Lysosome
A lysosome is a membrane-bound organelle found primarily in animal cells that contains hydrolytic enzymes capable of breaking down all types of biological polymers, including proteins, nucleic acids, carbohydrates, and lipids. Lysosomes maintain an acidic internal pH of around 4.5–5.0, which is optimal for the activity of their digestive enzymes. They play a central role in autophagy, phagocytosis, and cellular homeostasis by recycling cellular components.
Key Properties and Functions of Lysosomes
| Property | Value / Detail | Significance |
|---|---|---|
| Internal pH | 4.5–5.0 | Optimal for hydrolase enzyme activity |
| Number per cell | 300–400 (varies) | Higher in immune cells |
| Membrane | Single phospholipid bilayer | Protected by glycocalyx coating |
| Key enzymes | Lipases, proteases, nucleases | Break down all biomolecules |
| Origin | Golgi apparatus | Budded off as vesicles |
| Related process | Autophagy | Self-digestion of damaged organelles |
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Related Terms
Vacuole
A vacuole is a membrane-bound organelle found in the cytoplasm of cells that stores water, nutrients, waste products, and other materials. In plant cells, the large central vacuole can occupy up to 90% of the cell's volume and plays a critical role in maintaining turgor pressure. Vacuoles help regulate cell volume, pH balance, and the disposal of cellular waste through a process called autophagy.
Endocytosis
Endocytosis is the process by which a cell engulfs extracellular material by invaginating its plasma membrane to form an intracellular vesicle, allowing large molecules, particles, or even other cells to be taken up without crossing the membrane directly. The three main types are phagocytosis (engulfment of solid particles), pinocytosis (engulfment of fluids and dissolved solutes), and receptor-mediated endocytosis (selective uptake via surface receptors such as clathrin-coated pits). Endocytosis requires energy (ATP) and plays critical roles in immune defense, nutrient uptake, and signal regulation.
Exocytosis
Exocytosis is the process by which a cell secretes or exports materials by fusing an intracellular vesicle with the plasma membrane, releasing its contents into the extracellular space. It is the reverse of endocytosis and requires ATP to drive vesicle trafficking along cytoskeletal tracks to the cell surface. Exocytosis is fundamental to neurotransmitter release at synapses, hormone secretion by endocrine cells, secretion of digestive enzymes by pancreatic acinar cells, and delivery of membrane proteins to the cell surface.
From Greek "lysis" (loosening, dissolution) and "soma" (body). The term was coined by Belgian biochemist Christian de Duve, who discovered lysosomes in 1955 and later received the Nobel Prize in Physiology or Medicine in 1974.