We look for some form of movement like walking, talking, breathing etc. or growth in the body part like height, size to confirm the organism is live. But this is not always true. For example, we can not detect a movement in plants and unicellular organism easily.
A cell of a unicellular organism is in direct contact with the environment and intake of oxygen occurs directly by diffusion. While Many cells of multi-cellular organisms like human are not in contact with the external environment.