endocytosis (phagocytosis, pinocytosis, receptor-mediated)
/ EN-doh-sy-TOH-sis /
Channels and pumps are great for moving small molecules and single ions, but a cell sometimes needs to take in something far too big for any doorway — a whole bacterium, a large droplet of fluid, a clump of cholesterol. So it does what a soap bubble can do: it wraps a piece of its own membrane around the target, pinches it off, and swallows it whole inside a little bag. This bulk swallowing is called endocytosis.
There are three flavors, sorted by what gets eaten. Phagocytosis (cell eating) is gulping a large solid particle, like an immune cell engulfing a bacterium. Pinocytosis (cell drinking) is taking in tiny sips of the surrounding fluid and whatever is dissolved in it, fairly indiscriminately. Receptor-mediated endocytosis is the precise, picky version: specific receptor proteins on the surface grab exactly the wanted molecule, gather into a coated dimple, and that dimple buds inward — so the cell collects only what it has marked, very efficiently.
Endocytosis is how your immune cells clear infections, how your cells take up cholesterol-carrying particles from the blood, and how many nutrients and hormones get inside. It is also a back door that some viruses and bacteria exploit, tricking the cell into swallowing them. Because it costs energy and rebuilds membrane each time, it is an active, deliberate process, not a passive leak.
A white blood cell hunting an invader reaches its membrane around a bacterium, swallows it whole into a bubble, and then dissolves it inside — phagocytosis, your body's microscopic monster-eating.
When a target is too big for a doorway, the cell wraps and swallows it.
Familial high cholesterol can be caused by broken receptors for receptor-mediated endocytosis: the cells cannot pull cholesterol particles out of the blood, so it builds up dangerously.