Osmosis
All cells contain a plasma membrane or cell membrane that forms the boundary of the cell. Among its many functions, it will act as the gatekeeper to regulate what materials can move into and out of the cell.
Water moves easily across this border. Water movement is essential for the cell to maintain homeostasis. The direction of water movement across the cell membrane is determined by the concentration of solute on either side. Water always moves to the side where a higher solute concentration exists. (Another way to think about this is that water will move from a higher water concentration to a lower one to make the concentrations equal on both sides of the membrane.) The movement of water across a semipermeable membrane from a lower solute concentration to a higher solute concentration is called osmosis.
In this activity, you will be given a practical example of osmosis in action.
Explain what is happening in your mini case study by answering the questions following the story. Use technical biological vocabulary (such as hypotonic, hypertonic, isotonic, and solute concentration) to clearly answer the questions. Do the practice questions before looking at the case study.
Practice Questions:
You have an artificial cell containing 35% glucose placed in a 15% glucose solution.
(Note: the cell is the circle; the rectangle is the solution.)
Which way will the water move?
Will the cell shrink, swell, or stay the same size?
What type of solution is the cell in?
You have an artificial cell containing 12% NaCl placed in a solution that is 15% glucose and 10% NaCl.
Which way will the water move?
Will the cell shrink, swell, or stay the same size?
What type of solution is the cell in?
Case #6 – A Fishy Tale
You read a news release indicating that a species of saltwater fish was found in Lake Michigan. How it got there was a mystery. Unfortunately, the fish has died. In another instance, a saltwater fish was found living in a lake a bit upstream from the Gulf of Mexico. It was assumed that the fish swam from the ocean to a freshwater lake. This fish survived and is doing well in the lake. You wonder why one fish survived and the other did not. (Be sure to include specific biological terms such as hypotonic, isotonic, or hypertonic in your explanation. Also talk about the water relationship between the cells of the fish and the water outside.)
Questions:
What happened to the saltwater fish that was found in Lake Michigan?
Explain what happened to the survivor fish that swam from the Gulf of Mexico to the freshwater lake. (Hint: the journey from the ocean to the lake will take the fish through some brackish water—water that has more salinity than freshwater but less than salt water.)
What would happen to a freshwater fish that suddenly found itself in the Atlantic Ocean? Explain.
