Final answer:
Signaling proteins that are unique to animal cells are associated with gap junctions, while those absent in animal cells are linked to plasmodesmata in plants. These structures underlie the differences in direct intercellular signaling mechanisms between animals and plants.
Step-by-step explanation:
The types of signaling proteins that are common in animals but not found in plant cells, and vice versa, are differentiated mainly by the gap junctions and plasmodesmata structures in animal and plant cells, respectively. Gap junctions are specialized intercellular connections between animal cells that allow the direct transfer of small signaling molecules and ions, whereas plant cells communicate via plasmodesmata, channels through cell walls that connect the cytoplasm of adjacent plant cells.
Signaling proteins absent from animal cells, such as those related to plasmodesmata, facilitate extensive communication within plant tissues, whereas signaling proteins unique to animal cells are associated with the gap junction mechanism and other intercellular communication systems not present in plants. This reflects a fundamental difference in how multicellular organisms communicate internally, with the main categories of chemical signaling being autocrine, paracrine, endocrine, and direct signaling across gap junctions or through plasmodesmata.