Question

Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R). Assertion (A): Some cell signalling receptors are at the cell surface with transmembrane domains that help them embed in the plasma membrane. Reason (R): They are localized at the cell surface as their activating ligands cannot enter the cell interior. In the light of the above statements, choose the most appropriate answer from the options given below:

• A. Both (A) and (R) are correct and (R) is the correct explanation of (A) \
• B. Both (A) and (R) are correct but (R) is not the correct explanation of (A) \
• C. (A) is correct but (R) is not correct \
• D. (A) is not correct but (R) is correct

Hint:

If a ligand is hydrophilic, it cannot cross the lipid membrane. It needs a cell-surface receptor to catch the signal and pass it inside.

Answer 1

The Correct Option is A

Step 1: Concept Cellular communication relies on specific signaling molecules interacting with matching protein receptors. The chemical properties of a signaling molecule determine whether it targets receptors on the surface of the cell or inside the cell.

Step 2: Meaning Hydrophilic or large polar molecules cannot easily diffuse through the hydrophobic core of the plasma membrane, meaning they must pass their signals through surface-bound receptors.

Step 3: Analysis * Assertion (A) is correct: Many signaling receptors are embedded in the cell surface. They feature hydrophobic alpha-helical transmembrane domains that anchor them firmly into the plasma membrane lipid bilayer. * Reason (R) is correct: These receptors are positioned on the outer surface because their target signaling ligands (like peptide hormones or neurotransmitters) are large or highly charged. Because they cannot cross the lipid membrane, they must bind to external receptors to pass their message inside. * Explanation assessment: The cellular location of these receptors described in (A) is directly explained by the chemical traits of the ligands described in (R).

Step 4: Conclusion Both statements are true, and (R) provides the correct logical explanation for (A). Final Answer: (A)