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Vacuoles Definition and Meaning

A vacuole is a membrane-bound cell organelle consisting of an empty space filled with fluid and other constituents. Vacuoles are present in both plant and animal cells. Vacuoles do have a definite shape and assume the shape based on their constituents.

Almost all eukaryotic organisms contain vacuoles. Singled-celled eukaryotes such as the protozoa contain vacuoles made up of cytoplasm. Vacuoles in protozoa perform various functions such as ingestion, food storage, digestion, excretion, and water expulsion.

Vacuoles found in plant cells are larger than that of animal vacuoles and occupy more than 80% of the cell volume. The vacuoles of plants, algae, and fungi are concealed with potent secondary metabolites like tannins and other biological pigments. The primary function of vacuoles in plants, algae, and fungi is to safeguard the cell from self-toxicity.

Vacuoles present in fungi perform functions such as regulation of the osmotic pressure, maintaining ion concentration, amino acid storage, maintaining cell homeostasis, and degradative procedures.

Animal vacuoles are smaller than plant vacuoles and differ in their sizes. The animal vacuole is known to perform functions such as storage, cell protection, endocytosis & exocytosis, recycling, and turgor pressure.

Structure of vacuoles

The fundamental structure of the vacuole present in any eukaryotic cell consists of two parts - the membrane lining the cell organelle and the cell sap.

A vacuole is a cell organelle present in the cytoplasmic matrix of a eukaryotic cell. It is bound by a thin membrane called the tonoplast. This membrane consists of phospholipids and is embedded with proteins. The membrane proteins of the vacuole are responsible for transporting various molecules across the tonoplast. Different protein combinations help the vacuole retain different structures.

The interior of vacuoles is filled with a watery fluid along with some constituents. All the components of the vacuole are together called the cell sap. The composition of the cell sap differs from that of the cytoplasm.

Functions of vacuoles

Vacuoles perform a wide variety of functions in protozoa, plants, algae, and fungi. In animals, vacuoles are confined to a few specific functions. The functions performed by a vacuole in human cells are explained below.

1. Storage

Vacuoles store materials such as salts, proteins, minerals, and pigments within the cell. The cell sap of the vacuole additionally consists of protons that help maintain the acidic environment inside the cell. The movement of protons into and out of the vacuole stabilizes the acidic pH of the cell plasma. When an acidic medium is created, a proton motive force is generated. This force further promotes the displacement of nutrients inside and outside the vacuole. Vacuoles also store a large number of lipids.

2. Turgor or hydrostatic pressure

Vacuoles are composed of watery fluid components. This fluid exerts pressure on the cell membrane. The pressure exerted by the fluid on the cell membrane is called the turgor pressure or hydrostatic pressure. The turgor pressure is essential for maintaining the shape of the cell and assists it in withstanding extreme conditions.

3. Endocytosis and Exocytosis

Endocytosis and exocytosis are fundamental processes seen in intracellular digestion. Endocytosis is a process where the food materials are uptaken by the cell. Endocytosis is a process where the cell excretes the waste material after the food is digested.

The animal vacuole also uptakes the food material by endocytosis. After it is digested in the presence of enzymes, the waste materials are discharged out of the vacuoles by the process of exocytosis. Lysosomes are small vesicles present and are found attached to the vacuole. The primary function of lysosomes is to secrete digestive enzymes to derive nutrients.

They have got an extra feature of accumulating substances absorbed by the organism. Therefore, vacuoles are known to possess good waste management procedures too.

4. Defence

Vacuoles offer protection to the cell against several factors. A vacuole keeps away the cell from harmful substances. It additionally eliminates harmful substances from the body, if present. Vacuoles also help the cell attack the bacteria entering into it. Vacuoles, with the help of lysosomes, protect the cell against bacteria, viruses, and other microbes. Vacuoles also maintain the internal acidic pH of the cell, thereby protecting it from shrinking or bursting.

5. Recycling

As said earlier, vacuoles are composed of proteins, minerals, pigments, and salts too. Another significant function of a vacuole is the recycling of disintegrated proteins. Vacuoles destroy the disintegrated proteins and recycle them. The recycled proteins further build up the cell. This function is mostly observed in plant cells. Vacuoles are thus known to maintain an excellent balance between the degradation and production of important cellular components.

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