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.
A paragraph with unity develops one, and only one, key controlling idea, to ensure unity, edit out any stray ideas that donât belong and fight the urge to take interesting but irrelevant side trips; they only create confusion about your destination. 4.1.2 The Topic Sentence. The topic sentence states the main idea of the paragraph. Think of the topic sentence as a rallying point, with all supporting sentences developing the idea it expresses. A good topic sentence helps you gauge what information belongs in a paragraph, thus ensuring unity.