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Mendel and others looking down their microscopes at plant cells could perhaps be forgiven for thinking the cells were empty. A first glance, most of a plant cells seems nothing but hollow space, but this is an illusion.

Filling this space is an organelle called a central vacuole which is full of water. Bounded by a single membrane, this organelle functions as a combination of reservoir, waste dump, storage region and even as a means of keeping the cell in shape. Filled tight with water, the vacuole pushes the cytoplasm into a thin strip adjacent to the membrane and pushes outwards like a water filled balloon. It is this turgor pressure that holds the cell firm and provides the characteristic shape of plant structures such as leaves. When a plant has been without water for a long time, the central vacuoles lose water, the cells lose shape, and the whole leaf wilts.

Plants often store sugars, ions, some proteins and occasionally pigments inside the vacuole. Flower petal cells, for example, get their characteristic color from the pigments made and deposited in the central vacuole. Poisonous materials, that would harm the cytoplasm, are excreted into the vacuole, and these reservoirs of poisons also make the plant unattractive to herbivorous animals.

In free living single celled animals and plants such as Amoeba or Euglena that inhabit fresh water, osmosis is a big problem. These cells constantly take up water across the semipermiable membrane and if this process went on indefinitely, the cell would burst. Instead, these cells collect the excess water into a contractile vacuole, which, as its name suggests, can contract. When full of water the vacuole connects with the outside and pumps, forcing the water out of the cell.

Science@a Distance
© 2002, Professor John Blamire