Final answer:
Cells communicate directly through gap junctions in animals or plasmodesmata in plants, allowing signals like calcium ions to pass between them while larger molecules cannot.
Step-by-step explanation:
Some cells can communicate directly with each other through gap junctions in animals and plasmodesmata in plants. These are connections between the plasma membranes of neighboring cells which act as water-filled channels allowing small signaling molecules to diffuse between the two cells. This means that small molecules such as calcium ions (Ca2+) can move between cells, but larger molecules like proteins and DNA cannot. Direct signaling via these channels is critical as it allows cells to coordinate responses swiftly and efficiently, forming a network of communication throughout the organism.