Cold slows diffusion too
- The idea
- Predict
- See it
- Why
- Check
The idea
“If uptake drops in the cold, it must be a pump. Diffusion uses no energy, so it shouldn't care.”
Cold sounds like an energy problem, and pumps are the part that runs on ATP. Molecules themselves slow down in the cold, though, and diffusion is just molecules moving.
The short version
The mistake: If uptake drops in the cold, it must be a pump. Diffusion uses no energy, so it shouldn't care.
What's actually true: Cold slows both diffusion and pumps, so a temperature effect can't tell them apart. Cutting the ATP supply can.
- Temperature sets how fast molecules move, so cooling slows simple diffusion. It slows proteins too, including the carriers of facilitated diffusion, which use no ATP.
- Pumps slow in the cold as well. So lower uptake in the cold fits active and passive transport alike, and can't tell you which it is.
- To find out, change only the energy supply. Withhold oxygen or add a poison that blocks respiration. If uptake collapses at the same temperature, it needs ATP.
On the exam: Cold slows every kind of transport. To spot a pump, look for the row where ATP was cut.