>if we eat plants , then it has to be lost forever.

It is not lost "forever" the molecules just breakdown and get combined into different things which eventually breakdown themselves or converted into something else, eventually the moluces (hydrogen and oxygen) get back to being water and so the cycle continues.

Nothing is "lost forever" because it has to go somewhere but it'll come back eventually in some shape or form.

Physics - would a ship have to be under constant thrust to have gravity? As there's no friction in space surely a lack of thrust would just mean the momentum would carry on?

Biology - If humans were exposed to less gravity - lets say on mars and grew up in this condition, what would happen to their bones? How would they feel on a planet of different gravity? Would their skeleton be able to grow accustom to it?