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The following points highlight the four main types of vacuoles. The types are: 1. Sap Vacuoles 2. Contractile Vacuoles 3. Food Vacuoles 4. Air Vacuoles.
Type # 1. Sap Vacuoles:
They are fluid filled vacuoles or vesicles which are separated from the cytoplasm by a selectively permeable membrane called tonoplast. It has a number of transport systems for the passage of different substances. A number of small sap vacuoles occur in animal cells and young plant cells. In mature plant cells, the small vacuoles fuse to form a single large central vacuole which occupies up to 90% of the volume of the cell.
The large central vacuole spreads the cytoplasm in the form of a thin peripheral layer. This is a device to facilitate rapid exchange between cytoplasm and the surrounding environment. The fluid present in the sap vacuoles is often called sap or vacuolar sap. It contains mineral salts, sugars, amino acids, esters, proteins, waste products and water soluble pigments called anthocyanin’s.
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Some crystalline deposits may also occur:
(i) Tonoplast has sites for passage of a number of ions and other materials into vacuole against their concentration gradient,
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(ii) They may store food reserve, e.g. sucrose,
(iii) Solutes present in cell sap maintain a proper osmotic pressure in the cell for its turgidity and water absorption,
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(iv) They play an important role in cell enlargement,
(v) The sap vacuoles store and concentrate waste products. The same are segregated from the living part of the cell,
(vi) Water soluble pigments provide colouration to the cell. The most common water soluble vacuolar pigments are anthocyanins (red, blue, purple) and anthoxanthins (ivory to deep yellow). They provide colouration to flowers in Rose, Violet, Dahlia, etc. The pigments attract pollinating and dispersing agencies. They also absorb light radiations passing through them so that their intensity is decreased,
(vii) Some plant vacuoles have special transport proteins, an acidic pH, a battery of hydrolytic enzymes and function as lysosomes.
(viii) Tannins are stored in vaculoes, cytoplasm and cell walls,
(ix) Latex is stored in vaculoes or vacuolar canals,
(x) Alkaloids and tannins stored in vaculoes provide protection against herbivores.
Type # 2. Contractile Vacuoles:
They occur in some protistan and algal cells found mostly in fresh water. A contractile vacuole has a highly extensible and collapsible membrane. It is also connected to a few feeding canals (e.g., Paramecium). The feeding canals obtain water with or without waste products from the surrounding cytoplasm. They pour the same into the contractile vacuole.
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The vacuole swells up. The process is called diastole. The swollen contractile vacuole comes in contact with plasma membrane and collapses. Collapsing is called systole. This throws the vacuolar contents to the outside. Contractile vacuoles take part in osmoregulation and excretion. Osmoregulation is required in fresh water habitats where water has tendency to enter the living cells.
Due to the presence of higher osmotic concentration in the latter, continued entry of water shall cause bursting of the cells. This is prevented by throwing the extra water to the outside with the help of contractile vacuoles.
Type # 3. Food Vacuoles:
They occur in the cells of protozoan protists, several lower animals and phagocytes of higher animals. A food vacuole is formed by fusion of phagosome and a lysosome. The food vacuole contains digestive enzymes with the help of which nutrients are digested. The digested materials pass out into the surrounding cytoplasm.
Type # 4. Air Vacuoles (Pseudo-vacuoles, Gas vacuoles):
They have been reported only in prokaryotes. An air vacuole is not a single entity, neither it is surrounded by a common membrane. It consists of a number of smaller sub-microscopic vesicles. Each vesicle is surrounded by a protein membrane and encloses metabolic gases. Air vacuoles not only store gases but provide buoyancy, mechanical strength and protection from harmful radiations.
