To get from Boston to San Francisco, a person has a few choices: drive a car, hop on a bus, fly in an airplane, hitchhike from city to city, maybe even ride a bike. Circumstances will dictate which way works best. If the traveler is in a hurry, the bus might take too long. Carrying a large suitcase makes a bike impractical. And if the person is short on cash, a plane ticket may be too expensive.

In every type of living cell, materials jet around in a similar variety of manners. The way cellular cargo travels depends on its size, where it’s headed, how quickly it must arrive, and how much energy is available. Some chemicals circulate passively through a cell, with no need for energy or a road, but others—like building materials needed at the end of a growing cell or chemical messengers that must reach the nucleus—need to move quickly toward a set destination.

Illustration: Adam Simpson