This practical involves students looking closely at the cells that make up a raw potato using microscopes and then comparing with the cells of a cooked potato.
Technician preparation may be required before the lesson as the potatoes have to be sliced very thinly in order for the student to see the cells clearly. The problem with this is that the slices may dry out very quickly and need to be moist for the lesson. If preparing the slices in advance, keep them soaking in a bowl of water.
If you are required to prepare the slices of potato then try to do so just before the lesson. Slices need to be less that 1mm think if possible. This may prove difficult unless a very sharp scalpel is used.
Equipment required (per set):
Students take a slice of the raw potato, place it on a slide and add a drop of iodine to stain it. A cover slip is placed over the slice and it is observed through a microscope.
For the cooked sample of potato, students boil it for about 10 minutes and repeat the steps above.
- potato slices (pre sliced or potato and scalpel)
- microscope slide and cover slips
- 250ml beaker
- bunsen burner
- heatproof mat
- iodine solution (0.01mol) (low hazard)
Students should be able to see the differences in cell shape between the raw and cooked samples. As the potato cooks, the membrane around the starch grain bursts and the starch grain swells, the cell membrane bursts and the vacuole membrane bursts. The cell wall breaks down allowing starch to leak out.
The iodine staining should bring out the features of these cells.
Students can see without a microscope that the longer the potato sample is cooked for, the 'squishier' it becomes. This is caused by the cell walls breaking down. In the raw sample, these cell walls hold the shape of the potato and are much stronger.
A microscope or 'flex' camera may be used in this lesson to display the cells on a TV or projection. Blowing up the cells this way may make their structures easier to see. You may also be able to adjust the contrast and colours this way making the image stand out more.