Analysis of Cell Signalling Receptor Statements

This question involves evaluating two statements about cell signalling receptors, Assertion (A) and Reason (R), to determine their validity and relationship.

Evaluating Assertion (A)

Assertion (A) states that some cell signalling receptors are located at the cell surface and possess transmembrane domains.

• Cell surface receptors are a well-known class of receptors involved in transmitting signals from the extracellular environment into the cell.
• A key structural feature of many cell surface receptors is the presence of one or more transmembrane domains, which anchor the receptor protein within the lipid bilayer of the plasma membrane.

Therefore, Assertion (A) is factually correct.

Evaluating Reason (R)

Reason (R) suggests that these receptors are at the cell surface because their activating ligands cannot enter the cell interior.

• Many signalling molecules (ligands), such as peptide hormones and growth factors, are hydrophilic and cannot freely cross the hydrophobic lipid bilayer of the cell membrane.
• For these extracellular ligands to trigger a cellular response, receptors must be accessible on the cell surface to bind them.

This explains the necessity of cell surface receptors for such ligands. Therefore, Reason (R) is also factually correct.

Relationship Between Assertion (A) and Reason (R)

Reason (R) explains *why* receptors are located at the cell surface—because the signalling molecules (ligands) that activate them are unable to cross the plasma membrane. Assertion (A) describes the structural characteristic (transmembrane domains) of these cell surface receptors that allows them to be embedded in the membrane. The need for surface localization (explained by R) directly leads to the structural requirement for membrane anchoring (described in A). Thus, Reason (R) provides a correct and logical explanation for Assertion (A).

Conclusion

Both Assertion (A) and Reason (R) are correct, and Reason (R) is the correct explanation for Assertion (A).