Hormone — Safekipedia Adventurer

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. 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 saw that roosters without testes acted differently and looked smaller. He found that a chemical from the testes, called the hormone testosterone, caused these changes.

Later, Charles Darwin and his son Francis looked at how plants grow toward light. They thought a special substance helped this, which turned out to be the plant hormone auxin. In 1894, George Oliver and Edward Albert Schäfer found that something from adrenal glands could change the body, and they named it the hormone adrenaline. Finally, William Bayliss and Ernest Starling learned that a substance from the intestines, called secretin, made the pancreas work, and this led to the name "hormone" for these special chemicals.

Types of signaling

Hormones travel from where they are made to other parts of the body, changing how those parts work. They can be released into the blood or travel in other ways to reach their target. There are several main types of hormone signaling, each working a little differently.

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 attach to special parts called receptors on cells. These receptors help the hormone send messages to the cell.

Many hormones attach to receptors on the outside of the cell. This starts actions inside the cell.

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

Effects in humans

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

They also help with big changes, such as growing up. Hormones control when we feel hungry and work together to keep our bodies healthy and balanced.

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. Examples include estrogens and progestogens. These help with birth control and aging. Thyroxine, or levothyroxine, helps people whose bodies do not make enough thyroid hormone. Insulin is important for people with diabetes. Special amounts of hormones can help treat some diseases, but they can also have side effects.

Hormone-behavior interactions

Hormones can change how we act. The amount of hormones in our body can make us more or less likely to do certain things, but hormones alone don’t make us act a certain way. Our choices and what happens around us can also change hormone levels. This creates a cycle where our actions and hormones affect each other.

Scientists study how hormones and behavior are linked by looking for three clues: the behavior should happen when the hormone is there, the behavior should not happen when the hormone is missing, and adding the hormone back should make the behavior happen again. This helps scientists learn how hormones and behavior are connected.

Comparison with neurotransmitters

Hormones and neurotransmitters both help send messages in the body, but they work differently. Hormones travel through the blood to different parts of the body and work over longer periods. Neurotransmitters work over very short distances and act quickly.

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

Binding proteins

Hormone binding proteins help hormones travel in the body and stay useful for longer. When a hormone attaches to a binding protein, it creates a supply to replace hormones when they are used up. For example, thyroxine-binding protein carries most of the thyroxine in the body, which helps control how fast the body uses energy.