Molecular and Cellular Behavior

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Signal transduction pathways link signal reception with cellular response
Signal transduction pathways regulate what occurs in, between, and amid individual cells. They are incredibly crucial because they initiate large responses beginning with a single signal. These chemical signals are called ligands that enter the cell either through active or passive transport. If the ligand is small and nonpolar (such as nitric oxide, oxygen gas, and carbon dioxide), then it will simply diffuse through the cell membrane and will attach to an intracellular receptor. However, if the ligand does not fit into this category it will attach to a membrane receptor. These ligands are specific to each receptor. This ensures that no ligand will bind to an incorrect receptor. Once a receptor is bound, a signal is transduced in a way that the cell can acknowledge it. Signal transduction pathways will fundamentally affect gene expression, change enzyme or protein activity, and possibly cell death.

Membrane Receptors
Membrane receptors span the membrane of the cell. Since hydrophilic molecules cannot simply diffuse through the cells to send a signal to the cell, they must attach to these receptors to be able to send a signal inside the cell.

Ligand -Gated Ion channel receptor
Specific signal molecules open and close ion channel receptors that permit the passing of ions in and out of the cell. These ions include Na+, K+, Ca2+, or Cl-, necessary for sending chemical signals.

G-protein-coupled
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