Advanced Placement (AP) Biology Practice Exam

Which type of ligands typically do not enter the cell?

1. Water soluble ligands

2. Lipid soluble ligands

3. Peptide ligands

4. Hormonal ligands

The correct answer is: Water soluble ligands

Water-soluble ligands typically do not enter the cell because they cannot easily cross the lipid bilayer of the cell membrane. The structure of the cell membrane is composed of phospholipids, which create a hydrophobic (water-repelling) environment. As a result, water-soluble ligands, which include many proteins, peptide hormones, and neurotransmitters, tend to bind to specific receptors located on the surface of the cell instead of penetrating the membrane. By binding to these cell surface receptors, water-soluble ligands can initiate a signaling cascade inside the cell that can lead to a variety of cellular responses, such as changes in gene expression, metabolic activities, or cellular behavior. This process allows the cell to respond to external signals without requiring the ligand to pass through the membrane, thereby maintaining the integrity of the lipid bilayer and regulating the internal environment of the cell. In contrast, lipid-soluble ligands, such as steroid hormones, readily diffuse through the membrane and can directly enter the cell, where they often interact with intracellular receptors. Peptide ligands are a subset of water-soluble ligands and share the same properties and limitations in terms of membrane permeability. Hormonal ligands can be either water-soluble or lipid-sol