Hormone — Safekipedia Discoverer

A hormone comes from an ancient Greek word meaning "setting in motion." It is a special kind of signaling molecule that helps different parts of the body talk to each other. In both animals, plants, and fungi, hormones travel through the body to distant organs and tissues to control important processes and behaviors.

Hormones are used to share messages between organs and tissues. In animals with a backbone, called vertebrates, they help with many body functions like digestion, growth, sleep, and even mood. In plants, hormones control how they grow from tiny seeds to mature plants.

Left: A hormone feedback loop in a female adult human(1) follicle-stimulating hormone(2) luteinizing hormone(3) progesterone(4) estradiolRight: Auxin transport from leaves to roots in Arabidopsis thaliana

When a hormone reaches a target cell, it attaches to special proteins called receptors. This attachment changes how the cell works, often turning on genes that make new proteins. Some hormones work quickly on the outside of cells, while others enter the cell and work inside.

The body has special glands, called endocrine glands, that produce and release hormones into the bloodstream. These glands respond to signals in the body, like high blood sugar, by making hormones such as insulin to bring sugar levels back to normal. Plants don't have these special glands, but they still make hormones in different parts of their bodies depending on their age and environment.

Introduction and overview

Further information: Signal transduction

Hormones are special chemicals made in places in our bodies called endocrine glands, like the thyroid gland, ovaries, and testes. These hormones travel through our blood to reach other parts of the body, where they help control many important processes.

Here’s a simple way to think about how hormones work: First, a hormone is made in a specific place in the body. Then, it is stored and released into the blood. It travels to cells that have special receptors, like locks that only this hormone can open. When the hormone finds its match, it triggers a reaction in the cell, helping the body do things like grow, get energy, or stay balanced. Finally, the hormone breaks down when its job is done. This whole process helps keep our bodies working properly.

Discovery

Arnold Adolph Berthold was a German scientist who studied roosters in 1849. He noticed that roosters without testes behaved differently and had smaller features. Through experiments, he discovered that a chemical from the testes, later known as the hormone testosterone, was responsible for these changes.

Later, Charles Darwin and his son Francis explored how plants grow toward light. Their work suggested a "transmissible substance" was involved, which was later confirmed as the plant hormone auxin. In 1894, George Oliver and Edward Albert Schäfer discovered that extracts from adrenal glands caused physical changes, identifying the hormone adrenaline. Finally, William Bayliss and Ernest Starling found that a substance from the intestines, called secretin, stimulated the pancreas, leading to the coining of the term "hormone" to describe these chemical messengers.

Types of signaling

Hormones work by traveling from where they are made to other parts of the body, affecting how those parts function. They may be released directly into the blood or act in other ways to reach their target. There are several main types of hormone signaling, each with its own way of working.

Signaling Types – Hormones
SN	Types	Description
1	Endocrine	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.

Chemical classes

Hormones are special messages that help living things grow and stay healthy. They can come in many different forms and are found in animals, plants, fungi, and even some simple creatures. In insects and crabs, there are special hormones like the juvenile hormone that help them grow.

Different types of hormones are secreted in the human body, with different biological roles and functions.

Plants also use their own kinds of hormones. Some examples are abscisic acid, auxin, cytokinin, ethylene, and gibberellin. These help plants grow and stay strong.

Main article: List of human hormones

Further information: Plant hormone

Hormone types in Vertebrates
SN	Types	Description
1	Proteins and Peptides	Peptide hormones are made of a chain of amino acids 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 hydrophilic, 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. Steroids contain four fused rings. They are lipophilic and hence can cross membranes to bind to intracellular nuclear receptors.
4	Eicosanoids	Eicosanoids hormones are derived from lipids such as arachidonic acid, lipoxins, thromboxanes and prostaglandins. Examples include prostaglandin and thromboxane. These hormones are produced by cyclooxygenases and lipoxygenases. They are hydrophobic and act on membrane receptors.
5	Gases	Ethylene and Nitric Oxide

Receptors

Most hormones start by attaching to special parts called receptors on the outside or inside of cells. These receptors help the hormone send important messages to the cell. For example, many hormones attach to receptors on the cell’s surface, which then trigger a series of actions inside the cell.

Some hormones, like steroid and thyroid hormones, can pass through the cell’s outer layer because they dissolve in fats. Once inside, they attach to receptors and move into the cell’s nucleus. There, they help turn certain genes on or off, which changes what proteins the cell makes.

Effects in humans

Hormones help control many important processes in our bodies. They can affect how we grow, our sleep patterns, and even our moods. Hormones also help manage our energy use and get our bodies ready for different activities, like getting ready for school or playing with friends.

They play a role in major life changes, such as growing up and becoming an adult, and they help control when our bodies feel hungry. Hormones work together to keep our bodies balanced and functioning well.

Main article: Homeostasis

Regulation

Blood glucose levels are maintained at a constant level in the body by a negative feedback mechanism. When the blood glucose level is too high, the pancreas secretes insulin and when the level is too low, the pancreas then secretes glucagon. The flat line shown represents the homeostatic set point. The sinusoidal line represents the blood glucose level.

Hormones are carefully controlled in our bodies by a system called negative feedback. This means that when there is too much of a hormone, the body finds ways to reduce it. This helps keep everything balanced.

Hormones can be affected by many things, such as other hormones, nutrients in our blood, our brain activity, and even changes in our environment like light or temperature. Some special hormones, called tropic hormones, help other glands make more hormones. For example, thyroid-stimulating hormone (TSH) makes the thyroid gland more active. Hormones can also be stored in an inactive form until they are needed.

Therapeutic use

Many hormones and their related compounds are used as medicine. Common examples include estrogens and progestogens, which are used for birth control and to help with aging-related changes. Thyroxine, known as levothyroxine, helps people whose bodies do not make enough thyroid hormone. Insulin is important for people with diabetes. Sometimes, special amounts of hormones, called pharmacologic doses, are used to help treat diseases, although they can also have side effects.

Hormone-behavior interactions

Hormones can affect how we behave. The levels of hormones in our body can change how likely we are to do certain things, but they don't make us do them by themselves. Our actions and the world around us can also change hormone levels, creating a cycle where behavior and hormones influence each other.

To know if a hormone affects a certain behavior, scientists look for three things: the behavior should happen when the hormone is present, the behavior shouldn't happen if the hormone is missing, and adding the hormone back should bring the behavior back. This helps scientists understand how hormones and behavior are connected.

Comparison with neurotransmitters

Hormones and neurotransmitters are both important for sending messages in the body, but they work in different ways. Hormones can travel to distant parts of the body through the blood, acting over longer distances and times. Neurotransmitters, on the other hand, work over very short distances and act much faster.

Neurohormones are a special type of hormone that are influenced by both the nervous system and endocrine system. They are released when neurons send electrical signals, and then travel through the bloodstream like other hormones to reach their targets.

Binding proteins

Hormone binding proteins help hormones travel in the body and stay effective for longer periods. When a hormone attaches to a binding protein, it creates a reserve that can replace hormones when they are used up. For example, thyroxine-binding protein carries most of the thyroxine in the body, which is important for controlling how fast the body uses energy.