Cells are the basic unit of all living things. The different parts of cells include:
https://www.projectgcse.co.uk/gcse_biology/cells_tissues_and_organs
What does "unit" mean in the given context?
] regarded as single and complete but which can also form an individual component of a larger or more complex whole.
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See sense 1 at https://www.lexico.com/en/definition/unit:
An individual thing [...] regarded as single and complete but which can also form an individual component of a larger or more complex whole.