Chemical signaling molecules, such as hormones, can only act on cells that are in direct physical contact with the signaling cell.

Coordination systems, including the nervous and endocrine systems, are essential for maintaining a stable internal environment (homeostasis).

Cell-surface membrane receptors exclusively bind to lipophilic signaling substances that easily pass through the phospholipid bilayer.

Plants do not require internal communication systems because they do not respond to external environmental stimuli.