Question

Cells can terminate signal transduction by cell surface receptors by A. reducing agonist availability in the vicinity of the target cell. B. internalizing and degrading the receptor-agonist complex. C. modifying the receptor so that it is inactive or desensitized. D. all of the above. E. none of the above.

Short answer

Answer: All of the above (A, B, and C). Cells can terminate signal transduction by reducing agonist availability, internalizing and degrading the receptor-agonist complex, and modifying the receptor to make it inactive or desensitized.

Step-by-step solution

Understand the Cell Surface Receptor-Signal Transduction Process

First, it's essential to have a basic understanding of the cell surface receptor-signal transduction process. Cell surface receptors are proteins that help transmit signals across the cell membrane. These signals are typically initiated by extracellular molecules (agonists) that bind to the receptor. This binding event triggers a cascade of intracellular signaling events, which ultimately affect cellular processes and functions.

Analyze Option A - Reducing Agonist Availability

Option A suggests that signal transduction termination can occur by reducing the availability of the agonist near the target cell. This is accurate, as reducing the concentration of the signaling molecule would decrease the probability of binding and initiating the signal transduction process.

Analyze Option B - Internalizing and Degrading the Receptor-Agonist Complex

Option B states that internalizing and degrading the receptor-agonist complex can terminate signal transduction. This is also true. When the receptor-agonist complex is internalized, the signaling process is interrupted. Within the cell, the complex can be degraded, separating the agonist and receptor to end the current signaling event.

Analyze Option C - Modifying the Receptor

Option C mentions that modifying the receptor to make it inactive or desensitized can terminate signal transduction. This is also accurate. Cellular processes such as phosphorylation can lead to receptor desensitization or inactivation. When a receptor is desensitized or inactive, it is unable to initiate or transmit signals, effectively terminating the signal transduction.

Determine the Correct Answer

As we have analyzed each of the statements A, B, and C, we have discovered that all of them are accurate mechanisms through which cells can terminate signal transduction by cell surface receptors. Therefore, the correct answer is option D, which states "all of the above."

Key concepts

Cell Surface Receptors

Understanding signal transduction is rooted in understanding cell surface receptors, which are crucial for the communication of cells with their external environment. Imagine these receptors like radio antennas that catch specific signals—in the body, these signals are chemical messengers, or agonists, like hormones, neurotransmitters, and other signaling molecules.

Once an agonist is detected by these receptors, the cell surface receptor undergoes a conformational change. This shape-shift starts a sequence of reactions inside the cell, known as a signaling cascade. It's as if the cell suddenly wakes up and gets to work, executing specific instructions encoded by the signal. In a way, cell surface receptors can be seen as the translators of external messages into internal actions that maintain the body's balance and respond to changes in the environment.

Agonist Availability

The 'conversation' a cell has with its environment can be muted by decreasing the availability of the agonist, akin to lowering the volume on a loudspeaker. Agonist availability refers to the presence of those chemicals that bind to receptors to initiate a signal.

If the concentration of an agonist around a target cell drops, maybe due to enzymes breaking the agonist down or by its uptake into other cells, the chance of it binding to a cell's receptors plummets. This would effectively interrupt the signal transduction. Without an agonist, the receptor remains 'off,' so to speak, waiting for the next signal that may or may not come.

Receptor-Agonist Complex

When an agonist and receptor come together, they create what's known as a receptor-agonist complex. This complex is the starting gun for the signaling cascade. However, the body has a neat trick: after a cell 'reads' the signal's message, it can take this complex inside itself, a process called endocytosis, and then dismantle it.

This is where a cell's internal recycling system comes into play—breaking down the complex to either reuse the parts or dispose of them entirely. It's as if the cell has a 'read and shred' policy for some messages to ensure those signals stop when they have served their purpose, thus preventing overactivity that could lead to cellular chaos.

Receptor Desensitization

Like people tuning out a repetitive noise, receptors can also become unresponsive or 'tired' after being exposed to the same signal for too long, a process known as receptor desensitization. Desensitization can happen through several mechanisms, such as phosphorylation, where a phosphate group is added to the receptor to change its shape and functionality.

In this changed state, the receptor no longer listens to its regular agonist, essentially muting the signal. This is how the body regulates overly prolonged or intense signals, safeguarding cells from excessive stimulation and maintaining homeostasis—a stable and balanced internal environment.