VACUOLES



 VACUOLES:

The Vacuoles is the membrane _ bound space found in the cytoplasm. contain water, sap, excretory products, and other materials not useful for the cell. The vacuole is bound by a single membrane called tonoplast.

  A big central vacuole that forms in mature plant cells,often comes from the agglomerates of smaller vacuoles. 

■It  plant cells the vacuole can occupy upto 90 per cent of the volume of the cell. 

The solution inside the central vacuole called cell sap,is the plant cell's main repository of inorganic ions, including potassium and chloride.





Based on components, Eukaryotes are divided into Animal cells and plants cells

■ plant cells have a cell wall outside the cell membrane. 

■ plant cells often have a large, membrane _ based sac called central vacuole ,which stores proteins, pigments and waste materials. 

Location:

The vacuole occur in most, if not all, cells. They are small in animal cells .Fungi and plants have large vacuoles. In plants, the vacuole increases in size as the cell enlarges, and a mature cell is almost fully occupied by a central vacuole that is surrounded by a thin layer of cytoplasm wherein  nucleus and other organelles are squeezed. This is an adaptation to quicken the exchange of materials between the cytoplasm  and the surrounding environment. 



Structure:

The vacuoles are bubble_ like saca bounded by a single unit membrane called the tonoplast. They are filled with a fluid called  cell sap.The latter consist of free water and a variety of compounds in solution or suspension. The compounds include minerals, sugars, oxygen, carbon dioxide, soluble pigment and organic acids. The pigments include anthocyanins ( red, purple, blue) and anthoxanthins ( ivory to deep yellow). The tonoplast, like the plasma membrane, is differentially permeable and can maintain concentration of materials different from that found  in the cytoplasmic matrix. The vacuoles have an acidic pH.

       In general, the cell  sap is hypertonic to the surrounding medium and permits water to pass into the vacuole by osmosis. Thus, the vacuole normally remains fully distended, causing the cell contents to exert a pressure on the cell wall. The rigidity of the cellulose of cell wall checks bursting  of the cell and maintains the turbidity characteristic of plant cells. 

       Origin  : The vacuole of a mature plant cell is formed by enlargement and fusion of small vacuoles present in the meristematic cells; these small vacuoles themselves arise from  the  endoplasmic reticulum and Golgi apparatus. 





Types of Vacuoles

There are many kinds of vacuoles regarding their contents: sap vacuoles, contractile vacuoles, food vacuoles and gas vacuoles.

(i) Sap Vacuoles: They are fluid filled vacuoles or vesicles separated from the cytoplasm by a selectively permeable membrane called tonoplast.Animal cells and young plant cells have number of small sap vacuoles. In mature plant cells, the small vacuoles fuse to form a single large central vacuole which spread the cytoplasm in the form of a thin peripheral layer.In a plant cell, the vacuoles can occupy 90 cent of the volume of the cell.This is a device to facilitate rapid exchange between cytoplasm and the surrounding environment. These are the most common type of vacuoles. They contain cell sap. The sap vacuole has been described above.

       The fluid present in the sap vacuoles is often called sap or vacuolar sap

(i) They store and concentrate mineral salts

(ii) Sap vacuoles may store food  reserve, e.g., sucrose.

(iii) Solutes present in cell sap maintain a proper osmotic pressure in the cell for its turbidity and water absorption, thus playing an important role in the cell enlargement. 

(iv) They store and concentrate waste products segregating them from the living part of the cell.


(v) The sap vacuoles also store water_ soluble pigments which provide colouration to the cell. The most common water_ soluble vascuolar pigment are anthocyanins (red, blue pumple) and anthoxanthins (ivory to deep yellow) which provide colouration to flowers in  Rose, Dahlila, etc. These pigments attract pollinating and dispersing agencies and also absorb light radiations passing through them so that their intensity is decreased. 


(vi) Some pant vacuoles  contain  hydrolytic enzymes  and function as lysosomes

(vii) Latex is stored in vacuoles or vacuolar canals.


(viii) Alkaloids and tannins stored in vacuoles provide protection against  herbivores. 

 (ii) Contractile Vacuoles: Fresh water protozoans continuously receive water by osmotic inflow from the hypotonic environment. To get rid of excess water, they have special type of vacuoles that appear and disappear at intervals. These are called contractile vacuoles.The latte taken up water from the cytosol directly as in Amoeba or by way of feeding canals as in Paramecium and swell up. This phase is called  diastole. The enlarged vacuoles move to the surface and discharge their contents to the exterior periodically by fusion with the Plasma membrane. This phase is termed systole. The contractile vacuole prevents too much water from accumulating in the cell and swelling it to bursting point. The process is called osmoregulation. The water entering and leaving the contractile vacuole may contain some excretory matter, causing excretion as an additional role.

(iii) Food Vacuoles: These are found in protozoans protists and lower animals such as sponges and coelenterates. They contain food particles. A  lysosomes fuses with a food vacuole to digest its food particle.

(iv) Gas Vacuoles: These occur in some prokaryotes. A gas vacuole is, in fact , a group of submicroscopic vesicles, each having a  protein  membrane around it. The vesicles contain metabolic gases. The gas vacuoles store gases, regulate buoyancy of the cell, provide mechanical support, and protect from harmful radiation. 

Functions:

 The vacuoles  of plant cells, have many functions ___
(i) The maintain turgor of plant cells. This supports the leaves and twigs with small amounts of woody tissue. 

(ii) They play a role in growth by absorbing water, causing elongation of cells with a minimum investment of new cytoplasm. 

( iii) They provide an aqueous environment for the accumulation and storage of water __ soluble compounds ( sugars, minerals, pigments).

(iv) They may serve as a waste deposit bin in which unwanted materials, such as toxins, may be diverted. Some Acacia trees produce and store cyanides in their vacuoles. If these cyanides were present in the cytoplasm, they would poison the rest of cell.

 (v) They impart colours. The anthocyanins and anthoxanthins give colour to flowers, fruits, buds and leaves for attracting insects and birds for pollination and seed dispersal. The pigments also absorb light to lessen its intensity. 

(vi) They give protection against herbivorous. Alkaloids and tannins stored in vaculoes discourage the herbivores from eating plants.

(Vii) They may act as lysosome.Plant cell vacuoles sometimes contain hydrolytic enzymes. In dead cells, the tonoplast loses its partial permeability and enzymes escape, causing autolysis. 

(viii) They may form food reserves.Sucrose stored in vacuoles may be utilized by the cytoplasm when necessary. Vacuoles of the storage cells in seeds store proteins.


































































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