Final answer:
Cells secrete molecules as signals to communicate with one another. Paracrine signaling involves the secretion of chemicals that act on nearby cells, while endocrine signaling pertains to the secretion of hormones that travel through the bloodstream to reach distant cells.
Step-by-step explanation:
When cells need to perform less direct contact they will secrete molecules as their signals.
Cells communicate with each other using several mechanisms. For short distances, paracrine signaling is a process where signals act locally between cells that are situated close to each other. Paracrine signals move by diffusion through the extracellular matrix, eliciting quick responses that remain for a short duration. To prevent these signals from affecting distant cells, paracrine ligands are quickly degraded by enzymes or taken up by neighboring cells, maintaining the localized response. This allows a concentration gradient to be reestablished if the molecules are released again, facilitating rapid diffusion.
For long-distance communication, cells use endocrine signaling, where signals (often hormones) are carried through the bloodstream to distant target cells. Additionally, cells can signal to themselves via autocrine signaling, or directly to adjacent cells through gap junctions.