Vacuoles | NEET BIOLOGY

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CELL : STRUCTURE AND FUNCTIONS

Vacuoles

• Vacuoles are non - cytoplasmic areas present inside the cytoplasm which are separated from the latter by specific membranes.
• Vacuoles are believed to be formed by expansion and pinching off from ER.
• Depending upon the contents and functions, vacuoles are of four types- sap vacuoles, contractile vacuoles, food vacuoles and air vacuoles.

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 upto 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 pig ments called anthocyanins. 
• Some crystalline deposits may also occur 
• Tonoplast has sites for passage of a number of ions and other materials into vacuole against their concentration gradient 
• They may store food reserve, e.g. , sucrose 
• Solutes present in cell sap maintain a proper osmotic pressure in the cell for its turgidity and water absorption.
• They play an important role in cell enlargement.
• The sap vacuoles store and concentrate waste products . The same are segregated from the living part of the cell.
• 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.
• Some plant vacuoles have special transport proteins, an acidic pH, a battery of hydrolytic enzymes and function as lysosomes. 
• Tannins are stored in vaculoes, cytoplasm and cell walls.
• Latex is stored in vaculoes or vacuolar canals.
• Alkaloids and tannins stored in vaculoes provide protection against herbivores 

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. 
• 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 osmoticconcentration 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.

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.

Air Vacuoles (Pseudovacuoles , 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 submicroscopic vesicles. 
• Each vesicle is surrounded by a protein membrane and encloses metabolic gases. 
• Air vacuoles not only store gases but provide buoyancy, mechanical stength and protection from harmful radiations.

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