CoachA unicellular living thing is a complete organism of exactly one cell (Remark 29.9). In one membrane it feeds, moves, senses and reproduces — by dividing in two (Proposition 29.7’s division, as a whole life story). The yeast of Example 43.5 is one; so are the swimmers of any pond drop, and most microbes of soil, yogurt and hands.
Examples
Example 45.5 (A drop of pond water, toured)
One drop from the aquarium’s green wall, under the lens: a slipper-shaped cell rowing past with beating hairs, faster than anything else in view; green single cells — swimming producers, catching light like any leaf; a blob that flows around its food and swallows it whole. Each is feeding, moving, avoiding — a complete animal-or-plant’s life, budgeted into one cell.