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#water movement

6 public questions tagged with this topic.

Membrane Transportation

What happens to water movement in an isotonic solution?

Isotonic relationship exists when two compartments separated by semipermeable membrane possess identical effective osmolarity of impermeable solutes, typically around 300 milliosmoles per kilogram for mammalian cells corresponding to 0.9 percent sodium chloride or 5 percent dextrose clinically used for infusion. Osmotic pressure pi equals iCRT is identical on both sides, so water chemical potentials equal and thermodynamic driving force zero. Nevertheless molecular agitation does not cease; water molecules continuously cross lipid bilayer by solubility diffusion and through aquaporin-1 channel

Ref: Alberts et al., Molecular Biology of the Cell, Chapter 11: Isotonic Solutions and Dynamic Equilibrium of Water Flux.

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Membrane Transportation

In a U-tube experiment, if Side A has 4M NaCl and Side B has 10M NaCl, what will be the movement of water?

Demonstration using U-tube separated by membrane permeable only to water nicely illustrates colligative water movement. Osmotic pressure approximated by van't Hoff law pi equals i times C times R times T proportional to total particle concentration. Solution A holding 4 molar NaCl dissociates into sodium and chloride giving factor near two, so effective osmolarity roughly 8 osmoles, solution B 10 molar near 20 osmoles, markedly higher. Chemical potential of water mu equals mu naught plus RT ln a water minus V bar P, and a water declines with solute. Therefore water potential higher in dilute a

Ref: Nelson & Cox, Lehninger Principles of Biochemistry, Chapter 11: Osmosis and Water Movement Between Compartments.

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Membrane Transportation

What is the effect of hypertonic conditions on cells?

Tonicity governs cell volume by defining relative effective osmolarity of extracellular fluid compared with cytoplasm, usually near 300 milliosmoles. Hypertonic environment contains higher concentration of non-penetrating solutes like sodium chloride, mannitol or sucrose, leading to lower water potential described by psi equals psi solute plus psi pressure. Since water diffuses toward lower water potential via aquaporins and lipid bilayer, net efflux occurs from cell to surroundings. Loss of water concentrates intracellular proteins and solutes, decreases cell volume, increases ionic strength

Ref: Alberts et al., Molecular Biology of the Cell, Chapter 11: Hypertonic Effects on Cell Volume and Regulatory Mechanisms.

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Membrane Transportation

In reverse osmosis, water moves:

Natural osmosis describes spontaneous water flow from region of high water chemical potential, meaning low solute concentration, to low water potential, high solute, across semipermeable membrane until equilibrium. Reverse osmosis inverts this natural tendency by imposing mechanical energy. In purification devices impure feed solution with high total dissolved salts and industrial effluent contacts one side of thin film composite polyamide membrane with pores smaller than 0.1 nanometer that reject ions by charge repulsion and size exclusion. A high pressure pump applies hydrostatic pressure ty

Ref: Voet & Voet Biochemistry, Chapter on Membrane Transport, Section: Reverse Osmosis Hydrostatic Pressure Overcoming Pi.

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Membrane Transportation

In a U-tube experiment, water moves from:

A classical U-tube experiment visualizes osmosis by dividing tube into two arms with a membrane impermeable to solute like sodium chloride but permeable to water via narrow pores or aquaporin analogues. Each arm filled with aqueous solution of different concentration. Water activity and chemical potential are inversely related to solute concentration; lower solute means more free water molecules with higher escaping tendency. Consequently random thermal motion results in greater diffusive flux of water from low solute concentration side where water concentration high toward high solute side wh

Ref: Lodish et al., Molecular Cell Biology, Chapter: Osmosis and U-Tube Demonstration of Water Movement.

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