What is a secondary lysosome?

A secondary lysosome, also known as a heterophagosome, is a type of lysosome formed when a primary lysosome fuses with a vesicle that contains material to be digested, such as a phagosome or pinocytic vesicle. This fusion brings together the hydrolytic enzymes of the lysosome and the ingested substances, initiating the process of intracellular digestion.

Inside a secondary lysosome, complex biomolecules like proteins, lipids, and carbohydrates are broken down into simpler molecules such as amino acids, fatty acids, and monosaccharides. These simpler molecules are then released into the cytoplasm, where they can be reused by the cell for energy production or as building blocks for new cellular components.

Secondary lysosomes play a crucial role in maintaining cellular homeostasis. They not only digest foreign particles like bacteria (in phagocytic cells such as macrophages and neutrophils) but also recycle worn-out cell organelles through a process known as autophagy.

The main difference between primary and secondary lysosomes is that primary lysosomes contain inactive enzymes, while secondary lysosomes are active, as they participate directly in digestion.

Thus, secondary lysosomes act as the functional units of cellular digestion, ensuring efficient recycling and defense within the cell.