ap membranes
TRANSCRIPT
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