The structure and function of vacuole along with its types are discussed below.
What is a vacuole?
A vacuole is a membrane-bounded, fluid-filled vesicle present in the cytoplasm of animal and plant cells. It performs various functions according to cell requirements such as storage, digestion, turgor maintenance. It was first discovered by Dutch scientist Antonie Van Leeuwenhoek in 1676. The vacuole word comes from the Medieval Latin word “vacuus” meaning “vacuum or empty”. In biology, the vacuole term is borrowed from a French word meaning “little vaccum” to represent the space in the cytoplasm and was first proposed by the famous French biologist Félix Dujardin.
Structure of vacuole
The vacuole is fluid-filled sacs present in the cytoplasm. The fluid present inside the vacuole is called cell sap and the membrane that separates this cell sap from cytoplasmic content is known as tonoplast (“ton” = “tension” and “plast” from “plastos” = “molded”).
The cell sap is the fluid part of the vacuole which contains enzymes, water, several salts, glucose, and waste materials. Tonoplast is a phospholipid bilayer membrane that provides tensions and maintains the turgor pressure of the vacuole. It also controls the movement of ions in and out of the vacuole. The term tonoplast was first coined by a Netherland botanist de Vries in 1885.
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| Structure of vacuole. Image created in RioRender.com |
In animal cell, several vacuoles are present, while plant cell has only one large vacuole occupying more than 90% of plant cell volume. In a plant cell, small vacuoles are formed by several biosynthetic and endocytotic pathways and then fused to form a large vacuole.
Function of vacuole
The main functions of vacuoles are:- They maintain turgor pressure inside the cell by absorbing excess water and then diffusing it out of the cell.
- The acidic nature of vacuole and the presence of digestive enzymes allows it to perform an important role in the digestion of many substances.
- In seeds and fruits, vacuoles store proteins, amino acids, lipids, and sugars. These stored substances are used by plants during the germination or growth of a new plant.
- It also works in the defense of plant cells by storing heavy metals such as cadmium and arsenic.
- Vacuoles are involved in the process of autophagy in which cellular components are degraded and recycled.
Video lesson
Types of vacuole
There are many types of vacuoles based on the function they perform.
1. Contractile vacuole
This vacuole is present in cells living in freshwater and maintains the isotonic environment by regulating the water quantity. As the freshwater has high water potential compared to the inside environment of a cell, so water rushes inside the cell and may cause the bursting of the cell. Contractile vacuoles remove excessive water from the cell and prevent it from bursting. It is mostly present in freshwater protozoa such as amoeba, euglena, and paramecium.
2. Food vacuole
It stores foods that need to be digested. It can be regarded as the stomach of the cell. Food vacuole with the help of lysosome digest food and releases nutrients into the cytoplasm which is used by the cell for energy generation or growth. While the undigested waste material is removed out of the cell by exocytosis. In the absence of food, the cell will not be able to store and digest nutrients, will starve, and eventually die. Food vacuoles are present in animals, plants, fungi, and protozoa.
3. Storage vacuole
It is concerned with the storage of substances such as proteins, glucose, lipids, several metals, and waste products. Food vacuoles are present in plant cells and provide nutrients to the seed during germination.
4. Central vacuole
It is commonly known as sap vacuole. It is present in plant cells and occupies most of the space of the cell. It contains cell sap so the name sap vacuole is given to it.
5. Gas vacuole
It is a hollow cylinder-like vesicle that stores metabolic gases and provides mechanical strength to the cell. Gas vacuoles are found in bacteria and help them to float in the water at their desired level.
Some questions and answers
Q. What is central vacuole?
A. Central vacuole is present in the center of the plant cell. It occupies up to 90% of the plant cell space. It contains enzymes, water, several salts, glucose, and waste materials.
Q. What is a contractile vacuole?
A. This vacuole is present in cells living in freshwater and maintains the isotonic environment by regulating the water quantity.
Q. How many vacuoles in animal cell?
A. Animal cells contain several small vacuoles.
Q. What is glycogen vacuole?
A. Glycogen vacuole stores glycogen.
Q. What does a vacuole store?
A. Vacuole stores water, salt, enzymes, proteins, minerals, and waste materials.
Q. Is the tonoplast and plasma membrane the same?
A. Tonoplast is the membrane surrounding the vacuole, while the plasma membrane encloses cell content.
Q. State three main functions of a vacuole?
A. Vacuoles maintain turgor pressure, helps in the digestion of many substances, and also store the nutrients.
Q. Do eukaryotes contain gas vacuoles?
A. No, gas vacuoles are only present in prokaryotic cells like bacteria.
Q. How vacuole protects a cell from bursting?
A. Contractile vacuoles remove the excess water from the cell living in freshwater and prevent it from bursting. In this way, they maintain the internal turgor pressure of the cell.
Q. What is the difference between animal and plant vacuoles?
A. The only difference between animal and plant vacuoles is the size of the vacuole. A big central vacuole is present in a plant cell, while many small vacuoles are present in an animal cell.
Q. Define term vacuole.
A. The vacuole word comes from the word “vacuus” meaning “vacuum or empty”. In biology, the vacuole term is borrowed from a French word meaning “little Vaccum” to represent the space in the cytoplasm.
A. Plant vacuole is also known as central vacuole occupies about 90% of the volume of a plant cell.
References
- Tan, X., Li, K., Wang, Z., Zhu, K., Tan, X., & Cao, J. (2019). A review of plant vacuoles: formation, located proteins, and functions. Plants, 8(9), 327.
- Marty, F. (1999). Plant vacuoles. The Plant Cell, 11(4), 587-599.
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