BIOLOGY Volume 1 - A Guide to General Biology - 2004

5. CELLS

5.10. Cell Structures

5.10.12. Vacuoles

A vacuole is a fluid-filled membrane-bound sac enclosed by a single membrane. Animal Cells typically contain relatively small vacuoles, such as phagocytic, digestive, autophagic, and contractile vacuoles. A different pattern is observed in plant cells, particularly in mature parenchyma tissue, where cells contain a single large central vacuole surrounded by a membrane known as the tonoplast (Fig. 5.11). The fluid filling the central vacuole is called Cell sap. This concentrated solution contains mineral salts, sugars, organic acids, oxygen, carbon dioxide, pigments, and various metabolic wastes or secondary metabolites. The Functions performed by vacuoles are listed below.

1. Water generally enters the concentrated cell sap via osmosis, passing through the selectively permeable tonoplast. This creates turgor pressure within The Cell, pressing the Cytoplasm firmly against The Cell wall. Osmotic water uptake plays a crucial role in cell elongation during growth and contributes to the overall water balance of the plant.

2. Vacuoles sometimes contain soluble pigments. This group includes anthocyanins, which exhibit red, blue, or purple coloration, as well as related compounds that are yellow or cream-colored. These pigments are primarily responsible for the coloration of flowers (such as roses, violets, and dahlias), as well as fruits, buds, and leaves. In leaves, they contribute to the various shades of autumn foliage, which also depend on the Photosynthetic Pigments retained in the METABOLISM/14.html">Chloroplasts. Coloration plays a vital role in attracting insects, birds, and other animals involved in plant pollination and seed dispersal.

3. In plants, vacuoles may occasionally contain hydrolytic Enzymes, functioning analogously to Lysosomes. Following cell death, the tonoplast—like all other membranes—loses its selective permeability, allowing the enzymes to be released from the vacuoles to induce autolysis.

4. Plant vacuoles can serve as storage sites for end products and certain secondary metabolites. Among the end products, calcium oxalate crystals are sometimes found. Secondary metabolites, such as Alkaloids and Tannins, likely serve a protective function by deterring herbivores from consuming the plants. Plant vacuoles may also accumulate latex, a milky sap found in plants such as the dandelion. The milky sap of the Para rubber tree contains compounds used to synthesize rubber, while that of the opium poppy contains alkaloids such as morphine, from which heroin is derived.

5. Certain soluble Components of the cell sap, such as sucrose and mineral salts, act as nutrient reserves that can be utilized by the cytoplasm when needed.