In order to detect a signal (that is, to be a target cell), a neighbor cell must have the right receptor for that signal. When a signaling molecule binds to its receptor, it alters the shape or activity of the receptor, triggering a change inside of the cell. Signaling molecules are often called ligands, a general term for molecules that bind specifically to other molecules (such as receptors).
A chunk paragraph is just a way of developing your skill in writing - it is a style or format which forces you to explain your ideas and arguments. Here's you do chunk writing: Write your topic sentence.
The part of an essay, report, or speech that explains and develops the main idea (or thesis). Body paragraphs come after the introduction and before the conclusion. The body is usually the longest part of an essay, and each body paragraph may begin with a topic sentence. See Examples and Observations below. Coherence and Cohesion