A carrier only takes what fits its binding site
- The idea
- Predict
- See it
- Why
- Check
The idea
“A carrier will move any molecule of the same kind and size as its usual one.”
Sugars can share a formula and a size, so it's easy to picture one carrier taking any of them. The carrier actually goes by shape.
The short version
The mistake: A carrier will move any molecule of the same kind and size as its usual one.
What's actually true: A carrier moves only molecules whose shape fits its binding site.
- A carrier protein has a binding site, a pocket whose shape and charges match certain molecules. It carries only what binds there. That close match is called specificity.
- Molecules close enough in shape can share a site and compete for it. So the ones that slow uptake show what the site recognizes.
On the exam: Ask whether it fits the binding site. Same family, same size or a bigger supply won't get it carried.