To investigate the effect of variation in temperature on the permeability of cell membranes using fresh beetroot
In this investigation, you will subject fresh, washed beetroot discs to different temperatures to investigate the effect of temperature on the permeability of the plasma membrane.
The beetroot discs will be placed in distilled water at different temperatures. You will compare the intensity of the beetroot pigment that has leaked out from the cells subjectively and possibly objectively using a colorimeter. You can relate the findings to the effect of different temperatures on the permeability of cell membranes in beetroot (as well as in other organisms).
Features of experimental design
The investigation contains these main features of experimental design:
■ There is one independent variable (IV) with a good range. The IV is temperature.
■ The dependent variable (DV) is what is observed or measured as the IV changes. The DV is measured quantitatively and is the amount of beetroot pigment that leaks from the discs.
■ There are many controlled variables (CVs) or constants. They allow a fair test so that we can link the changes in the DV to changes in the IV more reliably.
■ There will be replication through three trials at each value of the IV.
■ There is a control in order to check that the procedure itself does not generate pigment. Not all factors can be controlled, especially the biological material (a confounding variable).
Fresh beetroot tissue is used in this experiment because it contains a dark red pigment within the vacuole of its cells. The dark red pigment is betacyanin. There is also a yellow pigment called betaxanthin in the vacuole. The pigments are retained in the vacuole by a selectively permeable membrane called the tonoplast. It has a similar fluid-mosaic structure to that of the plasma membrane with protein molecules embedded in a phospholipid bilayer. If these two membranes are damaged, betacyanin can leak out of the cells. Normally it does not.
Figure 1 A diagram of a section of a beetroot cell (a parenchyma cell)
■ Wear eye protection
■ Take care when you use a cork borer, glass rod and scalpel to obtain beetroot discs
■ Do not touch hot objects
Equipment and materials
|Eye protection |Test tube rack and test tube holder | |Fresh beetroot, enough for 90 mm of discs |Forceps | |Number 5 cork borer (8 mm diameter) |Petri dish | |Glass rod |5 cm3 pipette and pipette filler | |Paper towels |400 cm3 beaker to be used as a water bath | |Tile |Thermometer | |Scalpel |Heating equipment: Bunsen burner, tripod, gauze and bench mat | |Ruler |Stop clock | |Boiling tube and bung |Colorimeter and cuvettes (or use a subjective scale) | |Distilled water wash bottle | | |Small beaker for distilled water, waste | | |Labelling equipment...
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