The Immunity Syndrome

My friend recently completed watching all of the episodes and films of Star Trek made thus far. Inspired by her, I've decided to go back and re-watch all of them myself. I'm now on the second season of The Original Series of Star Trek. I just watched the episode called "The Immunity Syndrome". In this episode, the crew of the Enterprise finds their ship and their bodies losing energy due to a gigantic, single-celled space creature. The single-celled organism looks very much like an amoeba, but one that is 11,000 miles long by 3,000 miles wide (that'd be one very large cell!). 

I love sci-fi stories that consider what other types of life could exist. A single-celled organism that is larger than North America may seem far fetched, but could it be possible that life at some point could evolve to form organisms so large? Even though this episode treats this organism as a giant cell (Spock even announces that it's composed mostly of protoplasm), maybe there could be organisms that are gigantic, contiguous structures as large or even larger than planets. Perhaps they have their own biomes composed of multitudes of other organisms inside of them, or perhaps they could be composed of various connected biological organs and tissues. Would such organisms need to eat worlds to live? Would they be able to develop some way to propel themselves through space? I guess we really wouldn't know if this is possible, until we ourselves step out amongst the stars and take a look to see if there are other forms of life out there.