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A3 the resuit of active transport and metabobc processes, the celi contains a high concentration of smali organie moiecules, soch as sugers, amino acids, and nucleotides, to which its plasma membrano is impormoablc. Bccause most of thcsc motabolitos arc chargod. thcy alao attroct counterions. Bolh thc smali motaboiitcs and their counterions make a furthor major contribution to intracollular osmolarity.

Macromolecułes them$e»ve-9 contribute verv Httle To the osmolarity of the celi interior sińce, despite their large size. each one counts oniy as a single molecule and there are rełatiuely f©w of them compared to tłvc numbcr of smali molccutot in thc celi. Howcvor, most biologica! macromolccules arc highly chargcd. arul thcy atlract many i nor ganić ions o? opposite charge. Bccousc of their larg© numbers. thcsc counterions make a major contribution to intracollular osmolarity.

THE PROBLEM

docauso of trio above ractors. a ccii that doos nolhing to contro! its osmolarity will h»vo a higher conccntration of solutes inside than outside. As o resuit, 0 water will be higher in concentration outside the celi than inside, This dilference in water concentration across the plasma membrana will causa water to mov« conlinuously into the cel! by osmosis, causing it to rupłure.    0

The osmolarity of the ewracellulor fluid is usoally due mainly to smoli inorganic ions. These leak slowly across The plasma membranę into the celi. If they %vere not pomped out. and if there were no other moiecules inside the celi that interacted with thern so as to influence their distribution, thcy would uYuntualiy como to eguilibrium with ogual conccntrotions inside and outsido thc coli. Howovcr. thc prcscncc of chargcd macromolccules and motabolitos in thc ccii that attract thcsc ions givcs nse to thc Donnan effoct:

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THE SOLUTION

Pompa Na+- K+ jest wykorzystywana do utrzymania odpowiedniego ciśnienia osmotycznego w komórce. Jony Na+ sa wypompowywane na zewnątrz co zapobiega przenikaniu

wody do komórki i jej pękaniu.

Animal cclls and bacteria control their intracollular osmolarity by actiuely pumping out inorganic ions, such as Na*, so tria! their cyt opla sm contains a iower total concentration of inorganic ions than the extracelUiiar fluid, there by compensating for their excess oł organie solutes.

Plant cclls arc prcvcntcd from swclling by their rigid wallst and so can tclerate an osmotic difference across their plasma membranes: an interna! turgor pressurc is built up. which at equilibfium forces out as much water as enters.

Many proto/oa avokl bccomlng swollen with water. despite an osmot< difference across the plasma membrano, by periodically extruding water from specia! contraciile vacuoles.

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