Drinking salt water actually robs the body of hydration, because it creates a hypertonic environment in the GI tract, which pulls water out of our cells, dehydrating the body. Virtual Classrooms. See all of related teaching materials on the. MICRO 8wk. MICRO 15wk. Instructor's Corner. The definition based on tonicity as the effective osmolality is best.
Consider again the U-tube experiment but pure water on one side and a test solution of unknown osmolality on the other side of a semipermeable membrane which is permeable only to water. Water will move into the test solution. What would happen if further amounts of the test solution were added before any movement of water had occurred?
An equilibrium situation would be reached at which the hydrostatic pressure ie difference in fluid heights in the two limbs of the U-tube on the test solution side of the membrane would balance the osmotic tendency for water to move across the membrane into the test solution. At this equilibrium point, the hydrostatic pressure is a measure of the osmotic tendency in the test solution: indeed the opposing hydrostatic pressure needed to balance the osmotic forces is usually referred to as the osmotic pressure.
There would be practical difficulties in performing this experiment with body fluids as the test solution as the osmotic pressure to be measured is over 7 atmospheres and an extremely long-limbed U-tube would be necessary!
Alternatively, the pressure could be supplied from a piston or a compressed gas source rather than a column of fluid.
Tonicity is the effective osmolality and is equal to the sum of the concentrations of the solutes which have the capacity to exert an osmotic force across the membrane. U-Tube Experimental Setup coming. If the water levels were different in the two limbs of the U-tube at the start of the experiment then: What would be the equilibrium situation as regards particle ie solute concentration on the two sides of the membrane?
What would the difference if any be in the heights of the water levels on the two sides of the membrane? Is the equilibrium condition reached when the particle concentration ie osmolality is equal on the two sides of the membrane? Osmolality and Tonicity: Relationship to Membrane Osmolality is a property of a particular solution and is independent of any membrane. Tonicity is a property of a solution in reference to a particular membrane.
Comparison of Different Definitions of Tonicity Effective osmolality - The best definition as it accounts for permeant solutes and is quantitative. The red cell test - A practical qualitative definition that emphasises the requirement that tonicity is defined in reference to a membrane. Comparison with osmolality of plasma - Does not account for permeant solutes, and not quantitative.
A final point here regarding the meaning of the term "osmotic pressure". Useful Definitions Mole - A mole is the amount of a substance that contains the number of molecules equal to Avogadro's number.
The mass in grams of one mole of a substance is the same as the number of atomic mass units in one molecule of that substance ie the molecular weight of the substance expressed as grams. The mole symbol: mol is the base unit in the SI system for the amount of a substance Avogadro's number - this is the number of molecules in one mole of a substance ie 6. Cells in a hypertonic solution shrink as water exits the cell, becoming shriveled.
Key Points Osmolarity describes the total solute concentration of a solution; solutions with a low solute concentration have a low osmolarity, while those with a high osmolarity have a high solute concentration. Water moves from the side of the membrane with lower osmolarity and more water to the side with higher osmolarity and less water. In a hypotonic solution, the extracellular fluid has a lower osmolarity than the fluid inside the cell; water enters the cell.
In a hypertonic solution, the extracellular fluid has a higher osmolarity than the fluid inside the cell; water leaves the cell. In an isotonic solution, the extracellular fluid has the same osmolarity as the cell; there will be no net movement of water into or out of the cell. Key Terms osmolarity : The osmotic concentration of a solution, normally expressed as osmoles of solute per litre of solution. For example, it has been estimated that an amount of water equivalent to roughly times the volume of the cell diffuses across the red blood cell membrane every second ; the cell doesn't lose or gain water because equal amounts go in and out.
There are, however, many cases in which net flow of water occurs across cell membranes and sheets of cells. An example of great importance to you is the secretion of and absorption of water in your small intestine. In such situations, water still moves across membranes by simple diffusion, but the process is important enough to warrant a distinct name - osmosis.
Osmosis is the net movement of water across a selectively permeable membrane driven by a difference in solute concentrations on the two sides of the membrane. A selectively permiable membrane is one that allows unrestricted passage of water, but not solute molecules or ions. Different concentrations of solute molecules leads to different concentrations of free water molecules on either side of the membrane. On the side of the membrane with higher free water concentration i.
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