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.
The chunking theory is closely related to the computational model known as EPAM (Elementary Perceiver and Memorizer) derived by Edward Feigenbaum and Herbert Simon. This model suggests that in order to become an expert, information is obtained by learning a sizeable database of chunks cataloged by a discrimination net.