Hi,
I found this excerpt on a site called http://www.athabascau.ca/courses/engl/353/essay.html
I hope it will help you.
Best wishes, Clive
There are two primary methods of organization .....: block and alternating. As the name implies, block organization requires that the writer first write about topic A, and then topic B: if you are writing about men and women, you would write your first half of the essay about men, and the second half about women. The important part of this approach is that you must be sure to create a transition between the two, otherwise you will have written two separate and unrelated sections—this is not an essay.
The alternating method, as it suggests, alternates point by point: you would write about men, then women, then men, then women and so forth. This can be a trickier approach because it is not just about one successful transition, rather, about numerous smooth transitions. The danger of this method is that the essay can sound choppy if you switch between them too often.