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#osmotic pressure

32 public questions tagged with this topic.

Membrane Transportation

Which condition would cause the highest osmotic pressure?

Osmotic pressure pi is thermodynamically defined for ideal dilute solution by van't Hoff expression pi equals i times M times R times T, where i accounts for dissociation into ions, M molarity of total particles, R universal gas constant 0.082 liter atm per mol per K, T absolute temperature. Sodium chloride dissociates into two particles so i about two, doubling effective concentration. Consequently pi scales linearly with total osmolarity. Among listed options, 0.1 molar NaCl yields approximately 0.2 osmolar giving pi about 4.9 atm at 298 K, 0.5 molar yields 1.0 osmolar about 24.5 atm, 1.0 molar yields 2.0 osmolar about 49 atm, 1.5 molar yields 3.0 osmolar about 73 atm, assuming complete dissociation and ignoring activity coefficients. Hence highest concentration produces highest osmotic pressure provided temperature constant. Even though example marks 1.0 molar as answer, principle that osmotic pressure increases monotonically with molar concentration remains; 1.5 molar would generate greatest pressure as predicted from proportionality between solute particle number and colligative property.

Ref: Berg et al., Biochemistry, Chapter 2: Van't Hoff Osmotic Pressure Dependence on Concentration.

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Membrane Transportation

Which of the following conditions describes osmotic pressure?

Osmotic pressure is a colligative property describing tendency of water to move across semipermeable membranes that allow solvent but restrict solute permeation. When compartment containing dilute solution is separated from concentrated solution, water chemical potential is higher where solute activity is lower. Spontaneous diffusion drives water toward higher solute until chemical potentials equalize, causing volume increase and hydrostatic pressure rise on concentrated side. The additional hydrostatic pressure required to stop net influx and hold volumes constant at equilibrium defines osmotic pressure. Thermodynamically it can be approximated by van't Hoff equation pi equals i times M times R times T, where i is van't Hoff factor for dissociation, M molarity, R gas constant, T absolute temperature. It depends only on total number of solute particles, not identity. This concept explains cellular swelling, water absorption in roots, and clinical use of isotonic solutions. Reverse osmosis applies pressure exceeding pi to invert flow. It is not force exerted by water passing through membrane nor pressure generated by ion channels, but prevention pressure balancing osmotic tendency.

Ref: Alberts et al., Molecular Biology of the Cell, Chapter 11: Osmotic Pressure Definition and van't Hoff Equation.

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Microbial Physiology

In bacterial growth, water availability is controlled by:

Water availability parameter aw water activity influences growth because intracellular reactions require aqueous medium, turgor pressure drives expansion. aw equals relative humidity 100 percent in equilibrium. Bacteria regulate response to aw change mainly via osmotic pressure sensing osmosensors EnvZ. Hyperosmotic shock water exits cytoplasm plasmolysis outer membrane separates, cytoplasm shrivels, growth arrested. Compatible solutes accumulation potassium glutamate, proline, ectoine, trehalose synthesised or imported via ProP, BetT transporters balances osmolarity without denaturing proteins. Mechanosensitive channels MscL MscS open to release solutes during hypoosmotic downshock preventing lysis. Halophiles require high sodium for protein solvation acidic proteome. Temperature indirectly affects aw through evaporation, oxygen concentration affects respiration not hydration, barometric pressure affects gas solubility. Thus osmotic pressure is primary determinant controlling water availability defining limits in dried foods jerky aw 0.8, salted cod, brines, desert soils. Food industry reduces aw by adding salt sugar to inhibit spoilage microbes and extend shelf life without refrigeration.

Ref: Brock Biology of Microorganisms, 16th ed., Chapter 5: Water activity and osmotic pressure.

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