Vacuoles

Vacuoles are membrane-bound organelles found in many eukaryotic cells. They are enclosed by a single membrane called the tonoplast and typically store substances and regulate the internal cell environment. Vacuoles originate as vesicles that bud from the endoplasmic reticulum and Golgi apparatus, or by fusion of existing vesicles.

In plant and fungal cells, vacuoles can be large and occupy most of the cell’s interior. The plant central vacuole stores water, ions, sugars, organic acids, pigments, and defensive compounds, and helps maintain turgor pressure that supports cell shape and growth. The vacuolar membrane uses proton pumps and transporters to move solutes into the vacuole, which drives water uptake and maintains pH and ionic balance. In many cells, vacuoles also contain hydrolytic enzymes analogous to lysosomes, enabling intracellular digestion of macromolecules.

In animal cells, vacuoles are typically smaller and more varied in function, including storage of nutrients and waste products, sequestration of pigments and toxins, and involvement in endocytosis and exocytosis. Specialized vacuoles include lysosomes, which digest macromolecules, and contractile vacuoles in some protists that expel excess water to regulate osmotic balance.

Overall, vacuoles support cell physiology by storing substances, regulating osmotic conditions, and contributing to metabolism and defense. Their composition and size vary across organisms and cell types, reflecting adaptation to cellular needs.