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From MyMCAT

Contents 1 Introduction 2 Production 3 Permeability 4 Receptors 5 Action/Messaging Process 6 Duration

Introduction

Hormones generally fall into two categories, those that are peptides, and those that are steroids (usually derived from cholesterol).

Production

Peptide hormones are simply specific peptides (short proteins made of amino acids) that have evolved to act as message carriers. Steroid hormones, on the other hand, are derived from cholesterol, a poly aromatic hydrocarbon. While the differences between these two sources specifically determine how they can act in the body, they do not specify where in the body they are produced. Both peptide and steroid hormones are made by the glands of the endocrine system but neither are produced solely by any specific organ.

Permeability

Because peptide hormones are made up of amino acids, they are charged, and as such, do not penetrate the cell's membrane. Steroid hormones on the other hand, are small and hydrophobic, thus allowing easy diffusion through the lipid bilayer of the cell membrane.

Receptors

For cells to recieve signals from peptide hormones, they must have membrane bound receptors capable of binding the specific peptide. Receptors for steroid hormones, however, can exist as cytoplasmic or nucleoplasmic molecules freely floating inside the cell.

Action/Messaging Process

For peptide hormones, the signal from the surface receptor must be relayed into the cell to have any effect. This process is done through second message molecules, which also often result in an amplification or cascade effect. (One activated receptor will activate 50 fold second message proteins, who then activate 50 fold more other molecules, etc...)

Steroid hormones, when bound to their receptor can directly cause an action as they are already inside the cell. The action of receptor bound steroid hormones is usually direct binding to DNA causing direct regulation of transcription.

Duration

The onset and duration of peptide hormones is usually quick and short as they are broken down rapidly. Second messages also often are only active for very short periods before being destroyed or deactivated.

Steroid hormone effects often last long periods as changes in transcription rates (and thus future protein concentrations) effect the cell for a substantial length of time.