Final answer:
The statement that most signaling molecules are found in low concentrations in the extracellular fluid is true. They include both hydrophobic molecules that interact with intracellular receptors, and hydrophilic molecules that bind to cell surface receptors. Signaling can occur through various mechanisms in multicellular organisms, such as endocrine, paracrine, autocrine, and direct signaling via gap junctions.option a.
Step-by-step explanation:
The statement that most signaling molecules are found in low concentrations in the extracellular fluid is true. Cells use signaling molecules to communicate, and these are typically found in very low concentrations, yet they are highly effective at initiating a response. There are different types of signaling molecules and how they interact with cells. Hydrophobic signaling molecules can diffuse directly through the plasma membrane to interact with intracellular receptors that often act as transcription factors to regulate gene expression. On the other hand, hydrophilic signaling molecules bind to receptors on the cell's surface, with the signal being transduced across the membrane without the molecule entering the cell.
Additionally, signaling in multicellular organisms can be categorized into paracrine, endocrine, autocrine, and direct signaling across gap junctions. Endocrine signals involve hormones carried through the bloodstream over long distances, contrasting with paracrine signals which operate over shorter distances. Autocrine signaling involves signals received by the same cell that sent them or by other nearby cells of the same type, while gap junctions allow the passage of small signaling molecules directly between neighboring cells.