Final answer:
Gap junctions in animal cells and plasmodesmata in plant cells permit the passage of small molecules from one cell to its adjacent cell, facilitating direct signaling and coordination in large groups of cells.
Step-by-step explanation:
Gap junctions permit the passage of small molecules from one cell to its adjacent cell. These structural features in animal cells and plasmodesmata in plant cells create channels between the plasma membranes of neighboring cells. They facilitate the movement of small molecules and ions and are crucial for direct signaling across cells, allowing for electrical and metabolic coupling that coordinates function across groups of cells.
Gap junctions consist of two connecting protein rings embedded in the cell membrane of adjacent cells, allowing the passage of water, ions like calcium ions (Ca²+), and small solutes, but not larger molecules such as proteins and nucleic acids. The specificity of these channels enables cells to remain structurally independent while still being able to transmit signals efficiently, making the entire organism a giant communication network.
In contrast, the plasmodesmata are connections that pass between adjacent plant cells' cell walls, connecting their cytoplasm. Both gap junctions and plasmodesmata are pivotal for transporting materials from cell to cell, which is essential for the coordination of cellular activities.