"Mitochondria IS the powerhouse of the cell" - is this grammatically correct?

If you could give an example from a scientific source, that would be helpful.

The specific phrase "mitochondria is the powerhouse of the cell" or “the mitochondria is the powerhouse of the cell" appears to be a meme. Memes can proliferate even when they aren’t grammatical or well worded; it’s not exactly the same thing as “popular usage” of an ordinary, non-meme phrase or saying.

The choice of just one author to use this wording in some popular, highly imitated online post might have been the ultimate cause of why you see it so often on the internet.

You’re correct that “mitochondria” is a plural noun in standard usage, so the correct phrasing would be instead “mitochondria are the powerhouse of the cell”, “mitochondria are the powerhouses of the cell”, or “the mitochondrion is the powerhouse of the cell.”

It would be, The mitochondria is the power house of the cell. In your example you had used a singular noun. When using a singular objective noun, ore often than not you will add the. One other tip is to check for the type of noun and how it is being used (objective, preperative, general, subjcet, etc.).

You're right to think it's incorrect. It is incorrect. Verb form should agree with the subject, so if using "mitochondria" (plural) for the subject, the verb should use the plural form "are".

However, this usage is ironic in that the phrase is perpetuated by its absurdity. It lives on despite no apparent source in part because the American education system has perpetuated it and in part because students of the English language and students of organic chemistry live to bond over mocking it for its failings. That is, the expression is flawed both in it's grammaticality and in it's capacity to convey useful information about cells.