How Does Osmosis Work in Cells?

Imagine a semi-permeable membrane is like a picky bouncer at a club. The bouncer lets small things in, like water molecules, but keeps big things out. Inside the cell, there are dissolved salts and sugars. If there is more salt outside than inside, water wants to rush in to balance things out. This rushing of water is called osmosis.

Water moves from where there is less stuff dissolved to where there is more stuff dissolved. It is not that water is attracted to the salt. Instead, the salt molecules take up space. This makes it harder for water to move around. Water naturally flows toward the area with more crowding.

If too much water enters, the cell swells up. If too much water leaves, the cell shrinks. Cells have ways to control this balance. This process keeps plants standing tall and helps your kidneys filter blood. Without osmosis, cells would either burst or shrivel up.