Definition:
Vacuoles (L. vaccus = empty) are lifeless membranous fluid filled sacs present within the cytoplasm.
Types:
According to their contents vacuoles maybe classified into following types:
(i) Sap vacuole:
It is the most common types of vacuole, occurring in higher plants cells.
(ii) Protein bodies:
These are specialized protein storing vacuoles present abundantly in seeds. During germination, the stored proteins in protein are hydrolyzed to amino acids and released into the cytosol for use in protein synthesis.
(iii) Contractile vacuoles:
These are special vacuoles help in osmoregulation. They are found in freshwater protozoans, are found in fresh water protozoans, Chlamydomosas (an alga) etc.
(iv) Food vacuoles:
Such vacuoles contain ingested food particles. Many lysosomes merge with a food vacuole to form a digestive vacuole orgastriole (e.g. protozoan protists, sponges, coelenterates).
(v) Gas vacuoles (= Pseudo vacuole):
Such vacuoles, filled with metabolic gases, are found in some prokaryotes and help them in floating.
Structure of Sap vacuole:
Mature plant cell contain a large central vacuole called sap vacuole that occupy about 80-90% of total cell volume. The sap vacuole is filled with a liquid called cell sap and surrounded by a vacuolar membrane or tonoplast. In meristematic cells, small provacuoles are always found which are derived from Golgi complex. During maturation, the provacuoles fuse together to form a sap vacuole. It occupies a very large part of cell pushing the cytoplasm outwards as a thin layer against the cell wall.
The important components of cell sap is water, dissolved inorganic ions, organic acids, sugars, enzymes and a variety of secondary metabolites. In most cases, vacuoles do not synthesize the molecules they accumulate but instead receive them from other parts of the cytoplasm. The sap vacuole is thus a tiny reservoir of the cell from which the cytoplasm draws water and other materials according to its need.
Sometimes a particular substance is present at such a high concentration that it forms crystals. Calcium oxalate crystals, which can adopt several different forms, are especially common.
Cell sap is usually slightly acidic, some cell sap such as that of the vacuole in citrus fruits are very acidic – hence sour taste of the fruit.
Origin:
The vacuole may originate directly from endoplasmic reticulum, but most of the tonoplast and vacuolar proteins are derived directly from Golgi complex.
Functions:
Different kinds of vacuoles with distinct functions may be found in a single mature cell, and some of the principal functions performed by vacuoles are—
1. Growth of Cell:
They maintain turgor of the plant cells. Most of the increase in the size of the cell results from enlargement of the vacuole(s). A direct impact of this strategy is development of internal pressure and the maintenance of tissue rigidity, one of the principal roles of the vacuole and tonoplast.
2. As Storage Compartments for Primary Metabolites:
Vacuoles are important storage compartments for primary metabolites, such as sugars and organic acids and the reserve proteins in seeds.
3. Removal of Toxic Metabolites:
Vacuoles also remove toxic secondary metabolites, such as nicotine and tannin, from the rest of the cytoplasm. Such substances accumulate permanently in the vacuoles. The secondary metabolites contained in the vacuoles are toxic not only to the plant itself but also to pathogens, parasites, and/or herbivores, and they therefore play an important role in plant defense.
4. Site of Pigment Deposition:
The vacuole is often a site of pigment deposition. The blue, violet, purple, dark red, and scarlet colours of plant cells are usually caused by a group of pigments known as the anthocyanins. Unlike most other plant pigments, the anthocyanins are readily soluble in water and are dissolved in the cell sap. They are responsible for the red and blue colours of many vegetables (radish, turnips, cabbages), fruits (grapes, plums, cherries) and a large number of flowers (cornflowers, geraniums, delphiniums, roses and peonies etc.).
Sometimes, the pigments are so brilliant that they mask the chlorophyll in the leaves (e.g., ornamental red maple). Anthocyanins are also responsible for the brilliant red colours of some leaves in autumn. These pigments form in response to cold, sunny weather when the leaves stop producing chlorophyll.
5. Breakdown of Macromolecules:
Vacuoles are also involved in the breakdown of macromolecules and the recycling of their components within the cell.
6. Degradation of Cell Organelles.
Entire cell organelles, such as mitochondria and plastids, may be deposited and degraded in vacuoles. Because of this digestive activity, vacuoles are comparable in function with the organelles known as lysosomes that occur in animal cells.