21.6
Types of Hormones
Hormones can be classified into three main types based on their chemical structures: steroids, peptides, and amines. Their actions are mediated by the…
Hormones are signaling molecules that are carried via the bloodstream to various parts of the body. They act on target cells and organs to regulate numerous biological processes. Hormones are classified into three main types, based on their chemical structures: steroids, amines and peptides.
Steroid hormones are derived from cholesterol. Like cholesterol, they contain four fused carbon rings with different side chains attached. Steroid hormones include estradiol, released from ovaries, female reproductive organs, and testosterone, released from testes, male reproductive organs.
The second class, amine hormones, are synthesized from a single amino acid, either tyrosine or tryptophan. The hormone epinephrine, derived from the amino acid tyrosine, triggers the fight or flight response during a stressful situation, such as a predator attack. The hormone melatonin, derived from tryptophan, regulates the internal biological clock.
Melatonin levels control the sleep-wake cycle, with low levels stimulating wakefulness during the day and high levels causing drowsiness at night. Lastly, peptide hormones consist of a chain of amino acids. The pancreas produces the peptide hormone insulin.
After a meal, insulin is released by the increase in blood glucose which stimulates cells to absorb excess glucose. Steroid hormones are lipophilic, or lipid soluble, and easily diffuse across the plasma membrane. Amine and peptide hormones are hydrophilic, or lipid insoluble, and cannot cross the plasma membrane of cells, therefore, these hormones bind to surface receptors expressed on their target cells to elicit a response.
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Q1: What are the three main types of hormones based on chemical structure?
Hormones are classified into three types: steroids, amines, and peptides. Steroid hormones derive from cholesterol and contain four fused carbon rings. Amine hormones are synthesized from single amino acids like tyrosine or tryptophan. Peptide hormones consist of chains of amino acids. Each type has distinct chemical properties that determine how they interact with target cells.
Q2: How do steroid hormones differ from peptide hormones in crossing cell membranes?
Steroid hormones are lipophilic, or lipid soluble, allowing them to easily diffuse across the plasma membrane and bind to intracellular receptors. Peptide hormones are hydrophilic, or lipid insoluble, and cannot cross the membrane. Instead, peptide hormones bind to surface receptors on target cells to trigger responses. This structural difference determines their mechanism of action within cells.
Q3: What role does epinephrine play during a stressful situation?
Epinephrine is an amine hormone derived from the amino acid tyrosine that triggers the fight or flight response during stress, such as a predator attack. It binds to G-protein coupled receptors on the cell surface, initiating a signaling cascade that activates second messengers in the cytoplasm. This produces rapid physiological changes to prepare the body for immediate action.
Q4: How does melatonin regulate the sleep-wake cycle?
Melatonin is an amine hormone derived from tryptophan that regulates the internal biological clock. Low melatonin levels stimulate wakefulness during the day, while high levels cause drowsiness at night. This hormone controls the sleep-wake cycle by modulating circadian rhythms, allowing the body to maintain proper temporal organization of physiological processes.
Q5: What is the function of insulin as a peptide hormone?
Insulin is a peptide hormone produced by the pancreas that regulates blood glucose levels. After a meal, increased blood glucose stimulates insulin release. Insulin then binds to surface receptors on target cells, signaling them to absorb excess glucose from the bloodstream. This maintains healthy blood glucose concentrations and provides cells with glucose for energy.
Q6: How do thyroid hormones differ from epinephrine in their mechanism of action?
Although both epinephrine and thyroid hormone thyroxine derive from tyrosine, they use different mechanisms. Epinephrine binds to surface receptors and activates second messengers. Thyroxine is converted to its active form triiodothyronine and transported across the plasma membrane, where it binds to hormone receptors in a complex with nuclear DNA to alter gene expression.
Q7: Why are amine hormones considered unique among hormone classes?
Amine hormones are unique because they share mechanisms of action with both steroid and peptide hormones. Some amine hormones, like epinephrine, bind to surface receptors like peptide hormones. Others, like thyroid hormone, cross the membrane and bind intracellular receptors like steroid hormones. This dual capability makes amine hormones biochemically versatile signaling molecules.