- Vacuoles are membrane bound cell organelles found almost in all the cells.
- They are small in animal cells whereas fungi and plants have large vacuoles.
- In plants, the vacuole increases in size as the cell grows and enlarges.
- A mature cell is almost fully occupied by a central vacuole surrounded by a thin layer of cytoplasm.
- The large size of the central vacuole is an adaptation for faster exchange of materials between the cytoplasm and the surrounding environment.
Structure:
- Vacuole is a bubble-like sac bounded by a single layered membrane called tonoplast.
- It is filled with a fluid called cell sap that consists of free water and a variety of compounds in solution of suspension.
- The compounds include minerals, sugars, oxygen, carbon dioxide, soluble pigments and organic acids.
- The pigments include anthocyanins (red, purple, blue) and anthoxanthins (ivory to deep yellow).
- Like the plasma membrane, the tonoplast is also selectively permeable and can maintain concentration of materials different from that found in the cytoplasmic matrix.
- Cell sap has acidic pH.
- Cell sap is hypertonic to the surrounding medium and tonoplast permits water to pass into the vacuole by osmosis.
- Thus, vacuole normally remains fully distended, causing the cell contents to exert a pressure on the cell wall.
- The rigidity of cellulose of cell wall checks the bursting of cell and maintains the turgidity characteristic of plant cells.
- Central vacuole of a mature plant cell is formed by the enlargement and fusion of small vacuoles present in the meristematic cells; these small vacuoles arise from the endoplasmic reticulum and Golgi apparatus.
Types:
- There are mainly four types of vacuoles based on their contents. They are:
- Sap vacuoles
- Contractile vacuoles
- Food vacuoles
- Gas vacuoles
- Sap vacuoles:
- They are the most common types of vacuoles in plant cells.
- They contain cell sap which further comprises of many substances like; minerals, sugars, oxygen, carbon dioxide, soluble pigments and organic acids.
- Cell sap has acidic pH and is hypertonic in nature.
- Sap vacuoles remain distended because of osmosis and make the cell turgid.
Functions:
- They maintain turgidity or turgor of the plant cells. This supports the leaves and twigs with small amounts of wood tissue.
- They play a role in growth of plants by absorbing water, causing elongation of cells with minimum investment of new cytoplasm.
- They provide an aqueous environment for the accumulation and storage of water-soluble compounds (sugars, minerals and pigments).
- They may serve as a waste deposit bin in which unwanted materials, such as toxins, may be dumped. Some Acacia trees produce and store cyanides in their vacuoles and protect the cell from cyanide poisoning.
- They impart colors to the fruits, flowers, buds and leaves due to the presence of anthocyanins and anthoxanthins in the sap.
- Alkaloids and tannins stored in sap vacuoles discourage herbivores from eating such plants.
- Some sap vacuoles act as lysosomes having hydrolytic enzymes. In dead cells, the tonoplast becomes more permeable and enzymes escape the vacuoles, causing autolysis.
- 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.
- Contractile vacuoles:
- They are present in protozoans like amoeba and Paramecium.
- Fresh water protozoans continuously receive water by osmotic inflow from the hypotonic environment.
- To get rid of excess water, they have special types of vacuoles called contractile vacuoles, which appear and disappear in regular intervals.
- These vacuoles take up water from the cytosol directly as in amoeba or by the way of feeding canals as in Paramecium and swell up. This swelling up phase is called diastole.
- The enlarged vacuoles move to the surface and discharge their contents to the exterior contracting periodically by fusion with the plasma membrane. This phase is called systole.
Functions:
- The contractile vacuole prevents too much water from accumulating in a cell and swelling it to bursting point. This process is called osmoregulation.
- The water leaving the contractile vacuole may contain some excretory products. Hence, it has an additional role of excretion.
- Food vacuoles:
- They are found in protozoan protists and lower animals like sponges and coelenterates.
- A lysosome fuses with a food vacuole and helps in digestion of food as lysosome consists of digesting enzymes.
- Once the food is digested, the nutrients are extracted out into the cytoplasm.
Functions:
- Food vacuoles help in internalization and digestion of food.
- The remaining undigested food is expelled out of the cell by the fusion of food vacuole with the plasma membrane.
- Gas vacuoles:
- These occur in some prokaryotes like bacteria.
- A gas vacuole is actually a group of microscopic vesicles, each having a protein membrane around it.
- The vesicles contain metabolic gases.
Functions:
- Gas vacuoles store gases
- Regulate buoyancy of the cell
- Provide mechanical support
- Protect the cell from harmful radiation.