BCH 4123 Lecture Notes - Lecture 3: Ketonuria, Aldosterone, Collecting Duct System

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erinraven63

1 Mar 2017

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Semipermeable membrane allows water but not all solutes to cross. Osmosis: movement of water through a semipermeable membrane from a region with low solute concentration to a region with high solute concentration. Osmole: the number of moles of a compound that contributes to osmosis. Osmolarity: concentration of all particles per volume (osmole/l, dependent on temperature and pressure. Osmolality: total concentration of all particles per mass (osmole/kg) independent of temperature and pressure, normal body fluids is 295 mosm/kg. Osmotic pressure: drawing/pulling force on water exerted by particles across a semipermeable membrane, compartment with less particles will shrink and one with more particles will expand. Change in volume is proportional to osmolality. Hydrostatic pressure: push of fluid, due to weight of fluid or by pumping fluid distribution, 2/3 tbw is icf, 1/3 tbw is ecf. Ecf is interstitial fluid (isf: dependent on age and gender. Water % of tbw declines with age. Males have higher tbw % than females.

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1) This is a thermodynamics questions

In your body, normal osmolarity of blood serum is just under 0.3 molar. Because cell membranes are permeable to water, the osmolarity of intracellular fluid is approximately equal to that of extracellular fluid (though the intracellular and extracellular composition in terms of specific ions and proteins is quite different). As an aside, measurement of osmolarity of serum is an important clinical test used to check for dehydration or other imbalances of water and electrolytes.

a) In the supplemental information, the osmosis example presented involves sucrose as a solute, but the van't Hoff equation also applies for other solutes. Using this formula (â = cRT, where â is the excess pressure), calculate the osmotic pressure associated with 0.3 molar concentration of osmolytic particles (e.g., ions, proteins, or other dissolved particles) in contact with pure water through a semi-permeable membrane at physiological temperature of 310 K. Express your results in atmospheres of pressure (1 atmosphere of pressure is 1.0x10⁵ Pascal, where 1 Pascal is a Newton/m²).

b) A biological cell membrane is a bilayer of phospholipid molecules, only a few nanometers thick, and can basically be thought of much like a soap bubble, except immersed in aqueous solution. If cells are placed in pure water, they often swell and burst in just a few minutes. Explain why this would happen.

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BCH 4123 Lecture Notes - Lecture 3: Ketonuria, Aldosterone, Collecting Duct System

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