What type of ligands primarily use membrane-bound receptors?

• A. Hydrophobic/lipophilic ligands
• B. Hydrophilic/lipophobic ligands
• C. Hydrophilic/lipophilic ligands
• D. Hydrophobic/lipophobic ligands

Answer: (B)

Membrane-bound receptors are primarily utilized by hydrophilic or lipophobic ligands. These ligands cannot easily cross the lipid bilayer of the cell membrane due to their polar nature, which makes them soluble in water but not in lipids. As a result, they rely on specifically structured receptors embedded in the cell's membrane to generate a response.

When a hydrophilic ligand binds to its membrane receptor, it typically triggers a signal transduction pathway that can result in changes within the cell, such as opening ion channels or activating second messenger systems. This process is essential for cellular communication and enables the cell to respond to various external signals, including hormones and neurotransmitters.

In contrast, hydrophobic or lipophilic ligands can often pass through the lipid bilayer and bind to receptors inside the cell, such as in the cytoplasm or nucleus, leading to different mechanisms of action. This difference in solubility and receptor location underlines the significance of the types of ligands and their corresponding receptors in physiological processes.