lysosome
Meaning of lysosome: The cell's acid-filled recycling compartment, walled off because the enzymes inside would digest the cell that built them.
Definition of lysosome
A lysosome is a small sac inside a cell that holds digestive enzymes. Its interior is kept acidic, far more acidic than the fluid around it, because those enzymes only work properly in such conditions. Worn-out cell parts, swallowed bacteria and material taken in from outside are delivered here and broken down into building blocks the cell can reuse.
How does material reach it?
| Route | What it delivers |
|---|---|
| Autophagy | The cell's own worn-out parts, wrapped in a membrane |
| Endocytosis | Material scooped in from outside the cell, including cholesterol carriers |
| Phagocytosis | Whole bacteria engulfed by immune cells |
Each route ends the same way, with the delivery vesicle fusing with a lysosome so its contents meet the enzymes.
Why does the acid matter?
Pumps in the membrane push hydrogen ions inward, keeping the interior around a hundred times more acidic than the surrounding fluid. The enzymes, a family of proteolytic enzymes and other hydrolases, are shaped to work at that acidity and work poorly at the neutral pH outside. That is a safety feature: a small leak does little harm, because the escaped enzymes are largely inactive in the cell's own fluid. The inner face of the membrane also carries a dense sugar coat that shields it from digestion.
What happens when one enzyme is missing?
The substance that enzyme would have broken down accumulates instead. Lysosomes swell with undigested material, cells become distended and eventually fail, and organs enlarge. This is the mechanism behind the lysosomal storage diseases, a group of inherited conditions in which the pattern of illness depends on which substance builds up and in which tissues. Some affect the brain and nervous system, others chiefly the liver, spleen, bone or heart. Several are now treated by giving the missing enzyme as an infusion.
What else do they do?
They are not merely incinerators. They act as sensors of the cell's nutrient supply and help decide whether the cell grows or conserves. They can be dispatched to plug a tear in the outer membrane. Immune cells and bone-dissolving cells release their contents outward deliberately, using the same enzymes as a weapon or a tool. Recycling their output back into cellular metabolism is what allows a starving cell to survive on its own components for a time.
Used in a sentence
Electron microscopy showed enlarged lysosomes packed with undigested storage material.
Good to know
The membrane, rather than the cell fluid, is what keeps lysosomal enzymes in check, and inherited absence of any single one of these enzymes produces the group known as lysosomal storage diseases.
Word origin
Where “lysosome” comes from
From the Greek lysis, meaning a loosening or dissolving, and soma, meaning body.