Which structure directly connects the cytoplasm of neighboring cells to allow passage of small molecules?

• A. Gap junctions
• B. Tight junctions
• C. Desmosomes
• D. Plasmodesmata

Answer: (A)

Gap junctions create direct cytoplasmic connections between neighboring animal cells, forming channels that let ions and small metabolites pass from one cell to the next. Each gap junction channel is made of connexons, which are hexameric assemblies of connexin proteins; when two connexons from adjacent cells align, they form a continuous pore. This enables rapid electrical and metabolic coupling, such as the synchronized activity in heart tissue or coordinated signaling in other tissues, because small molecules can diffuse straight through from one cytoplasm into the other.

Tight junctions form seals that prevent paracellular leakage, desmosomes provide strong adhesion without sharing cytoplasm, and plasmodesmata are plant-specific channels through cell walls that connect plant cell cytoplasms. Hence, gap junctions are the structure that directly connects cytoplasm of neighboring cells to allow passage of small molecules.