15-1,585

Obviously ion concentrationnot the same insidecell as outside

How is this maintained?

What is magic aboutthese concentrations(similar for inverts,verts)??

15-1, 579

Properties of a pure phospholipid membrane

15-33, 611In real membranes watergets in via aquaporins

transporting epithelia (RBCs, kidney, tear ducts)poisoned by mercurynot ion permeable at least 5 isoformsIshida et al. 1996 Expression and characterization of lacrimal gland water channels in Xenopus oocytes BBRC 224:1-4inject aquaporin cDNA into oocytes-take up waterinject antisense cDNA into same eggs-stops water uptake

15-3, 581membrane transportersthat do not requireATP hydrolysis

Couple a reaction thatis thermodynamicallyfavorable to one that is not

Transport with samerules as enzymekinetics

15-5, 583

15-7, 584Glucose transporters -uniporters-7 genes, 6 proteinsGLUT 1-RBC, brain, ubiquitousGLUT 2-liverGLUT 3-brain and fetal muscleGLUT 4-skeletal and cardiac muscleGLUT 5-sperm, muscle, adipocytesGLUT 6-pseudogeneGLUT 7-hepatocytes

Can go in either direction-concentration driven

Normally glucose higher outside cell than in,but during starvationtransport reverses

Frozen wood frogs-upregulate glucose transporters dramatically as soon as toes freeze-freeze from outside in-thaw from inside out-highest concentration of glucose in organs

Ion Concentration vs. Charge Concentration across membranes

-resting potential of a cell is -70 mV (or -85 mV-depending on who you talk to)-that means the inside is negative relative to the outside- -70 mV over 3.5 nm of plasma membrane thickness = 200000 V over cm thickness -not trivial

-plasma membrane is a capacitor- two conducting surfaces separated by a nonconducting material -spontaneous movement of ions (not ATP involved) determined by both concentration gradient and electrical potential across membrane

Conducting phospholipid heads

Nonconducting fatty acyl tails

-concentration gradient and electrical potential can both work in the samedirection, as in the case of sodium

------

++

++++

Na +Na +

145 mM 12 mM

Concentration Gr adient

Electrical Potential

------

++

++++

K +K +

4 mM 139 mM

Concentrat ion Gradient

E lectrical Potential

-or-concentration gradient and electrical potential can work in oppositedirections, as in the case of potassium

At Nernst potential-concentration gradient perfectly balanced with electrical gradient (electrical potential) and ions don't flow in or out

R=gas constantT=temp in Kelvinz=valenceF=Faraday's constantE=electrical potential

Osmolarity-total particle concentration-has to be balanced on both sides of membrane or cell will shrink or swell.

Group Exercise #10

This is an old model of how ions are transportedacross membranes, which is now has virtually no support.

Why can’t this model work?