The Process of Osmosis (WJEC GCSE Biology)

Osmosis

• Osmosis is a special type of diffusion that involves the movement of water molecules

• Osmosis can be defined as:

The movement of water from an area of high water concentration to an area of low water concentration through a selectively permeable membrane

• Note that a partially permeable cell membranes can also be described as selectively permeable

Osmosis diagram

Osmosis occurs when two solutions are separated by a partially permeable membrane

Effect of solution concentration on osmosis table

Investigating osmosis in living material

• It is possible to study osmosis by investigating the effect of solute concentration on osmosis in plant tissue

• The procedure is as follows:

  1. Prepare sugar solutions at a range of different solute concentrations

  2. Use a cork borer to prepare a series of potato chips of the same length

  3. Weigh each potato chip and record the mass

  4. Place each potato chip into a solution at a different solute concentration and leave for a set period of time, e.g. 30 minutes

  5. Remove the potato chips and dry them using a paper towel

  6. Weigh each chip again and record the new mass

  7. Calculate the change in mass of each chip

Variables

• The independent variable is the solute concentration of the solution, e.g. in mol dm³

• The dependent variable is the change in mass of the potato chips

• Important control variables include:

  • Type and volume of solute solution

  • Temperature

  • Time for which chips are left in the solution

  • The surface area of the potato chips

Osmosis practical diagram

Osmosis in living material can be investigated practically to obtain quantitative results that can be analysed

Analysing results

• Calculate the percentage change in mass of each potato chip, remembering to indicate whether the mass increases or decreases

• Plot percentage change in mass against sugar concentration on a graph

A positive percentage change in mass indicates that the potato has gained water by osmosis, meaning the solution is more dilute than the potato cell contents

A negative percentage change suggests the opposite

• If the plant tissue gains mass:

  • Water must have moved into the plant tissue from the solution surrounding it by osmosis

  • The solution surrounding the tissue has a higher water concentration than the contents of the plant cells

• If plant tissue loses mass:

  • Water must have moved out of the plant tissue into the solution surrounding it by osmosis

  • The solution surrounding the plant tissue has a lower water concentration than the contents of the plant cells

• If there is no overall change in mass:

  • There has been no net movement of water molecules

  • The water concentration in the plant tissue and the surrounding solution must be equal

  • The point on the graph at which the line of best fit crosses the x axis can be used to determine the solute concentration of the plant tissue cells