system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do single celled beings "know" what they're supposed to do/"want" to do?
gpt
I personally like to think of simpler life forms as like tiny computer programs. Their DNA is their "source code," and the cell itself is the hardware it runs on.

So a tiny cell 'knows' to avoid being eaten by larger cells because it's "source code" told it to build receptors on its cell membrane that are programmed to detects certain nearby chemicals, and one particular chemical triggers a flight response.

Other chemicals do the opposite -- they indicate a source of nutrients, and cause a chemical chain reaction that makes the cell move toward the food source. Once in range, various other mechanisms begin to operate, drawing in the food and disassembling it for processing.

But everything was built by following the template in the DNA, and certain triggers result in certain behavior.