What is hormones: Definition and 18 Discussions

A hormone (from the Greek participle ὁρμῶν, "setting in motion") is a class of signaling molecules in multicellular organisms that are sent to distant organs by complex biological processes to regulate physiology and behavior. Hormones are required for the correct development of animals, plants 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. Among the substances that can be considered hormones, are eicosanoids (e.g. prostaglandins and thromboxanes), steroids (e.g. oestrogen and brassinosteroid), amino acid derivatives (e.g. epinephrine and auxin), protein or peptides (e.g. insulin and CLE peptides), and gases (e.g. ethylene and nitric oxide).
Hormones are used to communicate between organs and tissues. In vertebrates, hormones are responsible for regulating a variety of 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. 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 expression of target proteins. 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 steroids) generally pass through the plasma membranes of target cells (both cytoplasmic and nuclear) to act within their nuclei. Brassinosteroids, 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, endocrine glands are specialized organs that secrete hormones into the endocrine signaling 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 bond 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 prohormones, must be activated in certain cells through a series of steps that are usually tightly controlled. 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 spatial distribution of hormone production. For example, the hormone auxin is produced mainly at the tips of young leaves 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.

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  1. red65

    Medical Is Renin an enzyme or a hormone?

    Hello everyone, renin is mentioned sometimes as a hormone and others as an enzyme, is it both? can anyone tell me the final answer?
  2. mktsgm

    Medical Exploring the Physiology of Hunger: Hormones, Neurons, and Energy Levels

    I have been studying about the physiology of hunger. I have studied about the hormones that are involved in the process of hunger, like ghrelin, leptin, pancreatic polypeptide, cholecystokinin, cholecystokinin (CCK) and the neurons like AgRP, POMC and neuropeptide Y (NPY) etc. I understand...
  3. petrushkagoogol

    Stress hormones and limbic system?

    What effect do stress hormones like cortisol and adrenaline have on the limbic system?
  4. D

    Antibodies against hormones of the own body

    Recently I came upon the description of medical tests for hormones like cortisole, or tyroid hormones like T3 and T4, which use polyclonal antibodies from hamsters, pigs, sheep or thelike. As far as I know, they are obtained by vacunating these animals and harvesting the antibodies from their...
  5. CollinsArg

    Are sexual hormones realated with animal's instints?

    Thing is, I have a male cat which is very wild, sometimes when I'm walking on a room it would came towards me to attact me as a pray, sometimes it hurts me...I told a friend about it and she told me that if I castrate my cat it wouldn't behave that way again. So, as I know that this behavior is...
  6. Suraj M

    Placental Hormones: HCG, HPL & Relaxin

    Which are the hormones that the placenta secretes? I have contradicting facts from the web, from my books and from my teachers I know for certain HCG, HPL and relaxin are secreted, other than that?
  7. G

    Effects of Social Situations on Neurotransmitters and Hormones

    Does anybody know of any studies that effect how society regulates neurotransmitters and/or hormones in the body; especially long term. Specifically any studies about how they (especially neurotransmitters) regulate in social groups. Its a theory I have and I'd like to see if there is anything...
  8. D

    Resistance Training & Hormones

    Can someone please explain to me how resistance training affects hormones? And what effects insulin has on the body post-exercise? Thanks!
  9. lisab

    Medical Are There Still Undiscovered Hormones in Our Body?

    I was idly reading this article... http://www.msnbc.msn.com/id/43446129/ns/health-diet_and_nutrition/ ...when I came to this passage: Really, is it believed that we don't know all of the hormones in our system? If so, is it likely that there are "many" that we don't know about yet...
  10. S

    Chemistry of thyroid hormones doesn't make sense?

    Hello everyone, This is a DIT molecule. Now the book says there is a coupling reaction DIT + DIT ----> gives T4 molecule or tyroxine hormone Now this is how T4 looks like I don't understand how you can get this when there is a addition reaction between these two. Does...
  11. S

    Medical Sheehan's syndrome. Why there is no deficiency in posterior pituitary hormones?

    Hello everyone, Why is that in sheehan's syndrome there is no deficiency of posterior pituitary hormones. Ok they are synthesized in hypothalamus, but they are stored in posterior pituitary. So if the gland is damaged how are they secreted? Also there seems to be a trasient polyurea associatd...
  12. G

    Medical Personal conflict over whether or not to start hormones

    I'm trans, and am in a personal conflict over whether or not to start hormones. I know that counseling is required first, but it is such a major decision that I feel that I should have more of a settled direction or plan than I currently do. I'm wondering if anyone here knows anything about the...
  13. S

    Protein Hormones and Cell Membrane interaction

    Hi all, sorry if this has been asked before but i can't find a thread with the info I'm after. I'm doing an access course at lvl3 (AS/A2) which includes biology and I'm stuck on the following question as part of an assignment I'm to hand in next week. Q: Suggest an explanation for the fact...
  14. F

    What is the Connection Between Cortisol and Smoking?

    when you smoke there is a two stage action that takes place. It is hard to know where to start though. It is like the chicken and the egg. Take last week for instance, or maybe the last week you were a smoker. There will have been a cycle happening in your body that will have been dictated by...
  15. S

    Can we decrease STH in rabbits by modifying hormones?

    Can we modify hormones? I mean can I decrease the STH by stimulating another hormone?
  16. Y

    Which Kidney Hormones: Insulin, Adrenaline, Growth Hormone, EPO or Thyroxine?

    I have a question that says: Which of the following hormons are produced in the kidneys: Insulin, adrenaline, growthhormon, EPO or thyroxine ? Thanks again :D
  17. M

    Can hormones affect memory and tolerance to alcohol?

    I have noticed that on days 28-4 my memory is awful. I have also noticed that during this period if I consume alcohol, I seem become intoxicated rapidly, and forget things much more than I normally would drinking the same amount. I tried to look at charts, but could not really work out a reason...
  18. G

    Hormones in the menstrual cycle

    A few questions on hormones (again :biggrin: ): What stimulates oestrogen production? Is it FSH and LH, or just FSH Does oestrogen inhibit or stimulate LH production? The material I got says that ostrogen inhibits both FSH and LH but from the Edexcel board in the mark scheme it...