Autophagy

Autophagy is how a cell takes apart pieces of itself, and the name says as much: it means "self-eating." The cell gathers up portions of its cytoplasm, the inside of the cell where all its parts float, and delivers them to the lysosome. The lysosome is an organelle, one of the tiny compartments inside a cell that each have their own job, and its job is to break down whatever arrives. This is how the cell gets rid of organelles and long-lived proteins, the ones that stick around inside the cell for a long time.

Here's how it works in one form, called macroautophagy. A double membrane, a very thin wrapping, curves into a cup shape around a piece of cytoplasm, like a cupped hand slowly closing over something on a table. Once the cup closes all the way, you're left with a sealed little pouch with its cargo inside: that's the autophagosome. Next, the autophagosome fuses with the lysosome, meaning their outer wrappings merge, and the cargo ends up loose inside the lysosome, right where it will be broken down.

So what does the cell get out of it? Perhaps its best-known role is helping the cell survive when it runs short of nutrients. The cell recycles and reuses its own components, a bit like taking apart a piece of furniture to reuse the wood and keep working. Thanks to that, the cell keeps functioning almost normally even when nutrients aren't coming in.