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.
Visking Tubing
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.
Membrane Channels
This is a brilliant simulation although it does need some setting up. Firstly, Java needs to be installed on the computers being used. The simulation can though be downloaded and used off-line.
It looks at the movement across a membrane through membrane channels. In this way it is a model for diffusion across a semi-permeable membrane.
Open the simulation and drag some green leakage channels into the cell membrane. These will allow green circles through but will not allow blue diamond molecules to pass through. Describe this as being the same as the semi-permeable cell membrane or the Visking tubing used in the previous experiment.
Introduce green molecules above the membrane by selecting them and pressing the large red button on the dispenser. Note how they cross through the channels to the other side of the membrane.
Select the blue diamond molecules and introduce them above the membrane. Note how these are unable to pass through the membrane.
This is a powerful simulation. Set students the task of investigating the movement of molecules across a membrane.
• Explain how the simulation relates to movement in and out of a cell.
• What is the effect of differing concentrations of molecules and rate at which they get from one side of the membrane to the other?
• How does the number of channels (permeability) of the membrane effect movement?
• Do molecules move only in one direction through the channels or do they move back-and-forth? What is the overall effect? How does this relate to the definition of diffusion.
• What happens if molecules are introduced on both sides of the membrane?
Water Potential During Ripening and Storage
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.
Gaseous exchange in the lungs
This video looks at gas exchange and it can be used to illustrate an instance where diffusion takes place in the body. Students do not need to have prior knowledge of the lungs and breathing but if they do, it will also serve as a reminder of this topic.
Near the start of the video, the presenters talk about the concentration of carbon dioxide and oxygen in air inhaled and exhaled. Draw students’ attention to the difference. This will be needed when thinking about how diffusion happens at the alveoli.
It is worth stressing that there is some carbon dioxide in inhaled air and also oxygen in exhaled air. This is to avoid giving the impression that we breath in oxygen and breath out carbon dioxide.
You may also remind students that nitrogen makes up around 78% of air.
The animation of the alveolus is slightly confusing as the deoxygenated blood is represented as red and the oxygenated blood as blue. Pause the video and point this out.
Remind students about the concentrations of gases in the inhaled and exhaled air that they noted earlier in the video.
If students consider the carbon dioxide first. Where is it at its highest concentration (in the deoxygenated blood) and where is it at its lowest (the air space in the alveolus). Which direction should the carbon dioxide diffuse?
Repeat this line of questioning with the oxygen before continuing the video to see what happens.
The video goes on to look at surface area. Stress this is an important feature of areas where exchange happens. Ask students for any other examples where they have come across a large surface area being important.
Diffusion, osmosis and active transport
This is an animation showing active transport, diffusion and osmosis. It can be found by scrolling to the bottom of the page.
Active transport can be looked at first by reminding students that diffusion sees molecules move down a concentrations gradient. Suggest that there are times when cells need to move molecules up a concentration gradient.
What is moving up a gradient likely to need? The process is called active transport as it requires energy.
The animation can be used to point out how the transport protein carries the molecule into the cell where there is already a high concentration present. Stress that this carrier protein needs energy to do this. Without energy, the process will stop. Students can be reminded about the process of cellular respiration and that this is the process that provides the energy for active transport.
The processes of diffusion, active transport and osmosis can be summarised by having students produce a revision table that contains the similarities and differences of each process. This page is also useful in reminding students of the key features when they construct their table.