What are the differences between steroid and peptide hormones?

Steroid hormones are lipid-based, pass through cell membranes and act on intracellular receptors, while peptide hormones are protein-based and act on cell surface receptors.

Steroid hormones and peptide hormones are two types of hormones that play crucial roles in the body's physiological processes. They differ primarily in their structure, mode of action, and the way they are transported in the bloodstream.

Steroid hormones are derived from cholesterol, a type of lipid. This lipid-based structure makes them hydrophobic, meaning they can easily pass through the lipid bilayer of cell membranes. Once inside the cell, they bind to specific intracellular receptors, forming a hormone-receptor complex. This complex then moves into the nucleus, where it influences gene expression and protein synthesis. Steroid hormones include sex hormones like testosterone and oestrogen, and hormones that regulate metabolism like cortisol.

On the other hand, peptide hormones are made up of amino acids, making them protein-based. Unlike steroid hormones, they are hydrophilic and cannot pass through the cell membrane. Instead, they bind to specific receptors on the cell surface. This binding triggers a cascade of intracellular events, often involving the production of second messengers like cyclic AMP, which amplify the hormone's signal within the cell. Peptide hormones include insulin, which regulates blood sugar levels, and growth hormone, which promotes cell growth and division.

In terms of transport, steroid hormones, due to their hydrophobic nature, are usually carried in the bloodstream bound to transport proteins. This allows them to remain in circulation longer. Peptide hormones, however, are soluble in blood and do not require transport proteins.

In summary, while both steroid and peptide hormones play vital roles in the body's functioning, they differ significantly in their structure, how they interact with cells, and their transport mechanisms. Understanding these differences is key to understanding how hormones regulate the body's myriad physiological processes.