Each solute follows its own gradient
- The idea
- Predict
- See it
- Why
- Check
The idea
“Solutes move away from the side with more dissolved stuff in total.”
A side packed with solute looks like it should push everything toward the emptier side. Glucose, though, only responds to how much glucose is on each side.
The short version
The mistake: Solutes move away from the side with more dissolved stuff in total.
What's actually true: A diffusing substance moves down its own concentration gradient, whatever else is dissolved on either side.
- A concentration gradient belongs to one substance. Glucose diffuses from where glucose is more concentrated to where it is less.
- Most cells turn incoming glucose into glucose phosphate, which the carrier can't bind. Free glucose inside stays low, so glucose keeps coming in.
- Total solute does matter for water. At equal pressure, water moves by osmosis toward the side with more solute.
On the exam: For any solute, compare only its own concentration on each side. Total solute is for water.