Which part of the vesicle contents is responsible for cell communication upon fusion with the plasma membrane?

The part of the vesicle contents that is responsible for cell communication upon fusion with the plasma membrane is ligands. Ligands are signaling molecules that can bind to receptors on the surface of target cells, initiating a series of cellular responses. Upon the fusion of the vesicle with the plasma membrane, ligands are released into the extracellular space where they can interact with specific receptors on neighboring or distant cells. This process is essential for various signaling pathways, including hormonal signaling, neurotransmission, and immune responses.

Other components like enzymes, solutes, and electrolytes play different roles within the cell but do not directly participate in the communication process as ligands do. For instance, enzymes are typically involved in catalyzing biochemical reactions, solutes consist of dissolved substances that may not have signaling functions, and electrolytes assist in maintaining the electrical balance and osmotic pressure in the cell. Thus, while important for cellular function, these components do not directly facilitate cell communication like ligands do.