Onion cells - the purple spherical shapes in the cells are nuclei, a type of organelle
Individual cells are made out of even smaller parts called organelles.
Multicellular organisms have tissues: groups of similar cells. Tissues make up organs, which are groups of different types of tissues adapted for a particular function. Organs working together can perform a specific process, they are called organ systems.
An example in the heart:
Nucleus (organelle) > Muscle cell > Cardiac (heart) muscle tissue > Heart > Circulatory system > Human
So generally the order is:
Organelle > Cell > Tissue > Organ > Organ system > Whole organism
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