Popping giant cells

Traditionally, sections of potato are used to investigate osmosis. Potato tissue is familiar to all students, but some students will struggle to envisage the potato tissue as cellular and this could be a barrier to learning.

Rather than being able to explain the results of their investigation in terms of the net movement of water molecules into or out of the cells of potato tissue by osmosis, some students may envisage the potato soaking up the sucrose solution like a sponge and those students would be completely baffled as to why some potato cylinders gained mass while others lost mass.

Looking at the giant cells in pieces of pepper, before measuring mass change over time of sections of pepper in different sugar solutions, helps students to think about water movement in and out of the cells themselves as opposed to water being taken up by a “sponge-like” piece of tissue.

In this video you’ll see a very simple investigation that allows students to observe the giant cells found on the inside of a pepper when it is sliced open. The giant cells can be popped with a mounted needle, showing that the cell contains a lot of liquid and the cell contents is under pressure.

• SAPS protocol: osmosis in bell pepper pericarp

Comment

This practical helps to make an abstract idea more concrete, by letting students see that cells contain water. Can you think of any other practicals in biology which help students to visualise an abstract concept.