Plastids are organelles that have specific pigments which impart colours to the organism. They are found in plants and euglenoids. On the basis of pigment contained they can be divided into three types that are : i. Chloroplasts ii. Chromoplasts iii. Leucoplasts.
Receptor molecules are specific proteins in the cell's plasma membrane that receive chemical signals from outside the cell. When such chemical signals bind to a receptor, they cause some form of cellular response.