Question

Choose one out of paracrine signalling, endocrine signalling, autocrine signalling and direct signalling and describe briefly the process by which this occurs.

Pick a representative signal molecule from the pathway you have chosen and indicate its function, the type of cell in which it is made and the specific manner in which it acts as a signalling molecule, including the receptor that it binds to.

Answer 1

Endocrine signaling- This type of signaling occurs over long distances in the body. Signal is produced in the form of chemical hormones. These hormones are released from specialized glands, and are carried to target cells via the bloodstream. Some examples of endocrine glands present in our body are- thyroid, pancreas, pituitary, hypothalamus, etc.

Insulin signaling is an example of this type. Here, insulin acts as a signal molecule, which functions to lower the blood glucose level, by promoting it's uptake by fat and muscle cells. Insulin is made by the beta cells of Islets of Langerhans present in the Pancreas. A brief description of how insulin acts a signaling molecule has been given below-

Insulin binds to it's Receptor (IR). The receptor has an extracellular ligand binding domain, and an intracellular catalytic domain. Binding of the ligand causes autophosphorylation of tyrosine residues in the catalytic domain of the receptor which recruits IRS protein. Tyrosine residues of IRS protein is phosphorylated, which then acts as a docking site for other molecules. This activates one of the two signaling pathways in the target cell- PI3K-Akt pathway or Ras MAP Kinase pathway, which ultimately leads to decrease in blood glucose level, by promoting Glucose absorption.