This word is acceptable for play in the US & UK dictionaries that are being used in the following games:

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n. A differentiated structure within a cell, such as a mitochondrion, vacuole, or chloroplast, that performs a specific function.

n. A specialized structure found inside cells that carries out a specific life process (e.g. ribosomes, vacuoles).

n. a specialized part of a cell performing a specific function, usually visible under the microscope as a distinct object; it is analogous to an organ{2}, but on a microscopic scale.

n. a specialized part of a cell; analogous to an organ

"You would conjecture that some organelle is designed, say."

organ   

centriole    chondriosome    mitochondrion    cell nucleus    lysosome    karyon    nucleolus    ribosome    nucleole    nucleus   

cell organ