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.
Chunk paragraph has a specific meaning in the art of writing. This is what it means, but I don't know of any term for the concept in Spanish. There are one-chunk paragraphs, two-chunk paragraphs, etc. The one-chunk has a topic sentence (or thesis), a fact in support of the thesis, and commentary on that fact.