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.
In many languages, the fundamental unit of composition is the paragraph. A paragraph consists of several sentences that are grouped together. This group of sentences together discuss one main subject. In U.S. formal academic English, paragraphs have three principal parts. These three parts are the topic sentence, body sentences, and the concluding sentence.