Chemistry decides first, size second
- The idea
- Predict
- See it
- Why
- Check
The idea
“Small molecules get through membranes and into proteins more easily, so size predicts what they do.”
Sand slips through a sieve and pebbles don't. Membranes and proteins sort molecules by charge and polarity before size.
The short version
The mistake: Small molecules get through membranes and into proteins more easily, so size predicts what they do.
What's actually true: A membrane's nonpolar core blocks tiny ions but passes big nonpolar steroids. Side chains bind by charge and polarity.
- Steroids, O2 and CO2 are nonpolar, so they cross the tails by simple diffusion. Na+, K+ and even tiny H+ need channels or carriers.
- Size matters only among similar molecules. Polar water crosses slowly, and larger polar glucose barely at all.
- Proteins work this way too. Each R group is nonpolar, polar or charged. A positive one in an active site can hold a negative group on a substrate.
On the exam: Ask first whether it's charged, polar or nonpolar. Size only matters between chemically similar molecules.