Osmotic Concentrations

Solutions can be described by comparing their water potential with that of a cell:
• Isotonic: the solution has the same water potential as the cell, so there is no net movement of water.
• Hypotonic: the solution has a higher water potential (it is more dilute) than the cell, so water enters the cell.
• Hypertonic: the solution has a lower water potential (it is more concentrated) than the cell, so water leaves the cell.These terms describe the solution relative to a particular cell, so the same solution can be hypotonic to one cell and hypertonic to another.The water potential of a plant tissue can be found experimentally. Equal cylinders of potato are blotted, weighed and placed in a range of sucrose solutions, for example 0.0 to 1.0 mol dm⁻³. After some hours they are blotted and reweighed, and the percentage change in mass is calculated:% change = (final mass − initial mass) ÷ initial mass × 100Worked example: a cylinder of 2.50 g has a mass of 2.80 g at the end. Change = 0.30 g, so % change = 0.30 ÷ 2.50 × 100 = +12%. It gained water, so the solution was hypotonic to the tissue.Percentage change is plotted against sucrose concentration. Cylinders gain mass in dilute solutions and lose mass in concentrated ones.Where the line crosses zero change, the solution is isotonic with the tissue. If this happens at 0.30 mol dm⁻³, the tissue has the same water potential as 0.30 mol dm⁻³ sucrose solution, which can be read from a table of values.