IB Biology Topic C2.1.4: Chemical Diversity of Hormones and Neurotransmitters Notes

Chemical Class Sets a Messenger's Solubility, and Solubility Decides Where Its Receptor Sits

1. Hormones and neurotransmitters are chemically diverse, and they fall into a few broad classes based on their structure.
2. The three main classes are amines and amino acid derivatives, peptides and proteins, and steroids.
3. A messenger's chemical class fixes whether it is hydrophilic (water-soluble) or lipophilic (lipid-soluble).

Three Chemical Classes Cover Almost Every Hormone and Neurotransmitter

1. Amines and amino acid derivatives
   1. These are small molecules made by modifying a single amino acid, and most are hydrophilic.
   2. Examples include the hormones adrenaline (epinephrine) and thyroxine, and the neurotransmitters dopamine and serotonin.
2. Peptides and proteins
   1. These are chains of amino acids, from short peptides to larger proteins, and they are hydrophilic.
   2. Examples include the hormones insulin and growth hormone, and neurotransmitter peptides such as endorphins.
3. Steroids
   1. These are built from cholesterol, so they are lipophilic.
   2. Examples include cortisol, testosterone, and oestrogen.

Solubility Determines Whether a Messenger Can Cross the Cell Membrane

1. Hydrophilic messengers
   1. Water-soluble amines and peptides cannot pass through the hydrophobic core of the phospholipid bilayer.
   2. They therefore bind to receptors on the cell surface.
2. Lipophilic messengers
   1. Lipid-soluble steroids dissolve through the membrane and enter the cell.
   2. They bind to intracellular receptors inside the cytoplasm or nucleus.

In short, chemical class sets solubility, and solubility decides whether the receptor is on the membrane surface or inside the cell.

Chemical Diversity Lets One Body Run Many Independent Signals

1. Each messenger has a distinct shape and chemistry, so it fits only its own specific receptor.
2. This lets many signals travel through the same blood or tissue at once without interfering.