Define the mechanism that hormones modify the biochemical activity of a target cell.

Hormones modify the biochemical activity of a target cell or tissue. Two basic mechanisms are involved.

1. Fixed – Membrane – Receptor Mechanism

The fixed – membrane – receptor mechanism, applies to hormones that are proteins or amines. Because they are water – soluble and cannot diffuse across the plasma membrane, these hormones _initiate their response by means of specialized receptors on the plasma membrane of the target cell. Fig. 3.5.

2. Mobile – Receptor Mechanism

The mobile – receptor mechanism, applies to steroid hormones. These hormones are lipid – soluble and diffuse easily into the cytoplasm, where they initiate their response by binding to cytoplasmic receptors. Fig. 3.6.