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.
paragraph in British English. paragraphnoun [ C ] B1 a short part of a text, consisting of at least one sentence and beginning on a new line. It usually deals with a single event, description, idea, etc. Can you pick out the three deliberate mistakes in this paragraph? Cut that paragraph and then paste it at the end of the page.