Skip to 0 minutes and 6 secondsBEVERLEY GOODGER: This investigation is a version of the well-used investigation into osmosis in potato tissue, where the change in mass of pieces of potato tissue is measured after they have been placed in different molarity sucrose solutions. To carry out the investigation, each student or group
Skip to 0 minutes and 24 secondsof students needs: quarter of a bell pepper; though they could make do with less if cut carefully; 50 cubic centimetres of one molar sucrose solution; 75 cubic centimetres of distilled water; test tubes-- here, we're using five; two 10 cubic centimetre graduated pipettes and a pipette filler; a cutting tile; kitchen knife or scalpel; Petri dish with a lid; paper towels; laminated graph paper; blunt forceps. They also need access to a two decimal place balance. Students should have access to or be able to make up a series of different molarity sucrose solutions. Each tube needs five cubic centimetres of sucrose solution in it. The pieces of pepper can then be prepared.
Skip to 1 minute and 15 secondsPlace a piece of laminated graph paper onto the cutting tile to provide a cutting grid. Orientate the pepper so that the giant cells are running from the top of the paper towards the bottom. Avoiding sections of white pith, cut a strip of pepper into pieces one centimetre long and 0.5 centimetres wide. Place the pieces into a Petri dish and cover with a lid until ready to record their masses. This reduces any drying out of the tissue. Quickly rinse the pieces in distilled water and gently, yet thoroughly, pat dry with a paper towel just before weighing.
Skip to 2 minutes and 3 secondsOnce a piece of pepper has been weighed and its mass recorded in a results table, place it onto a paper grid where it's mass and the molarity of the sucrose solution that is going to be placed in can be recorded. This helps to keep track of which piece of pepper is which. When all the masses have been recorded, place each piece of pepper into the correct test tube of sucrose solution and leave for 30 minutes.
Skip to 2 minutes and 30 secondsAfter 30 minutes, remove the pepper pieces from the sucrose solution, pat dry, and reweigh. Record the data. Students can now calculate the percentage change in mass for each piece of pepper. They can also observe whether they can feel any difference in the texture of the pieces, which could be used to guide them towards an explanation of the results of their investigation. The students can then plot a graph showing molarity of sucrose solution versus percentage change in mass of the pepper tissue.
Taking osmosis practical work a step further
This video will introduce you to a version of the classic osmosis practical, measuring the change in mass of plant tissues in solutions with a range of concentrations, using pieces of bell pepper (capsicum annum) instead of potato.
By using bell pepper tissue with its giant cells that are visible to the naked eye, and by carrying out the cell-popping investigation (seen in the last step) which introduces them to the idea of the turgor pressure of cells, students may be more likely to envisage the movement of water in and out of the cells in the pepper tissue by osmosis, allowing them to apply their knowledge when interpreting their experimental data.
This experiment has a significant pause in the lesson, as students wait for thirty minutes to allow osmosis to occur. With this in mind, how would you structure this practical lesson in terms of when you would cover theory and when you would require students to undertake practical tasks?
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