'Cells were grown' is a passive sentence, but 'cells grown on a rotary shaker' is a noun phrase. In other words, the first expresses a complete thought, with a subject and verb, but in the second 'grown' works like an adjective -- it describes what kind of cells you mean. It is a short form of 'cells that were grown on a rotary shaker' (and this phrase has a passive verb), but in the form you ask about, 'grown' works like an adjective. You can read more about how this works in the Simplifying defining relative clauses section of our Defining relative clauses page.
This is the active and passive voice worksheets section. Active voice is when the subject performs the action expressed in the verb. Passive voice is when the subject is no longer active, but is, instead, being acted upon by the verb. In most English sentences with an action verb, the subject performs the action denoted by the verb. However, it is important to learn how to use both the active and passive voice while writing or speaking. Voice ultimately expresses the relationship of the subject to the action. Our active and passive voice worksheets may be used for a variety of grade levels.