What does the vacuole do in a cell? Functions of vacuoles

A vacuole is a membrane-bound organelle found in the cells of plants, fungi, animals, and some protists. However, the prominence of vacuoles is greater in the cells of plants, fungi, and certain protists than in the cells of animals.

A vacuole can be defined in biology as a space within a cell that is lined with a membrane that lacks cytoplasm but is filled with fluid. In other words, it is said to be a membrane-bound organelle that is lysosome-like. In mature plant cells, their vacuoles seem to be larger compared to animal vacuoles. Many plant cells have a large single central vacuole that takes up 80% or more of the cell serving functions such as waste disposal, protection, storage, and growth.

Structure of the vacuole in a cell

Vacuoles are formed from the fusion of multiple membrane vesicles. They are the larger forms of these membrane vesicles and have no shape or size. Moreso, the structure of a vacuole varies based on the requirements of the cell.

The structure of a vacuole in a cell can be described as a chamber that is surrounded by a membrane that keeps the cytosol from being exposed to the internal contents. Vacuoles only allow certain molecules to pass through because they are surrounded by semi-permeable membranes.

Structurally, a vacuole includes a variety of membrane-bound sacs that are composed of phospholipids. However, each organism may use slightly different phospholipids. There are proteins embedded in the membranes that function to transport molecules across the membrane or give the vacuole structure.

The various combination of these proteins embedded in the membrane is responsible for different vacuoles handling and holding different materials. Moreso, the different genetics in each organism cause different proteins to be embedded in the membrane of the vacuole.

This allows different molecules to pass through thus giving the vacuoles different properties.

The structure of this organelle is an enclosed compartment filled with water that contains inorganic and organic molecules. It functions to hold various solutions or materials that have been created and are being stored or excreted, as well as those that have been engulfed or phagocytized (see phagocytosis) by the cell. Therefore, enzymes in solution, as well as solids that have been engulfed, can be contained in the vacuole.

Functions of vacuoles in cells

1. The vacuole in cells isolates materials that might be harmful or of a threat to the cell.
2. Vacuoles stores waste products and small molecules in the cells.
3. In plant cells, the central vacuole stores water.
4. The vacuole function in the maintenance of the internal hydrostatic pressure or turgor within the cell.
5. Vacuoles maintain the acidic internal pH of the cell.
6. What the vacuole does in the cell is mainly to export unwanted substances from the cell.
7. As a result of the turgor pressure of the central vacuole, plants are able to support structures such as leaves and flowers.
8. In plant cells, what the vacuole does is to increase in size which allows the germinating plant or its organs e.g leaves to grow rapidly, mostly using up just water.
9. Plant cells have modified vacuoles known as “protein bodies” which keeps the stored proteins needed for germination in the seeds of plants.
10. Another function of the vacuole is its assistance in the lysis and recycling of misfolded proteins that have started to build up within the cell.
11. What the vacuole does in animal cells is to help in the larger processes of exocytosis and endocytosis.
12. Vacuole function in autophagy by maintaining a balance between the biogenesis (production) and degradation (turnover) of several substances and cell structures in certain organisms.
13. It was proposed by Thomas Boller and others that the vacuole plays a role in the destruction of invading bacteria.
14. Robert B. Mellor also proposed that the organ-specific forms of vacuoles function in housing symbiotic bacteria.
15. What the vacuole does in protists is to function in the storage of food that has been absorbed by the organism as well as assist in the digestive and waste management process that occurs within the cell.

The vacuoles in animal cells are commonly far less morphologically developed than those in plants and fungi. However, animal cell vacuoles are usually greater in number. There are recent studies that show that some animal cells possess well-developed prominent vacuoles. For instance, large vacuolar structures are seen in highly differentiated mammalian tissues such as embryonic visceral endoderm and absorbing epithelium.

Functions of vacuoles in animal cells

1. Storage functions
2. Endocytosis
3. Exocytosis

Storage Functions of vacuoles

Storing different types of molecules is what the vacuole does in an animal cell. The vacuole can store several types of molecules. For example, fat cells can store large amounts of lipids in specialized vacuoles. This in turn enables single cells to be able to store a large amount of fat, that organisms can use when resources are low.

The expandability of the vacuole in an animal cell indicates that an organism can gain or lose weight without too many cells being created or lost. Furthermore, the vacuoles of organisms are used to create an entire ecosystem where symbiotic organisms (see symbiosis) can live. Take the coral polyps for instance that often eat algae through endocytosis. These algae live in the vacuoles within the coral and allow the coral to gain the oxygen and nutrients that they give off.

Endocytosis

The vacuole is used whenever a huge amount of substance is taken in through endocytosis or excreted through exocytosis. Organisms take in substances and have to store them separately from the cytosol. This could be because the substance can interfere with cellular processes being too large or because the substances are reactive and can cause unwanted reactions. Endocytosis is said to occur when the lysosomes that take in substances to be digested sometimes fuse to form a large digestive vesicle. This large digestive vesicle can digest nutrients in an acid environment and then transfer them for usage in the cytosol or other organelles. Endocytosis occurs and varies among different types of cells.

Endocytosis can occur in a variety of forms, either via phagocytosis or pinocytosis. In the process of phagocytosis (“cell eating”), bacteria, dead tissue, or other materials visible under the microscope are engulfed by cells. Before being engulfed, the material comes in contact with the cell membrane, which then invaginates. The engulfed material is left in the membrane-enclosed vacuole with the cell membrane intact as the invagination is pinched off.

The process of pinocytosis (“cell drinking”) is essentially the same as phagocytosis. The only difference is that the substances ingested are in solution and not visible under the microscope. Phagocytosis and pinocytosis both occur in association with lysosomes which complete the breakdown of the material that has been engulfed. Some bacterial cells like Salmonella when engulfed can survive and reproduce in the vacuoles of many mammal species.

Exocytosis

Several cells function as secretory cells and tend to produce and excrete large amounts of different substances. The extrusion process of proteins and lipids from the cell is known as exocytosis. It is a reverse process of endocytosis. In this process, the materials are absorbed into the secretory granules within the Golgi apparatus and are then transported to the cell membrane. Then later secreted into the extracellular environment. In this process, the vacuole serves as a storage vesicle that allows the containment, transportation, and disposal of selected lipids and proteins to the extracellular environment of the cell.

What really occurs during exocytosis is that the substances produced in the endoplasmic reticulum travel to the Golgi apparatus which are then modified and labeled for distribution. These substances are then put into vesicles that travel into the cytoplasm. The vesicles then merge into a larger vacuole before being excreted. Once vesicles fuse together they can be considered vacuoles. The vacuoles carrying different substances to and fro vary structurally in different cells. Also, the vacuoles have different functions even within cells. There may be many vacuoles that perform many functions in an animal cell.

Functions of vacuoles in a plant cell

1. Molecular degradation and storage of other substances
2. Transportation
3. Water storage and turgor pressure

Molecular degradation and storage of other substances

The vacuole in plant cells is enclosed by a membrane called tonoplast which is an important and highly integrated component of the plant endomembrane system. This large central vacuole develops slowly as the cell matures. Its development occurs from the fusion of smaller vacuoles gotten from the endoplasmic reticulum and Golgi apparatus. The vacuole is known to play a role in the molecular degradation and storage of substances in the plant cell.

These functions are sometimes carried out by different vacuoles in the same cell. One vacuole could serve as a compartment for the breaking down of materials while another vacuole could store waste products, nutrients, or other substances. Many of the substances that are usually stored in the vacuoles of plants have been found to be helpful to humans and are usually harvested e.g rubber, opium, and garlic flavoring.

The central vacuole is a very selective transportation medium, hence, the chemical palette of the vacuole solution also known as cell sap differs markedly from the solution of the surrounding cytoplasm. For instance, some vacuoles have pigments that give certain flowers their color. Therefore, vacuoles function in the storage of pigments which gives certain flowers their colors. The flower color aids the plant to attract bees and other pollinators.

Moreso, the central vacuole contains plant wastes that are unpalatable to insects and animals. These molecules can be poisonous, unpalatable, or odoriferous to several insects and animals, thereby discouraging them from consuming the plant. Furthermore, the developing seed cells of plants use the central vacuole as a repository for protein storage.

Transportation

The large vacuole that occupies the majority of the plant cell is surrounded by the tonoplast. This tonoplast is a type of cytoplasmic membrane that can stretch and fill itself with cell sap. The vacuole can also fill itself with protons from the cytosol. Hence, it produces an acid environment inside the cell. The vacuole can then make use of the chemical gradient created, to transport materials in and out of the vacuole. This transportation movement is called the proton motive force which involves the movement of water and other molecules.

Water storage and turgor pressure

The majority of plant cells have evolved to use vacuoles as water storage organelles that render a variety of functions to the cell. They use their vacuoles to exert turgor pressure which is essential to all plants. The vacuole can become pressurized when filled with water exerting a force on the cell wall of the cell. Even though the force is not much, the turgor pressure permits the cells to take a form and stand environmental factors such as rain, wind, and hail. Woody plants may produce additional proteins and fibers that enable them to stand tall, but many non-woody plants have the ability to reach heights of several feet just on turgor pressure alone.

Turgor pressure determines the cell rigidity and is associated with the difference between the intracellular and extracellular osmotic pressure of the cell. Osmotic pressure is the pressure needed on a solution to prevent the inward flow of its pure solvent across a semipermeable membrane. The plant cells’ response to water is a prime example of the significance of turgor pressure. The central vacuole of the plant cell swells once plants take in adequate amounts of water. As the liquid collects within the vacuole, a high level of turgor pressure is created. This helps to maintain the structural integrity of the plant together with the cell wall’s support.

Therefore, the turgor pressure is an efficient way for plants to structure themselves even though they can droop when the pH balance is off or they lack enough water. Without the right conditions (such as the right soil pH level), the root of plants are unable to take up water or nutrients that they store in the vacuole which can lead to the plant wilting. When water is not sufficient, the central vacuole shrinks, and the turgor pressure within the cell is reduced. Thereby affecting the rigidity of the plant which results in wilting. This means the structural relevance of the vacuole in plant cells is related to its ability to control turgor pressure.

Functions of Vacuoles in a Single Celled Organism

Single celled organisms do not have a cell wall and thus have specialized vacuoles. The most important one is the contractile vacuole. The contractile vacuole regulates water levels in single celled organisms.

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