Intracellular vs. extracellular concentrationsNote:Na+, K+, Cl-, phosphate,- & protein-

[IC] vs. [EC] important points*Intracellular cations = Intracellular anions (mEq/L)*Extracelluar cations = Extracellular anions (mEq/L)*miniscule, unmeasurable differences

Intracellular particles = Extracellular particlesi.e. IC osmolality = EC osmolality

Membrane transport overviewNo carrier:simple diffusion (lipid soluble substances)diffusion through ion channelsdiffusion through water channelsCarrier mediated transportfacilitated diffusion (passive)primary active transport (active, uses ATP)secondary active transport (active, uses ion gradient)Endocytosis & exocytosis

Simple diffusionThrough phospholipid bilayerLipid soluble substances e.g. O2, CO2, NH3, N2, fatty acids, steroids, ethanol, Passive (down concentration gradient)No carrier ( no saturation, competition)

Simple diffusionfig 4-2

Simple diffusion (flux)At equilibrium:compartment 1 concentration = compartment 2 concentrationone-way flux (left right) = one-way flux (right left)net flux = 0fig 4-3

Simple diffusion (graph of Ci vs. time)Graph shows that transport is passivei.e. over time Ci will reach, but never exceed Cofig 4-4

Simple diffusion (graph of rate vs. concentration)Graph shows that transport is not carrier mediated;because no saturation of transport rate

Transport through ion channelsfig 4-7

Properties of ion channelsUsually (not always) highly specific for the ionIon transport is passiveions are chargedtherefore, gradient depends on concentration & chargecombination is electrochemical gradientChannels open and close spontaneouslyPercentage of open time can be regulated (gating)Open time regulated by:binding of ligands to the channels (ligand gating)voltage difference across membrane (voltage gating)stretch of membrane (mechanical gating)covalent alteration of channel protein

Facilitated diffusionfig 4-8

Facilitated diffusion (properties)Passive, carrier mediatedExamples: glucose into most cells (not luminal membrane of kidney or intestine), urea, some amino acidsKinetics:shows: passiveshows: carrier mediated

Non-mediated vs. mediated transportfig 4-9

Primary active transport (Na+/K+ ATPase pump)3 Na+s out, 2 K+s in, 1 ATP hydrolyzedfig 4-11

Primary active transport propertiesActive (energy from direct hydrolysis of ATP)Carrier mediated Used when:many ions moved (e.g. 5 for Na+/K+ ATPase pump)ions moved against steep gradient (Ca++ ATPase in muscle, H+/K+ ATPase in stomach, H+ ATPase in kidney)

Primary active transport kineticsshows active transportshows carrier mediated

Effect of Na+/K+ ATPase pumpfig 4-12

Secondary active transportfig 4-13

Secondary active transport propertiesActive (energy from ion gradient, usually Na+)Carrier mediated Can be cotransport (symport) or countertransport (antiport)Examples (many):Na+/amino acids, Na+/glucose (luminal membrane kidney, GI tract), *Na+/H+ kidney, *Ca++/3Na+ muscle, *Cl-/HCO3- red cell. (* = countertransport)Kineticssee primary active transport graphs

Transport, the big picturefig 4-15

Table 4-2

Water transport (aka osmosis)Water moves through aquaporin channelsWater moves passively down its own concentration gradientDissolving solute in water reduces the water concentrationWater therefore moves from a dilute solution to a more concentrated solutionThe solute concentration depends on the number of particlesThe number of particles is called osmolarity (?osmolality?)The units of osmolarity are milliosmoles/L (mOsm/L)

Calculation of osmolarityThe osmolarity of a 100 mM glucose solution is 100 mOsm/LA 100 mM NaCl solution dissociates into 100 mM Na+ and 100 mM Cl-; its osmolarity is therefore 200 mOsm/L

Assuming complete dissociation, calculate the osmolarity of the following solutions:100 mM NaCl, 50 mM urea2.200 mM glucose, 30 mM CaCl2Answer: 250 mOsm/LAnswer: 290 mOsm/L

Red cells in solutionNotes: nonpenetrating solutes, cell osmolarity ~300 mOsm/Lfig 4-19

Crenated red cells

Osmolarity and tonicityOsmolarity is a measure of the total number of particlesTonicity is a measure of the solute particles which do not cross the cell membrane non-penetrating solutesTonicity therefore depends on the properties of the solute and the cell membraneFor example, urea crosses most cell membranes, and will enter the cell down its concentration gradientA solution of 300 mM urea is isosmotic to red cells but is hypotonic

Osmolarity and tonicity problemsConsider a solution of 100 mM NaCl and 200 mM urea. How does its osmolarity and tonicity compare with red cells having an osmolarity of 300 mOsm/L?Answer: hyperosmolar and hypotonic2.Consider a solution of 125 mM NaCl and 50 mM urea. How does its osmolarity and tonicity compare with red cells having an osmolarity of 300 mOsm/L?Answer: isosmolar and hypotonic

Osmolarity (important concept)Because cells contain abundant aquaporin channels, water rapidly equilibrates across the cell membraneTherefore, the osmolarity of virtually all body cells is equal, and equal to the osmolality of extracellular fluid

Drinking water

Endocytosis and exocytosisfig 4-20

Endocytosis and exocytosis propertiesEndocytosis:pinocytosis, phagocytosisspecificity conferred by receptor mediated endocytosisroute: see next slideExocytosis:release of neurotransmitters, hormones, digestive enzymesroute: rough er Golgi secretory vesiclesrelease usually triggered by cytosolic [Ca++]insertion of glucose transporters (insulin), insertion of water channels (ADH)

Endocytosis routefig 4-21

Epithelial transport (Na+)fig 4-22

Epithelial transport (water)fig 4-24

Epithelial transport (glucose in kidney, GI tract)fig 4-23