Hormone structure determines whether it is water-soluble or lipid-soluble, which in turn is central to its mechanism of action.

Hormones are signals produced in the endocrine system and released into bodily fluids, like blood, which carry them to target cells. Target cells respond to a hormone when they express a specific receptor for that hormone. Hormones play a role in growth, metabolism, and maintain the body’s homeostasis. Although there are many different hormones in the human body, they are divided into three classes based on their chemical structure: steroids, amines (amino acid-derived), and peptide hormones (which include peptides and proteins). 

Peptide hormones, such as insulin and growth hormone, and most of the amines, such as epinephrine, are water-soluble molecules that cannot pass through the cell membrane. Accordingly, their receptors are found on the surface of the target cells. Water-soluble hormones are secreted by exocytosis and travel to its destination via the bloodstream. A water-soluble hormone binding to its target receptor induces a cell signaling pathway that either causes prompt cellular response or changes in the expression of certain genes. For example, epinephrine (a.k.a. adrenaline) binding to its receptor activates a cytoplasmic enzyme that inhibits glycogen synthesis. As peptide hormones cannot pass through the cell membrane but bind to membrane receptors, this means they rely on cascades of kinases and relays to transmit their message to the cell having a faster effect. 

Steroid hormones are lipid-derived thus insoluble in water. As they are lipid-soluble, steroid hormones and some of the amines can easily diffuse through the cell membrane. When they diffuse out of the cell, they need transport proteins to transport them in the blood to their target cells. Also, for the same reason, they remain in circulation longer than peptide hormones. Target receptors for lipid-soluble hormones are either in the cytoplasm or nucleus. As lipid-soluble hormone forms a complex with its receptor, they together bind the DNA and induce the expression of certain genes. This means the action of steroid hormones is slower as they need to promote translation and transcription of the mRNA at the target gene. 

Practice Questions
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Key Points

• All hormones in the human body can be divided into lipid-derived (steroids), amino acid-derived (amines), and peptide hormones.

• Peptide hormones and most of the amino-acid derived hormones (amines) are water-soluble proteins that cannot pass through the cell membrane; therefore, their receptors are found on the surface of the target cells.

• Lipid-derived hormones or steroid hormones are insoluble in water, and transport proteins move them in the blood; they can diffuse across plasma membranes.

Key Terms

target cell: any cell having a specific receptor for a hormone

receptor: a protein on a cell membrane that binds with specific molecules

hormone: a molecule produced by one tissue and conveyed by the bloodstream to another that generally induce a physiological response

exocytosis: a mechanism by which cells deliver cargo out of the cell; the cargo material is surrounded by a lipid membrane which eventually fuses with cell membrane and excrete its content out

homeostasis: the state of steady internal, physical, and chemical conditions maintained by living systems

steroids: lipids that have a structure composed of four fused rings

insulin: hormone that controls levels of glucose in the body 

lipid-soluble: the capability of a substance or compound to dissolve in lipids