recap

7.2 recap

Receptors are proteins that bind, or are changed by, specific ligands or physical signals. The changed receptor initiates a response in the cell. These receptors are located in the cell membrane or inside the cell.

learning outcomes

You should be able to:

  • Explain how and why different cells respond to different chemical signals.

  • Using equations, describe the chemical kinetics of receptor–ligand interactions.

  • Analyze dissociation constants as a measure of binding affinity.

  • Compare and contrast cell membrane receptors and intracellular receptors.

Question 1

What is the dissociation constant KD, and what does it tell us about the likelihood that a signal will produce a cellular response?

The dissociation constant is the ratio of the rate constant for dissociation of a ligand and its receptor to the rate constant for binding. The higher the KD, the less likely it is that a signal will bind to its receptor, and the less likely it is that a cell will respond to a signal at particular concentrations of ligand and receptor.

Question 2

What is the chemical nature and importance of specificity in the binding of a receptor to its particular ligand?

The chemical nature of receptor–ligand bonding is shape and the presence of noncovalent interactions, such as hydrophobic and ionic interactions. This provides specificity. Specificity is important to ensure an appropriate biological response to each signal.

Question 3

What are the differences between receptors located in the cytoplasm and those at the cell membrane?

Cytoplasmic receptors lack nonpolar regions that allow insertion into the cell membrane. Their exteriors are polar, so they can interact with water in the cytoplasm. While membrane receptors stay in the membrane, cytoplasmic receptors may be translocated to the nucleus after binding to their ligand.

Now that we have discussed signals and receptors, let’s examine the characteristics of the molecules (transducers) that mediate the cellular response.