Membranes

Plasma Membrane

• Funct as selective barrier– Allows some mols in while keeping others out

I. Cell Membranes

• Present in all cells (prok & euk)• Organize a cell• Form phospholipid bilayers– Phospholipids are amphipathic• Has hydrophilic & hydrophobic ends

• Form EMS (endomembrane system)

II. Plasma Membrane Models

• 1925 – Gorter & Grendel – phospholipid bilayer – No proteins

• 1935 – Davson & Danielli – sandwich model– Proteins as outer layer

• 1972 – Sanger & Nicolson – Fluid Mosiac Model– Proteins embedded in or attached to

phospholipis bilayer– Supported by fracture-freeze

Fracture Freeze

Fluid Mosaic ModelA. Fluidity

• Lateral movement of pplpds (phospholipid)• Unsaturated HC tails have kinks– Keeps tails from packing together when cold– Cholesterol also hinders close packing of pplpds

B. Membrane Proteins

• Integral – penetrate hydrophobic core of lipid bilayer– May be transmembrane

= completely span membrane

• Peripheral – not embedded in lipid bilayer at all

• Functions– Transport– Enzymatic– Signal transduction– Cell-cell recognition– Intercellular joining– Attachment to

cytoskeleton & ECM (extracellular matrix)

C. Carbohydrates

• Function in cell-cell recognition• Short branched chains– Glycoproteins – carbs cov’ly bonded to protein– Glycolipid – carbs cov’ly bonded to lipid– Membranes have distinct inside & outside faces

III. Selective Permeability

• Cell needs a way to move substances in & out• Cross with ease:

– Hydrophobic mols (HCs, CO2, & O2) by dissolving into lipid bilayer

• Cross slowly:– Polar mols (sugars & H2O)

• Other ways to get through– Transport proteins – span membrane

• Channel proteins – serve as tunnel for certain mols• EX: aquaporin – channel protein to let H2O thru

– Carrier Proteins - ∆ shape to move sp proteins across membs

IV. PASSIVE TRANSPORT

A. Diffusion

• Passive transport (no nrg req’d)• Net movement of mol from area of ↑[] to ↓[]• Substance diffuses down [] gradient• Osmosis = diffusion of H2O from area of ↑[] to ↓[]• Tonicity – ability of sol to cause cell to fain or lose

H2O • EX:– Animal cell = no cw, best isotonic– Pant cell = cw, best hypotonic

Hypotonic• Less solute in sol

than cell• Water moves in• Animal: swell & lyse• Plant: Turgid (ideal)

Isotonic• Equal solute• No net

movement• Animal: stable• Plant: flacid

Hypertonic• More solute in sol

than cell• Water moves out• Animal: shrivel• Plant: plasmolyze

Adaptations for Osmoregulation

• Osmoregulation = control of H20 balance• Paramecium – no cw – contractile vacuole

B. Facilitated diffusion

• Diffusion which reqs helper protein• EX: aquaporin– Ion channel – may act as gated channels– Gated channels • stimulus causes them to open & close• Stimulus can be elec’l or chem’l

• Facilitated Diffusion

V. Active Transport

• Req’s nrg (ATP)• EX: Na+/K+ Pump (Na out & K in)

Membrane Potential

• -50 to -200mV• Acts like a battery• Inside of cell more (-) than outside the cell• Cations move into cell & anions out

Ions move across membrane by:

• Chem’l F (force) – [] gradient• Elec’l F – membrane potential

Electrogenic Pump

• Transport protein that generated voltage across membrane– EX: • Na-K Pump in animals• P+ pump in plants, fungi & bacteria

– Actively transports H+ out of cell

Cotransport

• Mechanism where single AP powered pump that transports a sp solute can indirectly drive the active transport of several other solutes

• EX: plants – p+ pump – H+ out of cell; AAs, sugars, etc into cell

Bulk Transport

• Lg mols• Exocytosis – macromols out of cell by fusion of

vesicles w/PM• EX: insulin leaves pancreas• Endocytosis – takes macromol by forming vesicles

from PM– 3 Types

• Phagocytosis – cellular eating (by pseudopodia)• Pinocytosis – cellular drinking• Receptor mediated endocytosis