Paragraph structure creates coherence
We take paragraphs for granted. But look at an old printed text that lacks this typographical device, and you will find yourself lost in a “river of words.” Thank the “printers and grammarians who invented typographical devices to mark off meaningful segments of thought.”  Indentation (to mark a new paragraph), serves to telegraph that a shift in the development of the writer’s thinking is afoot. As writers, in thinking about how we construct paragraphs, we are thinking about our readers, working with a typographical device that helps to explicate and organize our thoughts and argument so that readers can follow our thought process.
Paragraphs: the basic unit of composition
The paragraph, as William Strunk says in The Elements of Style, is the basic unit of composition.  As with larger units, such as the paper itself, a paragraph should have a clearly defined structure, which readers have come to expect. First off, each paragraph should contain one idea. This one main idea (or claim or thesis sentence) is usually expressed in the first or second sentence and is known as the topic sentence. In the ideal paragraph, the author uses the rest of the paragraph to develop that idea, providing reasons and evidence, and finally provides a concluding sentence that might amplify or expand the original claim by addressing consequences or implications of the idea.
Five sentences in length
As a rule of thumb, a paragraph should be roughly five sentences. Longer paragraphs are visually daunting, and at the least suggest a writer nor mindful of the reader, who, it must be said, may not really want to read what you have to say that badly. In sum, the essential, well-structured paragraph would present a topic sentence, three to five sentences providing reasons and evidence, and end with a concluding sentence.
Topic sentences key to well-structured paragraph
Many authors overlook the basic rules of paragraph writing or may not know them well. Rowena Murray , author of Writing for Academic Journals describes lecturing on academic writing and discussing paragraph structure and its role in argumentation with her audience of academic authors. When she asked if they knew the meaning of the term topic sentence, she was frequently met with silence. She came to the conclusion that many academics have gaps in their knowledge about how sentences and paragraphs work. Gaps or not, all authors should keep the basic rules of paragraph writing in the forefront of their thinking as they write.
Importance of transition words
Many authors write paragraphs that are too long or too short, paragraphs that lack transition words or have the wrong transition word. Transition words help to unify your argument. Kate Turabian,  author of A Manual for Writers of Research, Papers, Theses, and Dissertations, recommends that as you structure the ideas that support your claim, ideally in an outline form, you label each idea with the transition word that identifies whether the paragraph will present a contrasting idea (however, on the other hand, in contrast, etc.) or if it adds more evidence (additionally, furthermore, in addition, etc.), and so on. These words should be organic, that is, they should fit and be the right word, so it is important to be clear when you write each paragraph what its function and purpose is in the whole paper.
Editing your work
When editing your first, second, or third draft, you can often improve your paragraph writing style by combining paragraphs that are too short; breaking up paragraphs that are too long. Consider varying length for the sake of pacing. For example, you might use a two-sentence paragraph to emphasize a point or mark a transition. Pay close attention to your topic sentences to make sure that they provide the main idea of the paragraph. The appropriate transition or connecting word will help with cohesion and flow. However, connecting words by themselves cannot provide coherence to a paper. True coherence comes from connecting the thoughts within the paragraphs and in using syntax and the sense of the sentence to advance the narrative.
1. Corbett, Edward and Robert Connors. 1999. Classical Rhetoric for the Modern Student. New York: Oxford University Press.
2. Strunk, William. 1918. The Elements of Style. Reprint of the 1918 edition, http://www.bartleby.com/141/strunk5.html#9.
3. Murray, Rowena. 2009. Writing for Academic Journals. Berkshire, England: Open University Press.
4.Turabian, Kate. 2007. A Manual for Writers of Research, Papers, Theses, and Dissertations. Chicago: University of Chicago Press.