Cells communicate with each other via small, signaling molecules that are produced by specific cells and received by target cells. This communication system operates on both a local and long-distance level. The signaling molecules can be proteins, fatty acid derivatives, or gases. For example, nitric oxide gas is part of a locally based signaling system and is able to signal to lower a human’s blood pressure.

Hormones are long-distance signaling molecules that must be transported via the circulatory system from their production site to their target cells. For example, plant cells, because of their rigid cell walls, have cytoplasmic bridges called plasmodesmata that allow cell-to-cell communication, whereas animals use gap junctions to transfer material between adjacent cells.