I'm writing a sentence about energy coupling:

Energy coupling is a phenomenon where cells use the energy released from an exergonic reaction to fuel a different, endergonic reaction.

I can see "endergonic reaction" being regarded as a noun by itself rather than an adjective and a noun, and therefore the sentence wouldn't need a comma. I can also see "endergonic" being an adjective and "reaction" being a noun, so a comma would be needed before "endergonic".

Do I need a comma after "different"?

  • Why include different in your sentence at all? It would be assumed, since you mention two reactions. (The second reaction can't be the same as the first.) Oct 11, 2018 at 5:55

1 Answer 1


The comma is necessary in this case.

Imagine I'm talking about a red car, and I bring up a different, blue car.

The new car is different, in it being blue (although there may be other differences).

Leaving out the comma would be fine if the new car were also red, and yet different from my first red car; it would be a different red car.

So when talking about reactions, you could have a different exergonic reaction or a different, endergonic reaction.

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