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Vacuoles

Vacuoles are organelles of plant, fungal, and algal cells. They are part of the internal membrane system and are separated from the rest of the cytoplasm by a membrane called the tonoplast. A single large vacuole occupies more than 80 percent of the volume of most plant cells, mature fugal hyphae, and some algal cells. Many smaller vacuoles are found in expanding plant cells and in the tips of growing fungal hyphae. These vacuoles can be less than one micrometer in diameter. As the cell in which they reside matures, smaller vacuoles fuse to produce larger vacuoles.

Vacuoles are multifunctional organelles, and individual cells may contain more than one kind of vacuole, each kind having a different function. Vacuoles play crucial roles in cell expansion, serve as storage compartments for nutrients, and function as lytic organelles that contain digestive enzymes. Compounds contained within vacuoles also protect cells against environmental damage and deter attack by herbivores.

Vacuoles take up water through specialized membrane transporters called aquaporins. The hydrostatic pressure that develops within each cell, known as turgor pressure, is required for cell expansion and growth. Turgor pressure is carefully regulated in plants, fungi, and many algae by controlling rates of water and ion movement through the tonoplast. In fresh water algae and fungi lacking cell walls, contractile vacuoles fill with excess water from the cytosol and their contents are expelled from the cell through specialized pores.

The vacuole is an acidic organelle, and the pH of most vacuoles is around 5 to 6. Vacuole acidity is important for its lytic function since many vacuo-lar enzymes work most efficiently at or near pH 5. Acidification of vacuoles is brought about by transporters embedded in the tonoplast. These transporters use the energy stored in adenosine triphosphate (ATP), or in some cases, pyrophosphate, to pump protons from the cytosol into the vacuole. In extreme cases, such as in the lemon fruit juice sac, the pH of the vacuole can be as low as 2.

Vacuoles store organic acids, carbohydrates, proteins, and minerals. Some of these compounds are important for human nutrition. These include proteins stored in the cotyledons of beans and peas or the grains of cereals; simple sugars such as sucrose found in many fruits, the stems of sugarcane and the roots of sugar beets; and minerals such as potassium. In the leaves and stems of forage grasses, vacuoles store complex polysaccha-rides that are the principal energy source for herbivores.

Many other compounds accumulate in vacuoles. These include the water-soluble anthocyanin pigments that give the blue or red color to red beets, grapes, and peonies. Anthocyanins are also contained in the vacuoles of leaves and stems and are important photoprotectants that absorb excess light. Alkaloids, enzyme inhibitors, and toxins are contained in some vacuoles. Although these compounds may deter herbivory, some have been used to produce medicines. Aspirin and morphine are two examples. Waste products and xenobiotics, including herbicides, are often shuttled into vacuoles by specialized membrane transporters. Once in the vacuole, these compounds are digested or detoxified.