Two factors decide it, and you only checked one
- The idea
- Predict
- See it
- Why
- Check
The idea
“The option with the most surface area, or the thinnest barrier, must work best.”
More area does speed up exchange, so the biggest surface looks like the answer. A thinner barrier speeds it up too, and that can count for more.
The short version
The mistake: The option with the most surface area, or the thinnest barrier, must work best.
What's actually true: When an outcome depends on two factors, like area and distance, combine them before you compare.
- Diffusion speeds up with more area and slows with a thicker barrier. At the same gradient, the rate is proportional to area divided by thickness.
- Lungs and gills pair a huge area with a thin barrier. Alveoli or gill lamellae add area, and walls one or two cells thick keep the path short.
- Island species counts depend on both area and distance from the mainland. A large, remote island can hold fewer species than a smaller one near shore.
On the exam: List every factor the outcome depends on, and combine them before you compare.