Hormone

Hormones are a diverse class of signaling molecules used by multicellular organisms. They function as chemical messengers that travel to distant organs or tissues. Through complex biological processes, they regulate essential physiology and behavior. This regulation is necessary for the normal development of animals, plants, and fungi. Because they are defined by their ability to exert effects far from their production site, many different types of molecules can be classified as hormones.

The mechanism of hormonal signaling follows a specific sequence of biological steps. First, a specific hormone is synthesized within a particular tissue. This hormone is then stored and eventually secreted from the producing cell. After secretion, the hormone must be transported to its intended target cell. Once it arrives, the hormone is recognized by a specific receptor protein. This recognition can occur at the cell membrane or inside the cell. This binding triggers a signal transduction pathway, which is a relay of signals within the cell. This process often activates gene transcription, leading to the increased expression of target proteins. Finally, the hormone is broken down to end the signal.

Hormones interact with cells in several distinct ways based on their distance and target. Endocrine signaling involves hormones being released into the bloodstream to reach distant targets. Paracrine signaling occurs when hormones diffuse through interstitial spaces to reach nearby tissues. Autocrine signaling happens when a hormone affects the very cell that secreted it. Intracrine signaling is a process where the hormone acts inside the cell that synthesized it.

Chemical structure also determines how a hormone moves and acts. Water-soluble hormones, such as peptides and amines, generally act on the surface of target cells. They often use second messengers to relay their signal. In contrast, lipid-soluble hormones, like steroids, can pass directly through the plasma membrane. These hormones often act within the cell nucleus. Some plant hormones, such as brassinosteroids, are lipid-soluble but still attach to receptors at the cell surface.

The history of hormone discovery involves several landmark experiments. In 1849, Arnold Adolph Berthold studied the behavior of roosters. He discovered that removing the testes changed their sexual behavior and physical traits. By transplanting a testis into a different part of the body, he proved that a chemical secretion, rather than nerves or genetics, drove these changes. In 1894, George Oliver and Edward Albert Schäfer published findings on adrenal extracts. Their work was among the first to show the physiological effects of substances like adrenaline. In 1902, William Bayliss and Ernest Starling discovered secretin. They proved that the pancreas was stimulated by a chemical from the intestines rather than nerve impulses. Finally, in 1905, Starling coined the term "hormone" from a Greek word meaning to arouse or excite.

Hormones are vital for maintaining homeostasis through negative feedback loops. Homeostasis is the process of keeping internal conditions stable. For example, high blood sugar levels promote the synthesis of insulin. Insulin then acts to reduce glucose levels in the blood. Once the glucose levels return to a normal range, insulin production is reduced.

This loop prevents the body from having too much or too little of a substance. This system is essential for regulating metabolism, respiration, and growth.

In vertebrates, specialized organs called endocrine glands secrete hormones. These include the thyroid gland, ovaries, and testes. These glands often use fenestrated capillaries to release hormones directly into the blood. Some hormones, like insulin, are released in a fully active state. Others are released as prohormones, which require several controlled steps to become active. Plants handle this differently because they lack specialized endocrine organs. Instead, they use spatial distribution, such as the hormone auxin being produced at the tips of young leaves.