From insulin receptor to GLUT4 at the membrane
When insulin binds its receptor, the receptor autophosphorylates and recruits insulin receptor substrate proteins such as IRS-1. Phosphorylated IRS-1 activates PI3-kinase, which generates PIP3 and turns on Akt (also called PKB). Active Akt drives the translocation of GLUT4 glucose transporters from internal vesicles to the cell surface in muscle and fat, allowing glucose to enter the cell.
This branch is the metabolic arm of insulin action. A separate Ras/MAPK branch handles more of the growth-related signaling, which is one reason insulin resistance can be selective rather than affecting every downstream effect equally.
How serine phosphorylation can blunt the signal
In many models of insulin resistance, intracellular lipid intermediates such as diacylglycerol (DAG) activate certain protein kinase C isoforms, including PKC-theta in muscle. These kinases can phosphorylate IRS-1 on serine residues such as Ser307, which interferes with the normal tyrosine phosphorylation and weakens PI3K/Akt signaling, so less GLUT4 reaches the membrane.
This is one widely studied mechanism rather than the whole story; inflammation, mitochondrial factors, and genetics also contribute, and the relative importance is still debated. This summary is educational and not a basis for diagnosing or treating any condition.