The nucleolus is a naked, round or slightly irregular structure in the nucleus. It lacks a membrane and its contents are in direct contact with the nucleoplasm. It is a site for active ribosomal RNA (rRNA) synthesis.
Microtubules take part in the spindle formation. Mitochondria, vacuoles, and plastids. etc. are membrane-bound structures. The dividing cells possess a large number of mitochondria.
