Friday, August 21, 2015

Vacuole



In plant cells, the vacuole occupies large space that is  more than 90% of the plant cell space. It is said that they are usually formed by the fusion of many membrane vesicles. Vacuole does not have any specific size or shape. Its structure is such that it complements its function. Many mature and grown plant cells usually have a single large vacuole, surrounded by a structure known as a tonoplast. This is said to be a very active and dynamic membrane of this all important part of plant cell structure.
The tonoplast is also called as the vacuolar membrane, and it is a cytoplasmic membrane that surrounds a vacuole, thus separating the vacuolar contents from the cytoplasm of the cell.
It is a lipid bilayer, typical of other cellular membranes.A bilayer indicates that two layers,  hydrophillic head facing out into the cellular cytosol and the vacuolar cytosol.The hydrophobic  tails face inwards. The tonoplast is a stretchy membrane, and its main function is to protect the vacuole and isolate it from harmful substances. The tonoplast membrane controls ionic movement in and around the cell. Water flowing in and out of the vacuole is regulated by the tonoplast. Some animal and bacterial cells also contain vacuoles that are bound by vacuolar membranes. The size and structure of the vacuole varies depending on the cell.The tonoplast contains numerous proteins that are responsible in the transport of small solutes and salts.

Transport of protons from the cytosol to the vacuole stabilizes cytoplasmic pH, while making the vacuolar interior more acidic ,this helps transport nutrients into or out of the vacuole. Degradative enzymes are active at low pH of the vacuole. Although single large vacuoles are most common, the size and number of vacuoles may vary in different tissues and stages of development. For example, developing cells in the meristems contain small provacuoles and cells of the vascular cambium have many small vacuoles in the winter and one large one in the summer.
Young cells contains many small vacuoles and ample cytoplasm. As cells mature, the small vacuoles merge to form a single, large vacuole, the contents of  water.
Plant cells commonly appear empty after staining as their vacuoles do not retain stains. In roots and stems, the interiors of many empty cells contain starch granules, a storage molecule for glucose. 
Vacuoles store waste products that would be dangerous if they accumulated in the cell’s cytoplasm.
Many of these waste 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.

Vacuoles store many economically important products.

Proteins are store in vacuoles of the cells in seeds.Beet roots and sugarcane store large amounts of sugar n vacuoles. Latex is stored in vacuoles of rubber plants. Vacuoles of many plants store large amounts of amino acids, which are used as a reservoir of nitrogen. Large amounts of salt are also accumulated in vacuoles. The sap in most vacuoles has concentrations of salts similar to that of seawater.
Vacuoles  accumulate salts as potassium chloride and sodium chloride in marine algae and plants which grow in the salty soils,that salt content is  thousand times greater than plants grow normally soil.
That excess of salt converted int calcium oxalate crystals are common in vacuoles of many plants such as dumb cane .
Organic acids such as oxalic acid and malic acid are also accumulated. These acids make vacuoles slightly acidic. The vacuoles in citrus fruit contain large amounts of citric acid. Consequently, these vacuoles are very acidic,so fruit are sour taste.

Water Management

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.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. 

Plant Movement and Gas Exchange

Vacuoles are important for the movements of many plants. For example, leaf movements in the sensitive plant (Mimosa pudica) and Venus’s flytrap (Dioneae muscipula) are based on the tonoplast’s ability to absorb or lose water quickly.
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.

Enzymes present in the Vacuoles:Vacuoles contains a variety of digestive enzymes, such as phosphatases and esterases.

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 like turnips, radishes, and cabbage, fruits like cherries, plums, and grapes, and flowers like 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 betacyanin. Betacyanin is the a pigment present in vacuole.
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.