Practical Work for Learning: Model-based inquiry
Using a ‘pot model’ to represent osmosis
Teacher guidance
This lesson is designed to exemplify a model-based inquiry approach to practical work, in which students use and evaluate a model for osmosis.
Component resources: Teacher guidance, including lesson plan Student sheet and assessment item PowerPoint
Overview: Osmosis is an important process which plants use to take up water. A model can be used to help explain how this process works.
Summary of lesson: During the lesson students will be introduced to the model of osmosis make a ‘pot model’ using beans of different sizes to represent solvent and solute molecules, and a net to represent the cell membrane use the model to predict and explain what happens when they place potato pieces into a concentrated sucrose solution and distilled water, measuring the change in mass
The lesson follows this basic structure: Share the learning outcomes with students Set the context Revise relevant prior learning Define the key scientific terms Construct a ‘pot model’ Make a prediction and explain reasoning Test the prediction by manipulating variables and collecting data Analyse data Explain the data using the model Evaluate the fit of the model with the data Assess and evaluate learning
Age range: 14-16
Timing: 50 mins
Curriculum links: Osmosis is a key concept in GCSE Additional Science.
Prior knowledge: Students will already know: all matter consists of molecules or atoms which are constantly moving the random motion of atoms or molecules in liquids means that they spread from areas of high concentration to areas of low concentration plants need water to grow, and they get this water from the soil Practical Work for Learning: Model-based inquiry
cells are surrounded by a partially-permeable cell membrane. This allows some chemicals through, but is a barrier to other chemicals. The plant cell wall is freely permeable.
Learning outcomes: Students will be able to: explain the overall movement of water into and out of plant cells construct and apply a model of osmosis
Link to practical on Practical Biology / Chemistry / Physics: http://www.nuffieldfoundation.org/practical-biology/investigating-effect-concentration-blackcurrant- squash-osmosis-chipped-potatoes
Background information: The model of osmosis can be defined as follows:
When solutes dissolve in water, weak bonds form between the water and solute. For this reason, water molecules in a solution are less free to move across a partially permeable compared with water molecules in pure water.
Osmosis is the result of molecules colliding with pores in the membrane, water molecules going through, some solute molecules not.
Osmosis is the overall movement of water by diffusion through a partially-permeable membrane, from a solution of lower concentration to a solution of higher concentration of dissolved solutes.
Scientific terms: The scientific terms which are needed to understand and use this model are: atom – the smallest particle of a defined element molecule– a group of two or more atoms held together by covalent bonds partially-permeable membrane – a membrane which will only allow certain molecules (or ions) to pass through it by diffusion diffusion – the spread of particles through random motion from regions of higher concentration to regions of lower concentration solution – a mixture in which a solute (e.g. sucrose) is dissolved (forms weak bonds with) a solvent (e.g. water) concentration – the amount of solute (e.g. sucrose) in a certain volume of solvent (e.g. water) model - a simplified version of a theory which allows the theory to be discussed and used to solve problems. A model allows predictions to be made and tested through scientific inquiry.
Lesson outline:
Step Timing Details Resources Students will be able to: Share learning 2 min PowerPoint outcomes with explain the overall movement of water into presentation students and out of plant cells construct and apply a model of osmosis Practical Work for Learning: Model-based inquiry
Set the context 3 min Ask the question - when you water plants, the PowerPoint water goes in. How? presentation This is the phenomenon being studied.
Review 5 min Review plant cell structure so students understand PowerPoint relevant prior that this lesson concerns the passage of water presentation learning and solutes in and out of plant cells. Review a simple model of osmosis. This does not involve consideration of the pressure from plant cell walls or the reason why water passes from dilute to more concentrated solutions. Define the key scientific terms as part of this review. Emphasise how water passes in both directions across a membrane, so osmosis relates to the overall (net) flow over time.
Students 10 min Show students the presentation describing how PowerPoint construct and the model is made. presentation test a simple Students make their models. Equipment for model for making pot osmosis The student sheet asks them to sketch their models (alternatively findings and explain how the pot model represents this could be a osmosis. demonstration) The slide showing the simple model of osmosis can stimulate discussion about how well the pot model represents osmosis.
Refine the 5 mins Introduce the refined model for osmosis – then PowerPoint model of introduce the potato practical which can sit in presentation osmosis solutions while students make the revised pot model. The next two slides in the presentation introduce the idea that this pot model can’t account for the fact that water from a dilute solution is more likely to pass through a membrane than water from a concentrated solution. The more sophisticated diagram explains why this is. Ask for suggestions on how the pot model can be revised. Provide hints e.g. why did you need to turn the pots so they were equally on top and on the bottom while you shook? Lead to the idea that gravity can be used to represent the pressure (diffusion gradient) produced when solutions of different concentrations are separated by a partially permeable membrane. This pressure drives the overall flow of water in osmosis. Practical Work for Learning: Model-based inquiry
Set up the 20 min The potatoes can soak while students discuss and PowerPoint potato make their revised pot models. In this case the presentation practical and practical is set up before the prediction is made as make the students can be thinking while the potato pieces revised pot soak.
Analyse data 10 min If students only carried out the experiment with PowerPoint and explain one concentration, get pairs together into groups presentation using the of four so they can discuss both sets of data. model Discuss which potato sample has lost mass and which has gained mass. Link change in mass to net movement of water. Make sure students have engaged with the model to explain their results. e.g. The potato piece has shrivelled up/ reduced in mass. This is because water molecules moved from the more dilute solution in the potato cells into the more concentrated sucrose solution. The solute molecules and water molecules form weak bonds in a solution. This means there are less water molecules to move in and out of a cell when solute molecules are present.
Assess and For The questions ask students to transfer their evaluate homework understanding to a 2D diagram of osmosis. They learning / next are asked about how the model can be used to lesson predict outcomes for the potato experiment, and how it can be applied to plants taking up water from the soil.
Differentiation / optional extra activities: Be the molecules – students can be ‘water’ or ‘sucrose’ – differentiated by coloured bands or hats. When a sucrose molecule becomes surrounded by three or four water molecules forming a ring around them, they can’t get through the gaps in your semi-permeable membrane (made with chairs or other barriers). Unattached water molecules can get through the pores. Difficult to mimic random movement. Simple message: Sugar can’t cross the membrane, water can.
Taking it further: A full inquiry - Find the concentration equal to the concentration of the potato cells you have (or other plant tissue or red blood cells?)
Links to related practical activities on Practical biology / Chemistry / Physics: Investigating osmosis in chickens’ eggs Observing osmosis, plasmolysis and turgor in plant cells. A closer look at blood Practical Work for Learning: Model-based inquiry
Marking criteria:
Answers 1) sucrose solution inside potato cell
free water molecule
sucrose molecule surrounded by water molecules
cell membrane
overall movement of water by osmosis
Water moves in both directions across the partially permeable membranes of the potato cells.
The sucrose molecules can’t move across the membrane.
The overall movement of water is from the cells into the more concentrated solution.
2) The model shows that the potato will lose water, so the prediction is that it will lose mass. 3) Water in the soil is less concentrated than the solution inside plant cells. The overall movement of water by osmosis is into the plant from the soil water. 4) Encourage students to think about how the revised model allowed it to simulate solutions with different concentrations either side of the membrane.