UNIT 14 – Signal Transduction (Q&A) | MZO-001 MSCZOO | IGNOU

SAQ 1

Mark the correct option:

a) The cellular response to a particular signaling molecule depends on the number of
    i) receptor
    ii) ligand
    iii) receptor-ligand complexes
Answer: iii) receptor-ligand complexes

b) The binding of receptor to ligand binding is mediated through weak
    i) non-covalent forces
    ii) covalent forces
Answer: i) non-covalent forces

c) Different receptors of the same class that bind different ligands often induce the
    i) same cellular responses in a cell
    ii) different cellular responses in a cell
Answer: i) same cellular responses in a cell

d) Many signaling molecules bind to multiple types of receptors, each of which can activate different intracellular signaling pathways and thus induce
    i) same cellular responses
    ii) different cellular responses
Answer: ii) different cellular responses

SAQ 2

Fill in the blanks:
a) The binding of ligands to many cell-surface receptors leads to short-lived changes in the concentration of certain low-molecular-weight intracellular signalling molecules termed ......................... .
Answer: second messengers

b) The down regulation in receptor response with repeated or chronic exposure to ligand is called ....................... .
Answer: desensitization

c) The aggregated mass of certain lipids in plasma membrane of mammalian cells is termed as ................... .
Answer: caveolae

d) The time for which the switch protein remains in the active conformation and able to signal downstream depends on the rate of ..................... hydrolysis.
Answer: GTP

TERMINAL QUESTIONS

1. What are the common features of the signaling system?

Cell signaling is the way cells talk to each other and respond to their environment. It involves a series of biochemical steps where an external signal (like a hormone, neurotransmitter, or growth factor) binds to a specific receptor on the cell surface or inside the cell. This interaction starts a chain of events inside the cell which finally brings a proper cellular response. These signals help regulate important processes like cell growth, division, metabolism, immune defense and programmed cell death (apoptosis). Although there are different types of signaling pathways, most of them follow a common set of features that help in maintaining the accuracy, strength and timing of the signal inside the body.

Common Features of the Signaling System

1. Specificity

Each signaling molecule, also called a ligand, is recognized by only its specific receptor. These receptors are usually proteins present either on the cell membrane or inside the cell. This receptor-ligand specificity ensures that only the right cell receives and responds to the message and wrong or unrelated cells do not get affected. This high specificity prevents miscommunication between cells.

2. Amplification

One signal molecule can activate many internal signaling molecules through a cascade of biochemical reactions. For example, one hormone molecule can activate several enzymes and each of those enzymes can produce hundreds of molecules as response. This chain reaction is called a "signaling cascade" or "amplification" which helps to magnify the effect of even a small amount of signal.

3. Desensitization or Adaptation

If a signal remains present for a long time, the cell starts to ignore it. This is known as desensitization. For example, if a cell continuously gets stimulated, it might reduce the number of receptors or deactivate some pathway molecules. This helps protect the cell from overstimulation and allows it to maintain balance.

4. Integration

Cells often receive multiple signals at the same time. These may be positive (activating) or negative (inhibiting). The signaling system integrates all these different signals and gives a final, balanced response. This integration ensures that the cell responds correctly even when it is exposed to complex and changing conditions.

5. Termination

Once the signal has been transmitted and the required response is done, the signaling process is stopped or terminated. This is important to avoid overreaction or unnecessary continuation. Termination happens through degradation of signaling molecules, removal of the ligand, or deactivation of the receptor or downstream proteins. This reset allows the cell to be ready for the next signal.

2. What are the general principles of signal transduction, applicable to different signaling pathways?

Signal transduction is a fundamental process through which cells sense and respond to signals from their external environment. It enables communication between cells and helps regulate a wide range of cellular activities such as growth, division, differentiation, metabolism and survival. These signals are usually in the form of chemical messengers like hormones, neurotransmitters, or growth factors that bind to specific receptors either on the cell surface or within the cell. Once a signal is received, it is converted into a series of biochemical events inside the cell, ultimately leading to a specific functional response. This entire process ensures that cells can properly adapt and maintain homeostasis under changing physiological conditions.

General Principles of Signal Transduction

1. Specificity of Signaling

One of the fundamental principles of signal transduction is that signaling pathways are highly specific. Each receptor is designed to recognize and bind to a particular ligand, such as a hormone or neurotransmitter. The specificity of receptors ensures that the signals are accurately transmitted to the appropriate cellular machinery without interference from other signals.

2. Reversible Binding and Activation

Signal transduction often involves the reversible binding of ligands to receptors, which triggers the activation of intracellular signaling cascades (A cascade is a series of events triggered sequentially). The interaction between the ligand and the receptor is transient, ensuring that the signaling process remains flexible and responsive to changes. This allows for the dynamic regulation of cellular responses.

3. Amplification of Signal

A crucial feature of signal transduction is the amplification of the signal. A single receptor-ligand interaction can trigger a cascade of downstream events that result in the activation of multiple molecules within the cell. This amplification ensures that even low concentrations of a signal can produce a significant cellular response. A good example of this is the activation of second messengers such as cAMP (cyclic Adenosine Monophosphate), which can lead to the activation of various enzymes and proteins.

4. Integration of Signals

In multicellular organisms, cells often receive multiple signals simultaneously. Signal integration allows the cell to combine and interpret different signaling inputs to produce a coherent response. This integration is essential for processes like cell differentiation, immune response and maintaining homeostasis. Cells use complex networks of signaling pathways that intersect at various levels to ensure that responses are appropriate to the combination of signals received.

5. Signal Termination

To prevent overactivation and ensure that the response is appropriately controlled, signal transduction pathways must be terminated after the signal has been processed. This termination can occur through mechanisms like receptor desensitization, degradation of second messengers, or deactivation of signaling proteins. The ability to terminate signals helps cells reset their state and prepare for the next stimulus.

6. Modulation and Feedback Regulation

Signal transduction is not a one-way process; it involves feedback regulation. Positive feedback mechanisms can enhance the signaling pathway, while negative feedback mechanisms can suppress it. Feedback loops are important for maintaining the balance and appropriate intensity of cellular responses. This regulatory control ensures that signaling pathways are adaptable and respond to the needs of the cell and organism.




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