Question

Plasma membrane receptors must somehow let the inside of the cell “know” that a ligand/signal is present on the outside of the cell. In most cases, the ligand does NOT enter the cell. How does the inside of the cell “get the message”? Name two specific mechanisms for how this can be achieved. List and describe each mechanism. Then provide one example for each.  

Mechanism 1 (including example):

Mechanism 2 (including example):

Answer 1

For a cell to respond to a particular signal, it should have the necessary receptor for recognition and binding of the signal molecule. In most of the case signal molecule do not travel inside the cell. The signal is transduced inside the cell through following mechanisms :

Mechanism 1 :

Cell signalling through changes in conformation of the signal receptor : Cell-surface receptors are membrane-anchored proteins that bind to ligands on the outside surface of the cell. In this type of signaling, the ligand does not need to cross the plasma membrane. A typical cell-surface receptor has three different domains, or protein regions: a extracellular ("outside of cell") ligand-binding domain, a hydrophobic domain extending through the membrane, and an intracellular ("inside of cell") domain, which often transmits a signal. There are many kinds of cell-surface receptors, here three common types: ligand-gated ion channels, G protein-coupled receptors, and receptor tyrosine kinases. Recognition of the signal molecule by these receptors and its binding leads to a conformational change in these receptors which initiates the cascade response inside the cell.

Mechanism 2 :

Cell signalling through second messenger system : Second messengers are intracellular signaling molecules released by the cell to trigger physiological changes such as proliferation, differentiation, migration, survival, and apoptosis. Examples of second messenger molecules include cyclic AMP, cyclic GMP, inositol trisphosphate, diacylglycerol, and calcium. The cell releases second messenger molecules in response to exposure to extracellular signaling molecules—the firstmessengers. Most peptide hormones and neurotransmitters typically are biochemically hydrophilic molecules, hence, these first messengers may not physically cross the phospholipid bilayer to initiate changes within the cell directly. This functional limitation necessitates the cell to devise signal transduction mechanisms to transduce first messenger into second messengers, so that the extracellular signal may be propagated intracellularly. An important feature of the second messenger signaling system is that second messengers may be coupled downstream to multi-cyclic kinase cascades to greatly amplify the strength of the original first messenger signal. For example, RasGTP signals link with the Mitogen Activated Protein Kinase (MAPK) cascade to amplify the allosteric activation of proliferative transcription factors such as Myc and CREB.