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
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- 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
| Solution | Alternative description | Direction of osmosis |
| High water concentration |
Low solute concentration Dilute solution E.g. the highest possible water concentration is found in pure water |
Water will move out of a solution with a higher water concentration into a solution with a lower water concentration |
| Low water concentration |
High solute concentration e.g. a concentrated sugar solution |
Water will move into a solution with lower water concentration from a more dilute solution |
Examiner Tip
Osmosis is a topic that causes a great deal of confusion, so remember the following essential points:
- Osmosis only refers to the movement of water molecules
- When describing the direction of water movement during osmosis you MUST be clear about whether you are referring to the water concentration or the solute concentration of a solution; just stating 'water moves from high to low concentration' will not be good enough
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:
- Prepare sugar solutions at a range of different solute concentrations
- Use a cork borer to prepare a series of potato chips of the same length
- Weigh each potato chip and record the mass
- Place each potato chip into a solution at a different solute concentration and leave for a set period of time, e.g. 30 minutes
- Remove the potato chips and dry them using a paper towel
- Weigh each chip again and record the new mass
- 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
