Hormone Definition

A hormone is a biological compound used by multicellular organisms to organize, coordinate, and control the functions of their cells and tissues. These chemicals can control everything from metabolism to behavior, and are necessary for organisms to survive and reproduce. A hormone can be made from many starting molecules, but every hormone has a specific receptor it is made for. Many hormones travel through the bloodstream to their target cell, which has a specific receptor necessary to receive the signal.

The signal may be received externally, or first it may be transported to the cytosol or nucleus of the target cell where it will bind to its specific receptor. If the signal is received externally, a second messenger chemical is released into the cytosol, enacting a process in the target cell. Hormonal systems are highly complex, and many different cell types may react in different ways to the same hormone. Below are some common functions of hormones in plants and animals.

Hormones fall into one of three categories. Endocrine hormones travel long distances through vascular tissues to the target tissue. Paracrine hormones travel to nearby cells through the interstitial spaces between cells. Some hormones are released outside of a cell, then directly affect that cell. These are known as autocrine hormones. Some hormones act in multiple categories, acting on both local cells and working their way into the bloodstream. Every cell in an organism is specifically made with receptors for various hormones that will determine their function in the body and which signals they react to.

Functions of a Hormone

Hormones in Animals

The neuroendocrine system is a complex arrangement of cells in animals which can pass messages via hormones. From the time of conception to the time of death, different hormones will affect the body and alter its development and course. These chemical signals operate on a variety of levels in animals.

Neuroendocrine system

A hormonal response in animals starts with input to the sensory systems. Light, touch, smell, taste and other physical inputs are processed by the central nervous system. The brain decides what to make from these inputs based on genetics and past experiences. In response to the inputs, the brain sends a signal to the hypothalamus, the central processing center for hormonal instructions. For instructions which need to be delivered quickly, the hypothalamus contacts the posterior pituitary gland through nerve connections. These impulses signal the posterior pituitary to quickly release a hormone. Arteries within the gland carry the released hormones directly through the blood to the tissues they are meant for.

Alternatively, the hypothalamus is connected by short blood vessels directly to the anterior pituitary gland. The anterior pituitary contains a large number of different hormones which can be activated by specific hormones from the hypothalamus. The signal hormone released by the anterior pituitary can travel to specific tissues through the blood. It may be directed at other endocrine system glands, such as the adrenal glands or the gonads. These glands will receive the signal, causing the cascade of a different hormone.

While there are hundreds of reactions caused by a hormonal cascade, there are only a few different actions caused by hormones at a biochemical level. Many hormones bind to a surface proteins, which extends through the cell membrane. The protein then changes shape, causing a conformational change on the inside of the cell as well. This change can activate a second messenger, which carries the message to another point within the cell. Other hormones pass through the cell membrane and activate a process in the cytosol or travel all the way to the nuclear envelope to deliver a message about the rate of transcription.

There are hundreds of different signals that can be enacted by the hormone system. Different hormones activate different systems. Steroid hormones, for instance, activate gene transcription and regulate the enzymes created from genes. Vitamin D, a hormone and vitamin, regulates calcium in the blood and bones. Other signals can activate enzymes already present within the target cell, quickly turning on a metabolic process. Neurotransmitters are a special form of hormones, which travel only short distances between neurons. Animals also have special hormones called pheromones which they release into the environment to stimulate behaviors in other animals. These hormones can be sexual, territorial, or instructional.

Hormones in Plants

Much like in animals, plants have many different hormones which control their life cycles and development. There several groups of plant hormone, including the auxins, gibberellins and ethylene, among others. Plant hormones have been studied for a long time, as a means of modifying and manipulating plant growth. Some plant hormones have been developed artificially, for use on commercial crops. For instance, tomatoes are often ripened through the release of the plant hormone ethylene. This insures that all the tomatoes are ready for picking at the same time. This allows commercial machines to quickly and easily process entire fields at the same time. Auxin and related hormones are used to promote rooting and develop tissue cultures. This hormone also inhibits the growth of many adult plants and can act as a weed killer.

Plants also have hormones which come from a variety of starting molecules. In fact, a hormone from the brassinolides family resembles animal steroid hormones such as testosterone. Others, like hormones from the gibberellin family, have hundreds of different forms and don’t clearly resemble a known animal hormone. Synthetic versions of many of these hormones have been created in the lab, so their effects and composition could be more easily studied.

Through this process, it was found that plants have developed many pathways that use a particular hormone as a signal between plants. Plants being attacked by grasshoppers might release a hormone which signals neighboring plants to ready their defenses. This is very similar to how animals communicate with pheromones.

Hormonal Diseases

There are a wide variety of hormonal diseases affecting both people and animals. Because of the complexity of the endocrine system, problems may arise at many places. Common hormone issues include problems with the hypothalamus, pituitary, or adrenal glands. Irregularities in these organs can majorly affect growth, development, and metabolism. Other diseases related to hormones include hyperthyroidism, osteoporosis, and diabetes, all of which are caused by a hormone or hormones being improperly regulated or accepted by the body. A hormone disease can be genetic, or can be induced by the environment or diet.


1. Cortisol is a hormone associated with stress. Cortisol is released when an animal is scared, threatened, or suffering. Cortisol affects many tissues, but they have different reactions. How is this possible?
A. Different cells process cortisol into different chemicals
B. The receptor proteins are the same on the outside, but initiate different processes
C. The cortisol each cell receives is different

Answer to Question #1
B is correct. Each hormone-specific receptor protein can contain different tail-pieces, which extend into the target cells. The differing tail sections stimulate different reactions in different tissues, coordinating a body-wide response.

2. A scientist has identified a new hormone and specific receptor. While the scientist does not know much about the hormone, it is clear that the receptor is present only on the nuclear envelope. What can the scientist infer about the action of the hormone?
A. The hormone is water soluble
B. It signals for a quick metabolic shift
C. The hormone signals for a change in DNA transcription

Answer to Question #2
C is correct. Hormones that bind to proteins on the nuclear envelope usually have functions related to the regulation of DNA transcription. These chemical signals are usually insoluble in water, to make it easier for them to pass through the cell membrane.

3. You are studying a signaling system, and must identify the elements of the system. A chemical originate in a gland and travels through the bloodstream. This is Chemical A. Chemical B is attached to the cell membrane of the target cell, and Chemical C is created by Chemical B after Chemical A attaches to it. Chemical C moves through the cell’s cytosol and activates an enzyme. What is Chemical C?
A. Hormone
B. Receptor
C. Second Messenger

Answer to Question #3
C is correct. Chemical C is created after the hormone is accepted by the receptor. Second messengers do not leave the cell, and activate whatever process the hormone is intended to initiate.


  • Lodish, H., Berk, A., Kaiser, C. A., Krieger, M., Scott, M. P., Bretscher, A., . . . Matsudaira, P. (2008). Molecular Cell Biology (6th. ed.)New York: W.H. Freeman and Company.
  • McMahon, M. J., Kofranek, A. M., & Rubatzky, V. E. (2011). Plant Science: Growth, Development, and Utilization of Cultivated Plants (5th ed.). Boston: Prentince Hall.
  • Nelson, D. L., & Cox, M. M. (2008). Principles of Biochemistry. New York: W.H. Freeman and Company.
  • 1
  • 1
Notify of
Scroll Up