A hormone is a substance that is produced in tiny amounts in one part of an organism and is transported to other parts where it has a definite physiological effect. Hormones also often affect the functioning of other hormone-producing tissues so that the effects can be complex and integrated. They influence the rate at which reactions proceed. They are important in both animals and plants, affecting growth, differentiation, metabolism, digestive function, mineral and fluid balance.
The rate of secretion, efficacy on target organs, and rate of removal are all affected by numerous factors include feedback from their metabolic effects, mineral or sugar concentration in the blood, and the action of controlling hormones. The latter usually originate in the pituitary gland and those controlling the pituitary in the hypothalamus.
Hormones do not fall into any one particular chemical class. They may be steroids, peptides, or relatively simple compounds such as adrenaline.
In animals, hormones are usually secreted by endocrine glands directly from gland cells into the bloodstream, where they often circulate in combination with serum proteins. They occur copiously in vertebrates but are also known in some invertebrates.
Plant hormones, called phytohormones (e.g., auxins and gibberllins), play a prominent role in plant growth.
Related categories• BIOCHEMISTRY
• ANATOMY AND PHYSIOLOGY
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