Chapter 5

All life-forms on Earth arguably exist either as single cells or as collections of cells. All such cells have a plasma membrane at their periphery (although many cells have cell walls outside their plasma membrane). Why do you think the plasma membrane exists in all living things? Why aren't there living things that don't have a membrane at their periphery?

Because materials move from high concentration to low concentration via sim- ple diffusion, and because they move the same way via facilitated diffusion, why do cells make proteins to carry out facilitated diffusion? Isn't this just a waste of energy?

Place a number of marbles on one end of a cafeteria tray. Shake the tray gently a few times, keeping it level. What happens to the marbles? Start over and shake faster the next time. What happens now? Explain how this is like chemical diffusion.