A single water sac can fill 80 percent of a plant cell
Most mature plant cells are dominated by one large vacuole, usually more than 30 percent of the cell's volume and sometimes as much as 80. Pumped full of water, it keeps leaves and flowers standing and lets young plants grow fast while spending little beyond water. Early microscopists mistook related structures in protozoa for breathing organs.
A vacuole is a membrane-enclosed compartment filled mostly with water and dissolved molecules, including enzymes, and occasionally engulfed solids. It forms when many small vesicles merge, and it has no fixed shape or size. Vacuoles occur in plants and fungi and in some protists, animals and bacteria, and the structure probably evolved several separate times, even within the green plant lineage.
The history runs back centuries. Antonie van Leeuwenhoek described the plant vacuole in 1676. Lazzaro Spallanzani spotted pulsing contractile vacuoles in protozoa in 1776, taking them for respiratory organs, and Felix Dujardin named these 'stars' vacuoles in 1841. Matthias Schleiden extended the word to plant cells in 1842, and in 1885 Hugo de Vries named the surrounding membrane the tonoplast, from Greek roots for tension and moulded. Christian de Duve's lysosomes, named from Greek words for digestion and body, share so many features with vacuoles that separating the two is now more a matter of history than function.
Plant vacuoles do a great deal. They isolate harmful materials, maintain turgor pressure, export unwanted substances and, in seeds, store proteins for germination inside modified vacuoles called protein bodies. The tonoplast regulates the movement of ions. Vacuoles also drive autophagy, balancing the building and breakdown of cell components and recycling misfolded proteins. Thomas Boller and colleagues proposed they help destroy invading bacteria, while Robert Mellor suggested some specialised forms house symbiotic bacteria. In protists they store and digest food.
Animal cells have smaller but usually more numerous vacuoles, and some have none. Their roles are supporting ones, in export and uptake. During phagocytosis, 'cell eating', the membrane folds around bacteria or debris and pinches off a vacuole; pinocytosis, 'cell drinking', does the same with dissolved substances, and lysosomes finish the breakdown. Salmonella exploits this route, surviving and multiplying inside vacuoles of several mammal species.
Source: Vacuole
