Learning objectives
By the end of this section, you will be able to do the following:
- List the different types of hormones
- Explain their role in maintaining homeostasis
Maintaining homeostasis within the body requires the coordination of many different systems and organs. Communication between neighboring cells, and between cells and tissues in distant parts of the body, occurs through the release of chemicals called hormones. Hormones are released into body fluids (usually blood) that carry these chemicals to their target cells. At the target cells, which are cells that have a receptor for a signal or ligand from a signal cell, the hormones elicit a response. The cells, tissues, and organs that secrete hormones make up the endocrine system. Examples of glands of the endocrine system include the adrenal glands, which produce hormones such as epinephrine and norepinephrine that regulate responses to stress, and the thyroid gland, which produces thyroid hormones that regulate metabolic rates.
Although there are many different hormones in the human body, they can be divided into three classes based on their chemical structure: lipid-derived, amino acid-derived, and peptide (peptide and proteins) hormones. One of the key distinguishing features of lipid-derived hormones is that they can diffuse across plasma membranes whereas the amino acid-derived and peptide hormones cannot.
Lipid-Derived Hormones (or Lipid-soluble Hormones)
Most lipid hormones are derived from cholesterol and thus are structurally similar to it, as illustrated in fig-ch37_01_01. The primary class of lipid hormones in humans is the steroid hormones. Chemically, these hormones are usually ketones or alcohols; their chemical names will end in “-ol” for alcohols or “-one” for ketones. Examples of steroid hormones include estrogens (such as estradiol) and androgens (such as testosterone), both of which regulate bone and tissue development in all humans.
Gonadal hormones, produced by the gonads, include both steroid and peptide hormones. Androgens and estrogens resemble one another in chemical structure and originate from the same molecule. Estrogens are chief drivers of sexual development in an ovarian reproductive system, while androgens drive development in a testicular reproductive system. The ovaries produce steroid hormones such as estradiol and progesterone. When androgens are produced, some of them are later converted to estrogens. Minute amounts of estrogen occur through aromatase actions in adipose, brain, skin, and bone, which convert testosterone to estrogen. The testes and the adrenal cortex both secrete testosterone.
Other steroid hormones include aldosterone and cortisol, which are released by the adrenal glands along with some other types of androgens. Steroid hormones are insoluble in water, and they are transported by transport proteins in blood. As a result, they remain in circulation longer than peptide hormones. For example, cortisol has a half-life of 60 to 90 minutes, while epinephrine, an amino acid derived-hormone, has a half-life of approximately one minute.
View original figure (external; image not reproduced in this text edition)
Original figure description: Part A shows the molecular structure of cholesterol, which has three six-carbon rings attached to a five-carbon ring. A hydroxyl group is attached to the first six-membered ring, and a branched carbon chain is attached to the five-membered ring. Two methyl groups are attached each to a carbon that links the rings together. Part B shows the molecular structure of testosterone, which has a hydroxyl group in place of the branched carbon chain found on cholesterol. A ketone instead of a hydroxyl group is attached to the six-membered ring. Part C shows the molecular structure of estradiol, which, like testosterone, has a hydroxyl group in place of cholesterol’s branched carbon chain. Estradiol also lacks one of the methyl groups found in cholesterol.
Amino Acid-Derived Hormones
The amino acid-derived hormones are relatively small molecules that are derived from the amino acids tyrosine and tryptophan, shown in fig-ch37_01_02. If a hormone is amino acid-derived, its chemical name will end in “-ine”. Examples of amino acid-derived hormones include epinephrine and norepinephrine, which are synthesized in the medulla of the adrenal glands, and thyroxine, which is produced by the thyroid gland. The pineal gland in the brain makes and secretes melatonin which regulates sleep cycles.
View original figure (external; image not reproduced in this text edition)
Original figure description: Part A shows the amino acid tyrosine on the left and epinephrine on the right. Epinephrine is similar in structure to tyrosine, with minor modifications. Part B shows the amino acid tryptophan on the left and the structurally similar melatonin on the right.
Peptide Hormones
The structure of peptide hormones is that of a polypeptide chain (chain of amino acids). The peptide hormones include molecules that are short polypeptide chains, such as antidiuretic hormone and oxytocin produced in the brain and released into the blood in the posterior pituitary gland. This class also includes small proteins, like growth hormones produced by the pituitary, and large glycoproteins such as follicle-stimulating hormone produced by the pituitary. fig-ch37_01_03 illustrates these peptide hormones.
Secreted peptides like insulin are stored within vesicles in the cells that synthesize them. They are then released in response to stimuli such as high blood glucose levels in the case of insulin. Amino acid-derived and polypeptide hormones are water-soluble and insoluble in lipids. These hormones cannot pass through plasma membranes of cells; therefore, their receptors are found on the surface of the target cells.
View original figure (external; image not reproduced in this text edition)
Original figure description: Oxytocin, growth hormone, and follicle stimulating hormone are all large, with complex three-dimensional structures.
Section Summary
There are three basic types of hormones: lipid-derived, amino acid-derived, and peptide. Lipid-derived hormones are structurally similar to cholesterol and include steroid hormones such as estradiol and testosterone. Amino acid-derived hormones are relatively small molecules and include the adrenal hormones epinephrine and norepinephrine. Peptide hormones are polypeptide chains or proteins and include the pituitary hormones, antidiuretic hormone (vasopressin), and oxytocin.
Review Questions
A newly discovered hormone contains four amino acids linked together. Under which chemical class would this hormone be classified?
- lipid-derived hormone
- amino acid-derived hormone
- peptide hormone
- glycoprotein
C
Which class of hormones can diffuse through plasma membranes?
- lipid-derived hormones
- amino acid-derived hormones
- peptide hormones
- glycoprotein hormones
A
Why are steroids able to diffuse across the plasma membrane?
- Their transport protein moves them through the membrane.
- They are amphipathic, allowing them to interact with the entire phospholipid.
- Cells express channels that let hormones flow down their concentration gradient into the cells.
- They are non-polar molecules.
D
Critical Thinking Questions
Although there are many different hormones in the human body, they can be divided into three classes based on their chemical structure. What are these classes and what is one factor that distinguishes them?
Although there are many different hormones in the human body, they can be divided into three classes based on their chemical structure: lipid-derived, amino acid-derived, and peptide hormones. One of the key distinguishing features of the lipid-derived hormones is that they can diffuse across plasma membranes whereas the amino acid-derived and peptide hormones cannot.
Where is insulin stored, and why would it be released?
Secreted peptides such as insulin are stored within vesicles in the cells that synthesize them. They are then released in response to stimuli such as high blood glucose levels in the case of insulin.
Glucagon is the peptide hormone that signals for the body to release glucose into the bloodstream. How does glucagon contribute to maintaining homeostasis throughout the body? What other hormones are involved in regulating the blood glucose cycle?
Glucagon acts in opposition to insulin, the peptide hormone that stimulates cells to take up glucose from the bloodstream, to maintain blood glucose within healthy levels. When glucagon is released into the blood in response to falling blood sugar levels, the liver catabolizes its glycogen stores to release glucose. If glucagon does not function properly, the blood sugar will drop too low from insulin signaling driving cellular uptake from the blood.
Glossary
- amino acid-derived hormone
- hormone derived from amino acids
- lipid-derived hormone
- hormone derived mostly from cholesterol
- peptide hormone
- hormone composed of a polypeptide chain