Fick's Law tells us that a given molecule's diffusive rate across a membrane is directly proportional to the area of absorptive surface and to the concentration gradient across the membrane, and is inversely proportional to the distance the molecule must travel to traverse the membrane. While diffusion is by no means the only mechanism by which biologically important molecules traverse biological membranes, it is a significant one.

1. How have various organisms solved the problem of requiring a large surface area (for a fixed volume) to maximize diffusion rates?

2. No individual gas-absorbing surfaces in biology are more than a few cells thick.  What aspect of Fick's Law might explain that?

3. Can you think of ways in which organisms have solved the problem of requiring a large concentration gradient between the environment and inside the organism?