If you add a little salt water or alcohol to a fresh onion preparation, then they wrinkle somewhat. Why?

Homeostasis ensures the constancy of the internal environment, including the constancy of the salt and water composition. Since the concentration of water in onions is higher than in salt water, then along the concentration gradient (from higher to lower), water from the cell will flow into the environment, causing the phenomenon of plasmolysis – the separation of the protoplast from the cell membrane, due to which the cells shrink.