In an osmosis model, water moves from the more dilute solution to the more concentrated solution, through a partially permeable membrane. To predict the direction, you compare the two solutions and follow the water, not the solute.
This lesson builds directly on explaining diffusion and is part of movement across membranes. It is the reasoning you need for the potato data in interpreting a mass-change dataset.
How do you decide the direction of water movement?
Think of water potential as a measure of how freely water molecules can move. A dilute solution has a lot of free water molecules, so a high water potential. A concentrated solution has more solute particles holding on to water, so a lower water potential.
Water moves down its own gradient, from high water potential to low water potential. So water goes from the dilute side to the concentrated side.
You can use a short routine:
- Read what the model says can cross. If only water can cross, the solute stays put.
- Decide which side is more concentrated in solute.
- Name the direction: water moves from the dilute side to the concentrated side.
- State the consequence: the concentrated side gains volume or mass, and the dilute side loses it.
Worked example
The setup is invented for practice.
A bag made of a partially permeable membrane holds 30% sugar solution. It is placed in a beaker of pure water. The model states that only water molecules can cross the membrane.
Question: Predict what happens to the mass of the bag and explain why.
Step 1, what can cross? Only water. The sugar is too large to cross, so it stays inside the bag.
Step 2, which side is more concentrated? Inside the bag is 30% sugar. Outside is pure water, so the solution inside has more solute and a lower water potential than the water outside.
Step 3, direction. Water moves by osmosis from the pure water (higher water potential) into the bag (lower water potential), through the partially permeable membrane.
Step 4, consequence. More water enters than leaves, so the bag gains water and its mass increases. The bag also becomes firmer.
Now reverse the model: the bag holds pure water and the beaker holds 30% sugar solution. Water now moves out of the bag, so the mass decreases. Same rule, opposite result, because the dilute side swapped places.
The mistake to watch for
The most common error is to say that water moves towards the side with “more water” or to follow the sugar.
Mistaken answer: The sugar moves out of the bag into the water because there is more sugar inside.
The model says only water can cross, so the sugar cannot move. The prediction has followed the wrong substance.
The correction is to ask “what is allowed to cross?” first, then follow only that substance. A second mistake is saying water moves “from a low concentration of water to a high concentration of water”. Stay with higher to lower water potential, or dilute to concentrated solution, and you will keep the direction right.
Check yourself
Try these, then open each answer.
1. A bag holds 10% salt solution and sits in 20% salt solution. Only water can cross. In which direction does water move net?
Show answer
The 10% solution is more dilute, so it has a higher water potential. Water moves out of the bag into the 20% solution, so the bag loses mass.
2. A bag and the beaker both hold 15% sugar solution. What is the net movement of water, and what happens to the mass of the bag?
Show answer
There is no net movement of water, because the water potentials are equal. Water molecules still move both ways at the same rate, and the mass of the bag stays the same.
3. Explain in one sentence why the bag in the worked example becomes firmer.
Show answer
Water enters by osmosis, the volume inside increases, and the membrane is stretched by the extra water, so the bag feels firmer.
Where this leads next
Next compare osmosis with energy-using movement in distinguishing active transport from diffusion, then practise with the movement across membranes practice set. A quick way to keep track of repeated slips is the mistake log and retest queue.
If direction questions still feel like a coin toss, it often helps to have a teacher listen to your reasoning one step at a time. That is something we can do in online one-to-one Biology tuition.