Vacuoles Function, Definition, Types and Structure
A vacuole is an organelle in cells that works to hold various solutions or materials. This includes solutions that have been created and stored or excreted and those that have been engulfed or engulfed by the cell. A vacuole is simply a chamber surrounded by a membrane, which prevents the cytosol from being exposed to the content inside. Because the vacuoles are surrounded by semi-permeable membranes, they only let certain molecules through.
- A vacuole is a membrane-bound organelle that is present in all plant and fungal cells and in some protist, animal, and bacterial cells.
- The most visible compartment in most plant cells is a very large, fluid-filled vacuole. Large vacuoles are also found in three genera of filamentous sulfur bacteria, Thioploca, Beggiatoa, and Thiomargarita.
- However, the function and meaning of vacuoles vary greatly depending on the type of cell that has much greater prominence in plant, fungal, and certain protist cells than those in animals and bacteria.
- There may be several vacuoles in a single cell. Each vacuole is separated from the cytoplasm by a single membrane unit, called a tonoplast.
- In general, they occupy more than 30 percent of cell volume; But this can vary from 5 to 90 percent, depending on the cell type.
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 initially arise in dividing young cells, probably due to the progressive fusion of vesicles derived from the Golgi apparatus.
- A vacuole is surrounded by a membrane called a tonoplast or vacuolar membrane and is filled with cell sap.
- The tonoplast is the cytoplasmic membrane that surrounds a vacuole, which separates the vacuolar content of the cytoplasm from the cell. As a membrane, it is mainly involved in the regulation of ion movements around the cell and in the isolation of materials that can be harmful or a threat to the cell.
- Vacuoles are structurally and functionally related to lysosomes in animal cells and can contain a wide range of hydrolytic enzymes. Furthermore, they generally contain sugars, salts, acids and nitrogenous compounds like alkaloids and anthocyanin pigments in their cell sap.
- The pH of plant vacuoles can be as high as 9 to 10 due to large amounts of alkaline substances or as low as 3 due to the accumulation of amounts of acids (eg citric, oxalic and tartaric acids).
Types of Vacuoles
- It has a series of transport systems for the passage of different substances. Several small sap vacuoles are produced in animal cells and young plant cells. In mature plant cells, the small vacuoles fuse to form a single large central vacuole that occupies up to 90% of the cell’s volume.
- The large central vacuole extends 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 liquid present in the sap vacuoles is often called the sap or vacuolar sap.
- They occur in some algae and protist cells that are found mainly in freshwater.
- A contractile vacuole has a highly extensible and foldable membrane. It is also connected to some power channels (eg Paramecium). The feeding channels obtain water with or without waste products from the surrounding cytoplasm. They pour the same into the contractile vacuole.
- The vacuole swells. The process is called diastole. The swollen contractile vacuole contacts the plasma membrane and collapses. The collapse is called systole. This throws the vacuolar content out.
- Contractile vacuoles participate in osmoregulation and excretion.
- They occur in the cells of protozoan protists, various lower animals, and phagocytes from 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 the nutrients are digested. The digested materials pass 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, nor is it surrounded by a common membrane. It is made up of several smaller submicroscopic vesicles. Each vesicle is surrounded by a protein membrane and contains metabolic gases.
- Air vacuoles not only store gases but also provide buoyancy, mechanical strength, and protection against harmful radiation.
A vegetable vacuole has a variety of functions. Different vacuoles with different functions are also usually present in the same cell.
- Plant vacuoles can store many types of molecules. It can act as a storage organelle for nutrients and waste products.
- Some of the products stored by the vacuoles have a metabolic function. For example, succulent plants open their stomata and absorb carbon dioxide at night (when transpiration losses are less than during the day) and convert it to malic acid. This acid is stored in the vacuoles until the next day when the energy of light can be used to convert it into sugar while the stomata are kept closed.
- In particular, they can sequester substances that are potentially harmful to the plant cell, if they are present en masse in the cytoplasm.
- The vacuole has an important homeostatic function in plant cells that are subject to great variations in their environment. For example, when the pH in the environment falls, the flow of H + to the cytoplasm is damped by increased transport of H + to the vacuole.
- Many plant cells maintain turgor pressure at remarkable constant levels in the face of large changes in the tonicity of fluids in their immediate environment by changing the somatic pressure of the cytoplasm and vacuole, in part due to controlled decomposition and resynthesis of polymers such as polyphosphate. in the vacuole, and partly altering
- By increasing in size, the vacuoles allow the germinating plant or its organs (such as leaves) to grow very quickly and consume mainly only water.
- In seeds, the stored proteins necessary for germination are kept in “protein bodies”, which are modified vacuoles.
In Other Cells
In fungal cells, they are involved in many processes, including cell pH homeostasis and ion concentration, osmoregulation, amino acid, and polyphosphate storage, and degradative processes.
In animal cells, vacuoles perform primarily subordinate functions, aiding in larger processes of exocytosis and endocytosis.