Freshwater fish dump water. Saltwater fish save it.
- The idea
- Predict
- See it
- Why
- Check
The idea
“A saltwater fish makes lots of urine to get rid of salt, and a freshwater fish drinks to stay hydrated.”
Salty water sounds like it needs flushing out, and fresh water sounds safe to drink. Both guesses skip which way osmosis moves water.
The short version
The mistake: A saltwater fish makes lots of urine to get rid of salt, and a freshwater fish drinks to stay hydrated.
What's actually true: Freshwater fish gain water, so they drink little and make lots of urine. Saltwater fish lose water, so they drink and save it.
- A freshwater fish's blood is saltier than the water, so water moves in by osmosis and salt leaks out. A saltwater fish has the reverse problem.
- So the freshwater fish makes lots of dilute urine to dump water. Its gills use active transport to take salt back in.
- The saltwater fish drinks to replace lost water. Gill cells pump out the salt that comes with it, and the kidneys make little urine.
On the exam: First ask whether the fish gains or loses water by osmosis. Its plan undoes that.