system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 How do microbial lifeforms with no brain like parameciums, amoeba, bacteria, etc know what to do and where to go?

I've seen videos of them chasing down food and eating it, if they have no brain and no sensory organs, how do they do this?
gpt
Generally, they dont. Many can't move intentionally at all (microbes in the ocean are totally at the mercy of water currents/turbulence). The ones who do intentionally move, like amoebas, are only complex enough to sense certain chemicals in one direction, or light vs shadow, and move toward/away from it.