Imagine you are being lifted gently into a gargantuan lake, filled with fresh water.
You sink into the water, and you and the membrane around you shrink to the size of a microbial cell, which is 1-2 microns in size, tinier than a dust mote, tinier than a blood cell. Now the membrane becomes your gatekeeper, permeable to the nutrients you need to take in or the oxygen you have to use for your metabolism.