Membrane Functions Cell Membranes 1. boundaries 6. Cell-cell Today’s Topics 2. Localize adhesion • Membrane Structure specific – Fluid Mosaic Model – How appropriate fluidity functions is maintained • Diffusion • Osmosis 5. Cell-cell communication http://library.thinkquest.org/C004535/media/cell_membrane.gif 3. Transport

Sept 17, 2012 4. Signal detection

Figure 7.5 Phospholipid bilayer Fibers of extra- cellular matrix (ECM)

Glyco- Carbohydrate protein Glycolipid Fig. 7-2 EXTRACELLULAR SIDE OF MEMBRANE WATER! Hydrophilic head! Cholesterol

Microfilaments Peripheral of cytoskeleton proteins Hydrophobic Integral protein tail! CYTOPLASMIC SIDE OF MEMBRANE WATER!

Membrane Structure: The Fluid Mosaic Model

1972 Singer & Nicholson • Proteins embedded and floating in a sea hydrophobic of phospholipids A

B Phospholipid bilayer

hydrophilic

Protein and Lipid raft Figure 7.3

1 • Membrane proteins and lipids are Membrane • Integral synthesized in the ER and Golgi proteins • Peripheral apparatus 1 • Lipid-anchored Transmembrane glycoproteins

ER Secretory protein ~25% of known genes code for membrane Glycolipid Golgi 2 proteins apparatus

Vesicle

Most drugs target membrane proteins 3 Plasma membrane: Cytoplasmic face 4 Extracellular face Transmembrane glycoprotein Secreted protein

Membrane glycolipid Figure 7.10

Roles of membrane proteins? Evidence for the Fluid Mosaic Model?

Hydrophilic region of protein • Transport • Links to structural proteins Hydrophobic region of protein • Receptors • Enzymes • Energy Generation

Figure 7.6 Figure 7.7 The Fluidity of Membranes Evidence for membrane fluidity?

Mixed proteins after 1 hour Lateral movement occurs Flip-flopping across the membrane Mouse cell Human cell !107 times per second. is rare (! once per month). Hybrid cell

2 Evidence for integral membrane proteins: Freeze-Fracture Electron Microscopy

Fluid Viscous

Extracellular layer

A cell is frozen and fractured with a knife. The fracture plane often follows the hydrophobic interior of a membrane, splitting the phospholipid bilayer into two separated layers. The membrane proteins go wholly with one of the layers. Knife Proteins

Plasma membrane Cytoplasmic layer Illustrates: asymmetry of membrane components Unsaturated hydrocarbon Saturated hydro- tails with kinks Carbon tails External Leaflet Cytoplasmic Leaflet (b) Membrane fluidity Figure 7.4 Extracellular layer Cytoplasmic layer Figure 7.5 B

1. Lipid bilayers are selectively permeable

• small,nonpolar

• small uncharged, polar

• larger uncharged, polar molecules

• ions

Cholesterol Decreasing Figure 7.5 (c) Cholesterol within the animal cell membrane Size – polarity - ions permeability

Figure 7.13a Diffusion Simple Diffusion:

Net diffusion Net diffusion Equilibrium

(a) Diffusion of one solute

3 Simple Diffusion example: Osmosis Oxygen crossing red cell membrane Low High [Solute]! [Solute]!

O H2O! Lungs 2 CO2

Semi- permeable O Tissues 2 membrane CO2 O2 More Less CO free free 2 water water Driving force: concentration gradient Always moves from high to low concentration Osmosis!

Tonicity

HypoHypotonictonic solution Isotonic solution HyperHypertonictonic solution (a) Animal cell. An animal cell fares best in an isotonic environ- H O H O H2O 2 H2O ment unless it has 2 Maintainingspecial adaptations to Osmoticoffset the balance: osmotic uptake or loss of water. Animal cells – Figure 7.13 pump out ions Lysed Normal Shriveled

(b) Plant cell. Plant cells are turgid (firm) and H O H O generally healthiest in H O H2O 2 2 Plants,a hypotonic bacteria environ- – 2 ment, where the haveuptake cell of water walls, is buildeventually up pressurebalanced by the elastic wall pushing back on the cell.

Turgid (normal) Flaccid Plasmolyzed

4