Chapter 8-Transport Across Membranes:Overcoming the Permeability Barrier

-Insoluble, an effective barrier. -Cells&organelles must accumulate substances strikingly diff from surrounding env. -Selectively permeable

-dissolved ions& small organic molecules (solutes), metabolites (substrates, intermediates & products) -more solutes on inside if cell or organelle than outside -controlled movement (ex: electircal signaling- nerve,muscle) -Maintain ability of cell reactions to proceed at reasonable rate.

-direct movement of small, nonpolar molecules. -high to low conc'n (DOWN conc'n gradient) toward equilibrium -NO ENERGY REQUIRED -ex: oxygen, carbon dioxide

-due to shells of hydration association with H2O -requires H2O to be removed before crossing the membrane -Imp for cell activity to allow ions to cross in controlled manner (ex: electrochemical potential- Na+, or H+)

Diffusion of H2O across semi-premeable membrane -Direction: High water/low solute to low water/high solute conc'n

:substances dissolved in a solution

1-Hypertonic/Hyperosmotic solution: contains higher conc'n of solutes than outside the cell. 2-Hypotonic/Hypoosmotic solution: contains lower conc'n of solutes than outside of cell. 3-Isotonic/Ipoosmotic solution: continues qual conc'n of solutes as the cell

:movement of H2O into a call creates osmotic pressure. -this can cause a cell to swell & burst

Animal Cell: Hypertonic Solution-cell becomes shriveled, only water out isotonic solution-normal, water in and out hypotonic solution-lysed, too much water in (pops) Plant Cell: Hypertonic: plasmolyzed, water out, cell wall disfromed. isotonic:flaccid, water in and out, normal shape hypotonic: turgid, water in,more circle-ish, rounded edges of plant cell wall (be able to draw)

-usually allow small molecules -highly selective for 1/small group of closely related compounds -Behaves like enzymes-exhibits saturation kinetics -subject to competitive inhibition (ex: glucose vs. other transportable sugars)

-Uniport: one direction Coupled transport: -Symport: (cotransport)-2 things,same direction -Antiport: (countertransport)-2 things, opp direction

-allow specific solutes- mainly ions cross with no conformational change.

-lined with hydrophilic amino acid side chains -highly specific (ex: Na+, K+, CA+,Cl-)

gated- opens/closes in response to stimuli 1- Voltage-gated: respond to membrane potential 2-Ligand-gated:respond to binding by specific substances. 3-Mechanosensitive:respond to mechanical forces

:closed cylindrical B(beta)-sheet -inside lined w/polar side chains-water filled pore -outside linked w/nonpolar side chains interact w/hydrophobic membrane interior -allow hydrophilic solutes, size delimited, to cross