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[[File:Hormone Transport.png|thumb|300px|Left: A hormone feedback loop in a female adult human
Right: Auxin transport from leaves to roots in Arabidopsis thaliana]]
A hormone () is a class of cell signaling in multicellular organisms that are sent to distant organs or tissues by complex biological processes to regulate physiology and behavior. (2014). 9781464151132, W. H. Freeman. ISBN 9781464151132 Hormones are required for the normal development of , and fungi.
Due to the broad definition of a hormone (as a signaling molecule that exerts its effects far from its site of production), numerous kinds of molecules can be classified as hormones. Substances that can be considered hormones include (e.g. and ), (e.g. Estrogen and brassinosteroid), amino acid derivatives (e.g. epinephrine and auxin), protein or (e.g. insulin and ), and (e.g. ethylene and nitric oxide).
Hormones are used to communicate between organs and tissues. In , hormones are responsible for regulating a wide range of processes including both physiological processes and behavioral activities such as digestion, metabolism, respiration, sensory perception, sleep, excretion, lactation, stress induction, growth and development, movement, reproduction, and mood manipulation. (2026). 9780521692014, Cambridge Univ. Press. ISBN 9780521692014 In plants, hormones modulate almost all aspects of development, from germination to senescence.
Hormones affect distant cells by binding to specific receptor proteins in the target cell, resulting in a change in cell function. When a hormone binds to the receptor, it results in the activation of a signal transduction pathway that typically activates gene transcription, resulting in increased gene expression of target . Hormones can also act in non-genomic pathways that synergize with genomic effects.
Water-soluble hormones (such as peptides and amines) generally act on the surface of target cells via second messengers. Lipid soluble hormones, (such as ) generally pass through the cell membrane of target cells (both cell membrane and nuclear membrane) to act within their cell nucleus. , a type of polyhydroxysteroids, are a sixth class of plant hormones and may be useful as an anticancer drug for endocrine-responsive tumors to cause apoptosis and limit plant growth. Despite being lipid soluble, they nevertheless attach to their receptor at the cell surface.
In vertebrates, are specialized organs that secretion hormones into the endocrine system. Hormone secretion occurs in response to specific biochemical signals and is often subject to negative feedback regulation. For instance, high blood sugar (serum glucose concentration) promotes insulin synthesis. Insulin then acts to reduce glucose levels and maintain homeostasis, leading to reduced insulin levels.
Upon secretion, water-soluble hormones are readily transported through the circulatory system. Lipid-soluble hormones must bind to carrier plasma glycoproteins (e.g., thyroxine-binding globulin (TBG)) to form ligand-protein complexes. Some hormones, such as insulin and growth hormones, can be released into the bloodstream already fully active. Other hormones, called , must be activated in certain cells through a series of steps that are usually tightly controlled. (1997). 9780721662787, Saunders. ISBN 9780721662787
The endocrine system secretes hormones directly into the bloodstream, typically via fenestrated capillaries, whereas the exocrine system secretes its hormones indirectly using ducts. Hormones with paracrine function diffuse through the interstitial spaces to nearby target tissue.
Plants lack specialized organs for the secretion of hormones, although there is a spatial distribution of hormone production. For example, the hormone auxin is produced mainly at the tips of young leaf and in the shoot apical meristem. The lack of specialised glands means that the main site of hormone production can change throughout the life of a plant, and the site of production is dependent on the plant's age and environment.
Exocytosis and other methods of cell membrane are used to secrete hormones when the endocrine glands are signaled. The hierarchical model is an oversimplification of the hormonal signaling process. Cellular recipients of a particular hormonal signal may be one of several cell types that reside within a number of different tissues, as is the case for insulin, which triggers a diverse range of systemic physiological effects. Different tissue types may also respond differently to the same hormonal signal. (2026). 9780073532219, McGraw-Hill. ISBN 9780073532219
In 1905 Starling coined the word hormone from the Greek to arouse or excite which he defined as “the chemical messengers which speeding from cell to cell along the blood stream, may coordinate the activities and growth of different parts of the body”.Jamshed R Tata One hundred years of hormones EMBO Rep. 2005 Jun;6(6):490–496. doi: 10.1038/sj.embor.7400444
| +Signaling Types - Hormones !SN !Types !Description | ||
| 1 | Endocrine system | Acts on the target cells after being released into the bloodstream. |
| 2 | Paracrine | Acts on the nearby cells and does not have to enter general circulation. |
| 3 | Autocrine | Affects the cell types that secreted it and causes a biological effect. |
| 4 | Intracrine | Acts intracellularly on the cells that synthesized it. |
| +Hormone types in Vertebrates !SN !Types !Description | ||
| 1 | Proteins/ Peptides | are made of a chain of that can range from just 3 to hundreds. Examples include oxytocin and insulin. Their sequences are encoded in DNA and can be modified by alternative splicing and/or post-translational modification. They are packed in vesicles and are hydrophile, meaning that they are soluble in water. Due to their hydrophilicity, they can only bind to receptors on the membrane, as travelling through the membrane is unlikely. However, some hormones can bind to intracellular receptors through an intracrine mechanism. |
| 2 | Amino Acid Derivatives | Amino acid hormones are derived from amino acids, most commonly Tyrosine. They are stored in vesicles. Examples include Melatonin and Thyroxine. |
| 3 | Steroids | Steroid hormones are derived from cholesterol. Examples include the sex hormones estradiol and testosterone as well as the stress hormone cortisol. (2026). 9780321861580, Pearson Education, Inc. ISBN 9780321861580 Steroids contain four fused rings. They are lipophilic and hence can cross membranes to bind to intracellular . |
| 4 | Eicosanoids | hormones are derived from lipids such as arachidonic acid, , thromboxanes and . Examples include prostaglandin and thromboxane. These hormones are produced by and . They are hydrophobic and act on membrane receptors. |
| 5 | Gases | Ethylene and Nitric Oxide |
Receptors for most peptide hormone as well as many eicosanoid hormones are embedded in the cell membrane as cell surface receptors, and the majority of these belong to the G protein-coupled receptor (GPCR) class of seven alpha helix transmembrane proteins. The interaction of hormone and receptor typically triggers a cascade of secondary effects within the cytoplasm of the cell, described as signal transduction, often involving phosphorylation or dephosphorylation of various other cytoplasmic proteins, changes in ion channel permeability, or increased concentrations of intracellular molecules that may act as second messenger (e.g., cyclic AMP). Some also interact with intracellular receptors located in the cytoplasm or cell nucleus by an intracrine mechanism.
For steroid hormone or thyroid hormone hormones, their receptors are located intracellular within the cytoplasm of the target cell. These receptors belong to the nuclear receptor family of ligand-activated transcription factors. To bind their receptors, these hormones must first cross the cell membrane. They can do so because they are lipid-soluble. The combined hormone-receptor protein complex then moves across the nuclear membrane into the nucleus of the cell, where it binds to specific DNA sequences, regulating the expression of certain genes, and thereby increasing the levels of the proteins encoded by these genes. However, it has been shown that not all steroid receptors are located inside the cell. Some are associated with the plasma membrane.
A hormone may also regulate the production and release of other hormones. Hormone signals control the internal environment of the body through homeostasis.
One special group of hormones is the that stimulate the hormone production of other endocrine system. For example, thyroid-stimulating hormone (TSH) causes growth and increased activity of another endocrine gland, the thyroid, which increases output of . (2026). 9789350250136, Jaypee Brothers Medical Publishers (P) Ltd. ISBN 9789350250136
To release active hormones quickly into the circulation, hormone biosynthetic cells may produce and store biologically inactive hormones in the form of prehormone or . These can then be quickly converted into their active hormone form in response to a particular stimulus.
are considered to act as local hormones. They are considered to be "local" because they possess specific effects on target cells close to their site of formation. They also have a rapid degradation cycle, making sure they do not reach distant sites within the body."Eicosanoids". www.rpi.edu. Retrieved 2017-02-08.
Hormones are also regulated by receptor agonists. Hormones are ligands, which are any kinds of molecules that produce a signal by binding to a receptor site on a protein. Hormone effects can be inhibited, thus regulated, by competing ligands that bind to the same target receptor as the hormone in question. When a competing ligand is bound to the receptor site, the hormone is unable to bind to that site and is unable to elicit a response from the target cell. These competing ligands are called antagonists of the hormone. (2026). 9780321981226, Pearson. ISBN 9780321981226
A "pharmacologic dose" or "supraphysiological dose" of a hormone is a medical usage referring to an amount of a hormone far greater than naturally occurs in a healthy body. The effects of pharmacologic doses of hormones may be different from responses to naturally occurring amounts and may be therapeutically useful, though not without potentially adverse side effects. An example is the ability of pharmacologic doses of to suppress inflammation.
Not only can hormones influence behavior, but also behavior and the environment can influence hormone concentration. Thus, a feedback loop is formed, meaning behavior can affect hormone concentration, which in turn can affect behavior, which in turn can affect hormone concentration, and so on. For example, hormone-behavior feedback loops are essential in providing constancy to episodic hormone secretion, as the behaviors affected by episodically secreted hormones directly prevent the continuous release of said hormones.
Three broad stages of reasoning may be used to determine if a specific hormone-behavior interaction is present within a system:Nelson, R. J. (2011). An Introduction to Behavioral Endocrinology (4th ed.). Sinauer Associates. ISBN 978-0-87893-244-6.
Neurohormones are a type of hormone that share a commonality with neurotransmitters. They are produced by endocrine cells that receive input from neurons, or neuroendocrine cells. Both classic hormones and neurohormones are secreted by endocrine tissue; however, neurohormones are the result of a combination between endocrine reflexes and neural reflexes, creating a neuroendocrine pathway. While endocrine pathways produce chemical signals in the form of hormones, the neuroendocrine pathway involves the electrical signals of neurons. In this pathway, the result of the electrical signal produced by a neuron is the release of a chemical, which is the neurohormone . Finally, like a classic hormone, the neurohormone is released into the bloodstream to reach its target.
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