Definition of Lysosome

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Lysosome: An organelle (a little organ) in a cell containing enzymes that degrade macromolecules (large molecules) and other items (such as bacteria) taken up by a cell during the process of endocytosis.

In endocytosis, macromolecules and particles outside the cell are taken up by the cell via a progressive invagination (inpouching) and eventual pinching off of a region of the cell membrane, forming a membrane-bound vesicle (bubble) within the cytoplasm of the cell.

The vesicle then fuses with the lysosome and the lysosomal enzymes carry out their appointed task of destruction (by hydrolysis). Amazingly, the lysosomal enzymes do not normally damage the cell itself.