Vacuoles are the largest organelles in most mature plant cells. Frequently constituting more than 90 percent of the volume of a cell, the vacuole presses the rest of the protoplasm against the cell wall. Vacuoles are surrounded by a single fragile membrane called the vacuolar membrane, or tonoplast.
The contents of the vacuole, referred to as vacuolar sap, is 90 to 98 percent water. The vacuole of a typical plant cell occupies approximately 500,000 cubic micrometers. It would take approximately two million of these vacuoles to equal the volume of a sugar cube.
Almost all plant cells contain vacuoles. Not all mature cells of plants, however, contain a single vacuole. For example, the cells of the tissue that produces the wood and bark of trees contain many small vacuoles during winter, when the tissue is dormant.When the tissue becomes active in spring and summer, these small vacuoles fuse into single, large vacuoles.
Vacuoles were discovered in 1835 and were thought to have relatively little function. It is now known that vacuoles are versatile organelles that have many important functions, including that of storage reservoirs. Vacuoles store waste products that would be dangerous if they accumulated in the cell’s cytoplasm.
Many of thesewaste products, such as nicotine, other alkaloids, and cyanide-containing compounds, are poisons that help protect the plant against predators. Vacuoles are also temporary, controlled repositories for useful materials such as potassium, chloride, and calciumions.
Ions such as sodium (Na+) and chloride (Cl–) are moved across the tonoplast by active transport, an energy-dependent means of transport in cells. These ions are important for cellular metabolism. For example, absorption or release of calcium from vacuoles regulates calcium-dependent enzymes in the cell’s cytoplasm.
Vacuoles store many economically important products. For example, proteins are stockpiled in vacuoles of storage cells in seeds. Latex is stored in vacuoles of rubber plants. Vacuoles ofmany plants store large amounts of amino acids, which are used as a reservoir of nitrogen.
Vacuoles of beet roots and sugarcane store large amounts of sugar. Large amounts of salt are also accumulated in vacuoles. The sap inmost vacuoles has concentrations of salts similar to that of seawater.
In marine algae and plants that grow in the salty soils of deserts and ocean shores, vacuoles often accumulate salts, such as potassium chloride and sodium chloride, to levels several thousand times greater than that of the soil or brackish water in which the plants grow.
Sometimes the concentration of a particular salt in the vacuole is so great that it precipitates as crystals. For example, calcium oxalate crystals are common in vacuoles of many plants such as dumb cane (Dieffenbachia), which has toxic levels.
Organic acids such as oxalic acid and malic acid are also accumulated. These acids make vacuoles slightly acidic. For example, the typical pH of a vacuole is near 5.5, while that of the cytoplasm is near 7.5. (A pH of 7.0 is neutral.)
The vacuoles in citrus fruit contain large amounts of citric acid. Consequently, these vacuoles are very acidic, thus accounting for the tart, sour taste of the fruit.
When vacuoles absorb salts, they also absorb water. This water swells the vacuole, much as air inflates a tire. The water entering the vacuole creates a pressure inside the vacuole called turgor pressure and presses the surrounding layer of cytoplasm against the edge of the cell.
Turgor pressure is what makes non woody plant tissue firm. When the vacuole loses water, the turgor pressure is lost, and the tissue wilts. Thus, leaves of plants that lack water wilt, while those of well-watered plants remain firm.
The turgor pressure generated in vacuoles is important for cell growth because it stretches the cell wall of the plant. During cell growth, cells secrete protons into their cell walls. These protons weaken chemical bonds in the cell wall and can stretch it to a larger size. Plant hormones, such as auxins, control the secretion of protons.
This type of pressure driven growth by plant cells is energetically “inexpensive,” because it involves little more than absorbing water. This contrasts sharply with the growth of animal cells,which lack vacuoles. Animal cells must expand by making energy-rich cytoplasm, including large amounts of proteins and lipids.
Plant Movement and Gas Exchange
Cells in specialized regions of the leaves quickly transport salts out of their cells. When they do, water from the cells’ vacuoles also leaves the cells. This “deflates” the cells, and the tissue shrinks, thus moving the leaf.
Gas exchange in the leaves is also influenced by vacuoles. Pores through which gases enter and exit leaves are called stomata, and they are bordered by specialized cells called guard cells.
When the vacuoles of these cells absorb water, the cells become turgid and bow apart, thereby creating a pore through which gases move.
Thus, water uptake by vacuoles of guard cells correlates with stomatal opening and gas exchange. When water leaves the vacuoles of guard cells, the cells wilt and the pore closes, which stops gas exchange.
Gas exchange is crucial because it brings carbon dioxide into the leaf for photosynthesis and releases oxygen into the atmosphere. Many factors control water absorption by guard cells, including light, wind, temperature, and water availability.
Vacuoles function as digestive centers of cells: They contain a variety of digestive enzymes, such as phosphatases and esterases, that can degrade (break down) many different kinds of molecules.
Vacuoles use these enzymes to degrade and recycle the parts of damaged or old, unneeded organelles. Small vacuoles fuse with old or damaged organelles and, by means of enzymes, digest the organelles. The parts of the digested organelle are then recycled by the cell.
Pigment Holders and Pumps
Many cells have vacuoles that contain water-soluble pigments called anthocyanins. These pigments are responsible for the red and blue colors of many vegetables (turnips, radishes, and cabbages), fruits (cherries, plums, and grapes), and flowers (geraniums, roses, delphiniums, peonies, and corn flowers).
Anthocyanins help attract pollinating insects to the flowers. Sometimes these pigments are so bright that they mask the chlorophyll, as in the ornamental red maple. The red color of garden beets is caused by another vacuolar pigment, called betacyanin.
Many protozoa and unicellular algae contain specialized vacuoles called contractile vacuoles. These vacuoles pump excess water from cells. As a result of this secretion, pressure does not build inside the cells. Contractile vacuoles are rare in marine algae and are absent in terrestrial plants.
Growth, Development, and Transport Models
Vacuoles perform many functions that are critical to plant growth and development. For example, the cellular expansion that produces leaf movements and tropisms results largely from water uptake by vacuoles. Similarly, the absorption and loss of water by vacuoles of guard cells regulates photosynthesis, the process that fuels life on earth.
Vacuoles are increasingly used as tools for studying transport across membranes. Ions and sugars move quickly across the tonoplast; this movement makes isolated vacuoles a model system for studying transport of materials across membranes. The movement is controlled by specific proteins that transport each ion or sugar across the membrane.
Many economically important chemicals, including drugs, dyes, spices, and other materials, such as rubber, are contained in vacuoles. Biologists are trying to understand how plant cells make and package these chemicals in vacuoles.
Biotechnologists hope to use this knowledge to increase production of these chemicals. Far from being the inert structures they were once believed to be, vacuoles are critical to many aspects of plant life.