Vacuoles: Composition, Structure, and Functions

The cell comprises many different cellular organelles bound by the cell membrane and cell wall. The cell organelles present in the cells, especially eukaryotic cells, are mostly membrane-bound. Almost all types of cells (bacterial, animal, plant, fungal, and protozoan cells) have a cell organelle called vacuoles.

Vacuoles are fluid-filled vesicle that usually that help in the storage of various components and lie in the cytoplasm of cells. The number of vacuoles can be one to many per cell separated from the cytoplasm by a single membrane. A single vacuole in a plant cell is centrally located and can take up almost 90% of the cell volume. However, the volume occupied by the vacuole can range from 0-90%. The vacuole is smaller in size in animal and immature plant cells and is numerous. 

Composition of Vacuole

The internal fluid content of the vacuole is called sap. The sap comprises hydrolytic enzymes, ions, sugars (carbohydrates), salts, acids, and nitrogenous compounds like alkaloids and anthocyanin pigments. The pH of the vacuole can be as low as 3 due to the accumulation of acids like citric, oxalic, and tartaric acids and as high as 9-10 due to large quantities of alkaline substances.  

Structure of Vacuole

The vacuole possesses structural similarities to lysosomes and contains a wide range of hydrolytic enzymes. The vacuole is surrounded by a single-layered membrane called a tonoplast. The tonoplast is made up of phospholipids. The tonoplast has transmembrane proteins embedded in it that act as transport channels. The composition of the internal part of vacuoles differs from that of cytoplasm.  

The animal cells either have smaller and fewer vacuoles or do not have any at all. In immature and growing plant cells, the size of vacuoles is small and numerous. After maturation, the size of the vacuole increases, and their number decreases to as low as only one large centrally located. 

Types of Vacuole

Depending on the internal contents of vacuoles, they are of the following types:

1. Food Vacuole

The food vacuole is present in animal cells, protists, plant cells, and fungi. It is present in the plasma membrane and captures food particles when it enters the cell. The food is digested and stored as energy for future use by the cell. 

2. Contractile Vacuole

The contractile vacuole is present in the protists like paramecium. These are membrane-bound and filled with fluid that can expand and contract. The expansion and contraction help maintain the cells’ homeostasis to prevent cell lysis.  

3. Gas Vacuole

The gas vacuoles are the combination of various gas vesicles. These are present in prokaryotic organisms like bacteria. The shapes and numbers of gas vacuoles differ from organism to organism. The gas vacuoles are protein membrane-bound, and the internal part of the vacuole is hydrophobic. The hydrophobic nature of the vacuoles helps maintain buoyancy in aquatic bacteria like blue-green algae.   

4. Lytic Vacuole

These vacuoles have lysosomal enzymes and perform lytic functions in plant and animal cells. These have also been helpful with autophagy due to the presence of cell organelles like mitochondria and plastids.   

Functions of Vacuole

The functions of vacuoles in cells are as follows:

1. Storage function: The vacuole is vital in storing various materials in plant and animal cells. It stores pigments like anthocyanin that give color to fruits and flowers. The vacuole also helps accumulate harmful components from the plant cell’s cytoplasm. Likewise, it stores some interesting products like rubber latex in the cells of plant-like Hevea brazilensis or opium in the plant cells Papaver sominiferum. In seeds, the vacuoles store proteins required for germination. 
2. Homeostatic function: The flux of hydrogen ions into the cytoplasm decreases by the transport of hydrogen ions into the vacuoles during low environmental pH.  
3. Turgor pressure: The sap of the vacuole contains a high amount of fluid that provides pressure to the cell wall, known as the turgor pressure. The turgor pressure helps in maintaining the osmotic activity of the cell. 
4. Metabolic function: Plant cells’ vacuole components perform metabolic functions. For example, the stomata of succulent plants store malic acid that converts into sugars during day time. 
5. Autophagy: Some vacuoles are also involved in the autophagosome due to their hydrolytic enzymes. The autophagosome leads to cell death.  
6. Lysosomal function: The vacuole performs functions similar to that of a lysosome, which removes toxins and waste materials from the cell. 
7. Structure and support: The vacuoles in the plant help enlarge the cell and provide support to the plant cell. 

References

• Verma, P., & Agarwal, V. (2005). Cell biology, genetics, molecular biology, evolution and ecology (23rd ed., pp. 240-243). S Chand and Company Ltd.
• Iwasa, J., & Marshall, W. Biologia cellulare e molecolare di Karp (8th ed., p. 293). Wiley.
• Vacuole. Genome.gov. (2022). Retrieved 7 September 2022, from https://www.genome.gov/genetics-glossary/Vacuole.