Hormones, a term derived from the Greek word hormao meaning to excite or arouse, are signaling molecules sent by cells to influence activity of other cells. They prompt responses to internal and external change.
In the bloodstream, tissues or environment, hormones travel to target areas. They attach to specific receptors to trigger a biological response and interact only with cells having these receptors.
This communication system maintains homeostasis, the internal balance of a body. It helps regulate growth, metabolism, reproduction and other biological processes.
Microorganisms
Bacteria use hormone-like molecules to communicate and coordinate their activities. Known as quorum sensing, this allows bacteria to collectively behave as a multicellular entity.
For example one bacterium is no match for the host's defenses. By releasing and sensing hormone-like signal molecules (autoinducers), they decide when their population density is enough for a coordinated attack.
Scientists explore the possibility of disrupting quorum sensing with quorum quenching molecules as an antibacterial ploy. It could prevent bacteria from coordinating but results are not known.
Yeasts are reported to produce plant hormones auxin or zeatin, or both. The importance of the compounds to the ecology of the yeast again is vague, though auxin is responsible for cell elongation, common in yeast.
Plants
Plants, despite lacking a nervous system, rely heavily on plant hormones, or phytohormones. These chemical signals originate in parts of the plant and travel to other areas to regulate growth, development and responses to environmental stimuli.
Auxins: The pioneers of plant hormones, auxins, are responsible for cell elongation, apical dominance (inhibiting lateral bud growth), and root formation. When plants reach for the light, it's based on action of auxin.
It triggers cells on the shaded side of a plant to elongate more than those on the sunlit side. This process drives phototropism, making plants grow toward light.
When a plant is cut, auxins accumulate at the injury site, stimulating spontaneous root formation. The ability of trees and plants to sprout roots from a branch or cutting in water is due to auxins.
Gibberellins: These hormones promote stem elongation, seed germination, and flowering. They're used to produce seedless grapes.
Cytokinins: Counteracting auxins to some extent, cytokinins promote cell division and differentiation. They delay senescence or aging and are important for lateral bud growth and leaf expansion. Lateral buds grow along the sides of a shoot to make a plant bushy.
Abscisic Acid (ABA): Often called the stress hormone, ABA helps plants cope with drought and other stressful conditions by inducing stomatal closure (reducing water loss), promoting seed dormancy, and inhibiting growth.
Ethylene: Unique as a gaseous hormone, ethylene functions in fruit ripening, leaf abscission or dropping; and response to wounds or pathogen attack. It accelerates ripening in nearby apples to spread rot.
Bananas emit ethylene when harvested, prompting the fruit to ripen. When unripe bananas are put in a paper bag with ripe ones, the ethylene from the ripe bananas accelerates the ripening process of the unripe.
Plant hormones are used in agriculture to manipulate crop growth and yield. Synthetic auxins are used as herbicides to selectively kill flora like dandelions, plantain and clover.
Animals
In animals, hormones coordinate countless physiological processes. Produced by endocrine glands or specialized cells, they travel through the bloodstream to reach target cells with specific receptors.
Insulin: Secreted by the pancreas, insulin promotes glucose uptake by cells. Its counterpart, glucagon, raises blood glucose levels, stimulating glucose release from the liver. The two hormones work to balance blood sugar.
Insulin also affects fat and protein metabolism.
Adrenaline (Epinephrine): Released by the adrenal glands in response to stress or excitement, adrenaline triggers the "fight-or-flight" response to increase heart rate, blood pressure and energy levels. Within seconds of release, it influences several organs.
Sex Hormones (Estrogen, Testosterone): These are produced by the gonads (ovaries and testes) and are responsible for the development of secondary sexual characteristics, reproduction and sexual behavior.
Growth Hormone: Secreted by the pituitary gland, growth hormone promotes growth and development, particularly during childhood and adolescence.
Thyroid Hormones: Produced by the thyroid gland, these hormones regulate metabolism, growth, and development.
Some hormones also function as neurotransmitters. Norepinephrine for instance is a hormone when released by adrenal glands and a neurotransmitter when released by neurons.
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