A bigger total can still be less per unit
- The idea
- Predict
- See it
- Why
- Check
The idea
“A cell with more membrane takes in more, so it must be better supplied.”
A big cell does take in more in total. It also has much more inside to feed.
The short version
The mistake: A cell with more membrane takes in more, so it must be better supplied.
What's actually true: Compare per unit, like membrane per µm³ or oxygen per gram. A bigger total shared by even more can be less per unit.
- Double a round cell's width and its membrane area grows 4 times, but its volume grows 8 times.
- Things come in across the membrane but get used throughout the cytoplasm. So what counts is the surface area-to-volume ratio, how much membrane serves each µm³.
- That's one reason most cells are small. Intestinal cells, which absorb a lot, fold their membrane into microvilli.
On the exam: Divide before you compare, whether per µm³, per gram or per square meter.