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.
Write a topic sentence. The first sentence of your paragraph needs to be the topic sentence. A topic sentence is an introductory line that addresses what the main idea or thesis of the paragraph is going to be. It should contain the most important and relevant point you wish to make regarding your topic, thus summarizing the paragraph as a whole. Don't: use an obvious fact as your topic sentence.