- A vacuole is a membrane-bound organelle that is present in all plant and fungal cells and some protist, animal and bacterial cells.
- The most conspicuous compartment in most plant cells is a very large, fluid-filled vacuole. Large vacuoles are also found in three genera of filamentous sulfur bacteria, the Thioploca, Beggiatoa, and Thiomargarita.
- However, the function and significance of vacuoles vary greatly according to the type of cell having much greater prominence in the cells of plants, fungi, and certain protists than those of animals and bacteria.
- There may be several vacuoles in a single cell. Each vacuole is separated from the cytoplasm by a single unit membrane, called the tonoplast.
- Generally, they occupy more than 30 percent of the cell volume; but this may vary from 5 percent to 90 percent, depending on the cell type.
Figure: Diagram of Vacuoles
Structure of Vacuoles
- They generally have no basic shape or size; its structure varies according to the requirements of the cell.
- In immature and actively dividing plant cells the vacuoles are quite small. These vacuoles arise initially in young dividing cells, probably by the progressive fusion of vesicles derived from the Golgi apparatus.
- A vacuole is surrounded by a membrane called the tonoplast or vacuolar membrane and filled with cell sap.
- The tonoplast is the cytoplasmic membrane surrounding a vacuole, separating the vacuolar contents from the cell’s cytoplasm. As a membrane, it is mainly involved in regulating the movements of ions around the cell, and isolating materials that might be harmful or a threat to the cell.
- Vacuoles are structurally and functionally related to lysosomes in animal cells and may contain a wide range of hydrolytic enzymes. In addition, they usually contain sugars, salts, acids and nitrogenous compounds such as alkaloids and anthocyanin pigments in their cell sap.
- The pH of plant vacuoles may be as high as 9 to 10 due to large quantities of alkaline substances or as low as 3 due to the accumulation of quantities of acids (e.g., citric, oxalic and tartaric acids).
- 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 the cytoplasm and the surrounding environment. The fluid present in the sap vacuoles is often called a sap or vacuolar sap.
- They occur in some protistan and algal cells found mostly in freshwater.
- 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 the plasma membrane and collapses. Collapsing is called systole. This throws the vacuolar contents to the outside.
- Contractile vacuoles take part in osmoregulation and excretion.
- They occur in the cells of protozoan protists, several lower animals and phagocytes of higher animals.
- A food vacuole is formed by the fusion of a 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 (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.
A plant vacuole has a variety of functions. Different vacuoles with distinct functions are also often present in the same cell.
- Plant vacuoles can store many types of molecules. It can act as a storage organelle for both nutrients and waste products.
- Some of the products stored by vacuoles have a metabolic function. For example, succulent plants open their stomata and take up carbon dioxide at night (when transpiration losses are less than in the day) and convert it to malic acid. This acid is stored in vacuoles until the following day when light energy can be used to convert it to sugar while the stomata are kept shut.
- In particular, they can sequester substances that are potentially harmful to the plant cell, if they are present in bulk in the cytoplasm.
- The vacuole has an important homeostatic function in plant cells that are subjected to wide variations in their environment. For example, when the pH in the environment drops, the flux of H+ into the cytoplasm is buffered by increased transport of H+ into the vacuole.
- Many plant cells maintain turgor pressure at remarkable constant levels in the face of large changes in the tonicity of the fluids in their immediate environment by changing the somatic pressure of the cytoplasm and vacuole—in part by controlled breakdown and resynthesis of polymers such as polyphosphate in the vacuole, and in part by altering
- By increasing in size, vacuoles allow the germinating plant or its organs (such as leaves) to grow very quickly and using up mostly just water.
- In seeds, stored proteins needed for germination are kept in ‘protein bodies’, which are modified vacuoles.
In Other Cells
- In fungal cells, they are involved in many processes including the homeostasis of cell pH and the concentration of ions, osmoregulation, storing amino acids and polyphosphate and degradative processes.
- In animal cells, vacuoles perform mostly subordinate roles, assisting in larger processes of exocytosis and endocytosis.
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- Alberts, B. (2004). Essential cell biology. New York, NY: Garland Science Pub.
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- Alberts, Bruce, Johnson, Alexander, Lewis, Julian, Raff, Martin, Roberts, Keith, and Walter, Peter (2008). Molecular Biology of the Cell (Fifth Edition), (Garland Science, New York), p. 781