Hello Jessica,
I can see how this could be confusing. Yes, I would say that requirements are part of the scope. Indeed, both the Scope Management Plan and the Requirements Management plan are outputs of the Plan Scope Management process! I suggest that reviewing the respective paragraphs in the PMBOK might be helpful here (5.1.3.1 for Scope Management Plan, and 5.1.3.2 for Requirements Mangement Plan -- PMBOK Guide 5th Edition).
The Scope Management plan covers things like how you are going to develop a project scope statement, developing and maintaining your WBS, how you will get your customer to formally approve deliverables, and how you will deal with change reqests to the scope statement.
The Requirements Management Plan deals more specifically with requirements -- how are you going to collect them, how you will prioritize them, how you will trace them, etc.
So while the two are intimately related, perhaps it is best just to think of it this way: the requirements are so important we dedicate an entire plan to covering them!
I hope this helps.