholipid bilayer Hydrophobic regions of protein Hydrophilic regions of protein

Concept 7.1 The membrane is fluid -membranes are not static sheets of molecules locked in place -the membrane is held together primarily by hydrophobic interactions

Concept 7.1 Lateral movement (107 times per second) Flip-flop ( once per month) (a) Movement of phospholipids

Concept 7.1 (b) Membrane fluidity Fluid Unsaturated hydrocarbon tails with kinks Viscous Saturated hydro- carbon tails

Concept 7.1 Membranes are mosaics of structure and function - proteins are embedded in the fluid matrix; the lipid bilayer is the main fabric of the membrane, but proteins determine its specific fcn.

Concept 7.1 Fibers of extracellular matrix (ECM) Glyco- Carbohydrate protein Microfilaments of cytoskeleton Cholesterol Peripheral proteins Integral CYTOPLASMIC SIDE OF MEMBRANE Glycolipid EXTRACELLULAR SIDE OF MEMBRANE Carbohydrate

Concept 7.1 Two major types of membrane proteins: - Integral proteins: penetrate the hydrophobic core of the bilayer; many are transmembrane proteins - Peripheral proteins: appendages loosely bound to the surface of the membrane

Concept 7.1 EXTRACELLULAR N-terminus C-terminus CYTOPLASMIC SIDE  Helix CYTOPLASMIC SIDE EXTRACELLULAR

Concept 7.1 Membrane carbohydrates are important for cell-cell recognition - cell-cell recognition is the ability of a cell to distinguish one type of neighboring cell from another - membrane carbohydrates are usually oligosaccharides (can vary greatly)

Concept 7.1 (a) Transport (b) Enzymatic activity (c) Signal transduction ATP Enzymes Signal transduction Signaling molecule Receptor

Concept 7.1 Glyco- protein (d) Cell-cell recognition (e) Intercellular joining (f) Attachment to the cytoskeleton and extracellular matrix (ECM)

Concept 7.2 Hydrophobic molecules can cross the bilayer with ease. However, ions and polar molecules cannot pass through because they are hydrophilic. - proteins play keys roles in regulating transportation.

Concept 7.2 Transport proteins: allow hydrophilic molecules to enter and exit the cell. The selective permeability of a membrane depends on the specific transport proteins built into the membrane.

Concept 7.3 Passive transport involves diffusion across a membrane. - Diffusion: the tendency for molecules of any substance to spread out into available space - any substance will move down a [gradient]. [high]  [low]

Concept 7.3 Molecules of dye Membrane (cross section) WATER Net diffusion (a) Diffusion of one solute Equilibrium

Concept 7.3 (b) Diffusion of two solutes Net diffusion Equilibrium

Concept 7.3 Passive transport: diffusion of a substance across a biological membrane. (no energy is used) Osmosis is the passive transport of water - sln. w/ a higher [solute] = hypertonic - sln. w/ a lower [solute] = hypotonic - slns. w/ equal [solute] = isotonic

Concept 7.3

Concept 7.3 Organisms without cell walls that live in hypertonic or hypotonic environments must have adaptations for osmoregulation, the control of water balance

Concept 7.3 Organisms with cell walls - turgid (very firm) when placed in a hypotonic sln. - flacid (limp) if the sln. is isotonic - plasmolysis (shriveled) occurs when put in a hypertonic sln.

Concept 7.3 cell cell Hypotonic solution Isotonic solution Hypertonic solution H2O H2O H2O H2O (a) Animal cell Lysed Normal Shriveled H2O H2O H2O H2O (b) Plant cell Turgid (normal) Flaccid Plasmolyzed

Concept 7.3 Facilitated diffusion: passive transport of molecules through transport proteins - each protein is specific for the solute it transports

Concept 7.3 Channel protein (a) A channel protein Solute EXTRACELLULAR FLUID Channel protein (a) A channel protein Solute CYTOPLASM Carrier protein (b) A carrier protein

Concept 7.4 Active transport: movement of molecules across a membrane against the gradient (uses ATP) - sodium-potassium pump: movement of 3 Na+ for every 2 K+ ions

Concept 7.4 2 3 6 5 4 1 EXTRACELLULAR [Na+] high FLUID [K+] low Na+ [Na+] low [K+] high Na+ CYTOPLASM ATP ADP P 3 K+ 6 5 4 1

Concept 7.4 Some ion pumps generate voltage across membranes - membrane potential: the voltage across a membrane - electrogenic pump: a transport protein that generates voltage across a membrane (ex. Sodium-potassium pump)

Concept 7.4 The main electrogenic pump for plants and fungi is a proton pump which transports H+ ion out of the cell.

Concept 7.4 EXTRACELLULAR FLUID H+ Proton pump + – ATP CYTOPLASM

Concept 7.4 In cotransport, a ATP powered pump can drive the transport of other solutes. 1) active transport of a substance against a gradient 2) cotransport through a protein w/ 2nd substance

Concept 7.4 – + ATP H+ Diffusion of H+ Proton pump Sucrose-H+ cotransporter Sucrose

Concept 7.5 Exocytosis: the secretion of macromolecules by the fusion of vesicles with the plasma membrane Endocytosis: the cell takes in macromolecules by forming new vessicles - 3 types: phagocytosis, pinocytosis, and receptor-mediated endocytosis

Concept 7.5 PHAGOCYTOSIS CYTOPLASM 1 µm EXTRACELLULAR FLUID Pseudopodium “Food” or other particle Food vacuole Food vacuole Bacterium An amoeba engulfing a bacterium via phagocytosis (TEM) of amoeba 1 µm