This has nothing to do with biology. An astronaut's cells aren't made any different or "altered by time".
In the astronaut's rest frame, one second passes just as any other second. The astronaut feels nothing different because nothing is different. But when the astronaut is reunited with someone on Earth, we find the two have aged different amounts. Why? They have taken different paths through spacetime. Just as driving two different ways to your friend's house may give different traveled distances, traveling between two spacetime events (astronaut's departure and reunion) in different ways may give different elapsed time.
For more details, you can see the FAQ or read about the twin problem in special relativity.