Osmosis, diffusion and active transport
The movement of molecules is quite abstract and difficult to imagine. This list gives a range of practical demonstrations and investigatiosn for students to carry out.
It is worth remembering that activities need to help students develop an understanding of the processes. The analysis, interpretation and discussion of observations should not be rushed. Time for reflection and checking that learning has taken place should be built into the topic.
These are fundamental concepts in biology and can be revisited and reinforced when they are encountered in other topics.
Whilst this list provides a source of information and ideas for experimental work, it is important to note that recommendations can date very quickly. Do NOT follow suggestions which conflict with current advice from CLEAPSS, SSERC or recent safety guides. eLibrary users are responsible for ensuring that any activity, including practical work, which they carry out is consistent with current regulations related to Health and Safety and that they carry an appropriate risk assessment. Further information is provided in our Health and Safety guidance.
Links and Resources
This is a chemistry practical but it serves to illustrate diffusion in a liquid.
After setting up this experiment, the demonstration below can be done which shows diffusion in a gas.
The diffusion takes place in water. How does this relate to diffusion within living organisms and indeed inside living cells? Stress that much of living things is made up of a watery medium and so this type of diffusion is the same as would happen in living organisms.
In a biology lesson, students need not write an equation for the reaction, as the worksheet requests.
Students could draw a diagram of their petri dish and label areas of high concentration and areas of low concentration. Students should realise that the concentration and movement of the potassium iodide and lead nitrate are considered separately.
Students can then be challenged to come up with the definition of diffusion as movement from high to low concentration. Introducing the term ‘concentration gradient’ may help as students can identify with the idea of something moving down a ‘slope’.
How could students measure the rate of diffusion? The relative speed in a liquid is slower than in a gas.
Have students suggest situations in biology where such diffusion may take place. For example, dissolved oxygen and carbon dioxide in capillaries.
The demonstration can be set up so that it can be viewed whilst students are waiting for the results of their investigation into diffusion in a liquid. It is a chemistry demonstration but equally applicable to biology.
Make sure that the demonstration is in a well-ventilated room or fume cupboard.
Explain that the molecules will react together to form a cloudy ammonium chloride but they must first come together from each end of the tube. The glass tube just prevents air currents from mixing the vapours.
Questions can be used to elucidate the idea of a high concentration of each molecule close to the cotton wool and a lower concentration away from the cotton wool. Both molecules should be considered separately. What does this tell us about the direction of diffusion? The term ‘concentration gradient’ is useful – students can identify with the idea of something moving down a ‘slope’.
How could the speed of diffusion be measured? What would eventually happen if the tube was left for a long period of time?
Discuss how diffusion in gases is very important in biological systems. For example, gas exchange in the lungs and stomata.
This video is for teachers and shows how to set up an experiment in which Visking tubing acts as a model gut. It illustrates diffusion and the action of a semi-permeable, partially permeable, or differentially permeable membrane. Take care to note what term your specifications use.
When setting up the experiment it is not necessary to be particularly accurate with measuring concentrations of glucose and starch solutions. Solutions can readily be put into the Visking tubing using a pipette.
In the video, the Visking tubing is kept open using a cut-off syringe barrel. This aids in access to the internal contents of the tube when being sampled. If this is not required, it is just as effective to take a sample from inside the tubing at the start of the experiment and then tie off the top.
Instead of using Benedict’s reagent, the presence of glucose can be indicated using glucose test strips.
Students can draw diagrams to illustrate why glucose can escape the model gut whilst starch is retained within the gut. Relate this to the digestion of starch in the diet. Remind students that the cell surface membrane is also semi-permeable and will act in the same manner to control substances getting into and out of cells.
Students can be challenged to assess the validity of using Visking tubing to model absorption in the small intestine. It could also be used to investigate factors such as the effect of temperature, or initial solute concentrations on the speed of diffusion.
This activity is designed for post-16 students. However, the first practical described is suitable for 14-16 students.
This practical sees cylinders of a vegetable (potato is the easiest to use) placed in different sucrose solutions. Depending on the concentration of the solution, the potato cylinder either gains or loses weight due to the movement of water in or out of the potato cells.
It is best to calculate the % change in weight for each potato cylinder. Plot this data on a graph. Loss in weight (negative change) is below the x-axis and a gain in weight above it.
Similar results can be obtained by measuring the change in length of the potato cylinder. This is then related to the cells either shrinking or expanding depending on water movements.
Students may need to be prompted to realise that any change in weight is due to the movement of water in or out of cells. Looking at the graph, challenge students to think about the overall direction of water movement in relation to the concentration of the sugar solution. Notes can be added to the graph. What would they suggest is happening when there is overall no change in weight (where the line crosses the x-axis)?
Using these observations, it is possible to challenge students to come up with a definition of osmosis. You may need to prompt them into including that osmosis requires the presence of a differentially permeable membrane.
Students tend to have a good understanding of the idea of a concentrated or a weak solution. Only once students have understood the process, consider introducing terms such as water potential, that may be required by your specification.