4.12 Vacuoles

An organelle that is very prominent in all plant and fungal cells (but usually not in animal cells) is the vacuole (Figure 1b). Vacuoles can be very large and they have a number of roles. In mature plant cells, the vacuole is typically a membrane-bound space that fills most of the centre of the cell and is basically a storage compartment for water, ions and small organic molecules. In combination with the cell wall, vacuoles also have a role in determining the shape of the plant cell. A cell in which the vacuole is completely full of water is said to be turgid, and this cell turgidity ensures that the plant remains rigid and upright. In dry conditions, loss of cell turgidity results in plant wilting. Water molecules pass freely in and out of the vacuole, but the vacuolar membrane (the tonoplast) controls the passage of other molecules with specialised transporters that are different to those of the cell membrane. Vacuolar pH is lower than that of the cytosol, and vacuoles can contain enzymes that digest large organic molecules in a similar way to lysosomes in animal cells (lysosomes are rare in plant cells).