Big cells run short because of distance, not material
- The idea
- Predict
- See it
- Why
- Check
The idea
“Big blocks exchange slowly because their insides are thicker and harder to get through.”
A big block looks thick, and thick things seem hard to get through. In a uniform material, molecules move just as fast in a big piece.
The short version
The mistake: Big blocks exchange slowly because their insides are thicker and harder to get through.
What's actually true: A big cell's center is farther from the surface, and each bit of surface has more volume to supply.
- Diffusion is quick over short distances and slow over long ones. Twice as far takes about four times as long.
- As a cell grows, volume rises faster than surface area, so its surface area to volume ratio falls. Each bit of membrane has more cytoplasm to supply.
- So cells stay small or flat to keep every part near the membrane, or fold it into microvilli for more surface.
On the exam: When the material is the same, look at geometry. How far is the center from the surface?