Final answer:
Receptors are protein molecules in the target cell or on its surface that bind to ligands. There are two types of receptors, internal receptors and cell-surface receptors. Internal receptors are found in the cytoplasm and respond to hydrophobic ligands, while cell-surface receptors have three main components and bind ligands on the outside of the cell.
Step-by-step explanation:
Receptors are protein molecules in the target cell or on its surface that bind to ligands. There are two types of receptors, internal receptors and cell-surface receptors. Internal receptors, also known as intracellular or cytoplasmic receptors, are found in the cytoplasm of target cells and respond to hydrophobic ligand molecules that are able to travel across the plasma membrane. Once inside the cell, many of these molecules bind to proteins that act as regulators of mRNA synthesis (transcription) to mediate gene expression.
Each cell-surface receptor has three main components: an external ligand-binding domain called the extracellular domain, a hydrophobic membrane-spanning region called a transmembrane domain, and an intracellular domain inside the cell. The ligand-binding domain is also called the extracellular domain. The size and extent of each of these domains vary widely, depending on the type of receptor.