Question

Explain why the Signal Transduction Pathway is exclusive for polar signal molecules.

Answer 1

Cell membrane is selectively permeable and it is composed of phospholipid, the polar molecules are not allowed to pass through the lipid bilayer. But for lipid soluble molecules they can pass through the cell membrane and enters the cytoplasm as well as nucleus. For example, steroid hormones which also acts as signal molecules are nit restricted by plasma membrane and they diffuse in to the where they bind with intracellular receptor and directly goes t o nucleus and affects transcription of specific genes. But as polar molecules cannot pass the lipid bilayer, they need a cell membrane receptor with which they bind and make conformational cchenges in the receptor. The receptor has both extracellular as well as intracellular domain. When extracellular part binds with polar signal molecules, there are conformational changes in the intracellular part of the receptor. This causes activation of secondary messenger in the intracellular cytoplasm. This second messengers convey the message from the polar signal molecule or primary messenger to the effector molecule inside the cell. Like a relay race, information from polar signal molecules pass through receptor to second messenger and then to third messenger and so on. As polar molecules cannot pass the plasma membrane, signal transduction pathways are exclusive for them.