Osmosis: Required practical activity 2: investigate the effect of a range of concentrations of salt or sugar solutions on the mass of plant tissue.

The idea

In this required practical, cut equal-sized pieces of plant tissue such as potato or beetroot, blot them, and record initial masses. Place one piece in each of several salt or sugar solutions of different concentrations, plus a distilled water control, for a set time. Remove, blot dry and reweigh. Calculate change in mass for each piece. In dilute solutions, water enters cells by osmosis, so mass increases; in concentrated solutions, water leaves, so mass decreases. The concentration giving no change is approximately isotonic with the tissue. Control variables include tissue type, size, temperature and time. Plot change in mass against concentration and interpret the graph.

What earns the marks

• Use plant tissue pieces of the same size, shape and type, and blot them dry before weighing to remove surface water.
• Prepare a range of salt or sugar concentrations, including distilled water as a control, and measure each concentration accurately.
• Keep temperature, soaking time and volume of solution the same for every piece to make the test fair.
• Record initial and final masses accurately, then calculate change in mass for each piece.
• Explain results using osmosis: water moves into cells in dilute solutions and out of cells in concentrated solutions.
• Identify the concentration at which change in mass is zero as approximately isotonic to the plant tissue.

Common mistakes and how to put them right

• Using different-sized tissue pieces.

  Put it right: Keep size and shape constant so mass changes can be compared fairly.

• Not blotting the tissue before weighing.

  Put it right: Surface water adds mass, so blot each piece before both weighings.

• Leaving pieces in solution for different times.

  Put it right: Fix the soaking time for all pieces so the comparison is valid.

Exam tips

• State the independent variable (concentration), dependent variable (change in mass) and at least two control variables when describing the method.
• Calculate percentage change in mass if asked, using (final mass − initial mass) ÷ initial mass × 100, and include the unit %.
• When explaining results, link each direction of mass change to net water movement by osmosis across the cell membrane.