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Popping giant cells

This short practical investigates the cells present within the bell pepper pericarp tissues, and the importance of cell turgor to plants.
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SPEAKER: This short practical investigates the cells present within the bell pepper pericarp tissue, and the importance of the cell turgor to plants. The pericarp is the flesh of the pepper. The practical aims to reinforce the importance of the movement of water into and out of cells, by osmosis, to the survival of plants. You can use any colour of bell pepper, though you may find that the cells are less pronounced in green peppers. You will need a bell pepper, a hand lens or microscope, and a mounted needle. If a pepper is sliced in half, specialised plant cells called giant cells, can be seen on the inside of the pepper. They are so big that they can be seen without a microscope.
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If the inner surface of the pericarp is examined using a hand lens or a binocular microscope, the giant cells can be seen as long, thin structures up to five millimetres long. If the point of a mounted needle is gently pushed against one of the giant cells, the cell will burst and cell contents will exude from the inside of the cell. This is a very simple and elegant demonstration of the fact that these giant cells are turgid and full of water. The giant cells of the pepper are thought to play an important part in the mobilisation and storage of water within the pericarp, or fruit tissue.
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Their ability to undergo large changes in cell turgor is important for the survival of the fruit, and hence seeds of the species. They also help to produce the succulence, which makes them such an attractive food source. This investigation provides firsthand experience of turgid plant cells, and reinforces the concept of movement of water into and out of cells by osmosis, and the importance of osmosis in maintaining turgor pressure. This, in turn, is important for cell shape and juiciness. Traditionally, sections of potato are used to investigate osmosis. But students can find it conceptually difficult to think of pieces of potato being made up of cells that are all taking up or losing water by osmosis.
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Looking at the giant cells in pieces of pepper before measuring mass change over time in different sugar solutions, helps students to think about water movement in and out of cells themselves, as opposed to water being taken up by a sponge-like piece of tissue.
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.

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.
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