37.3: How Hormones Work - Introduction
- Explain the ways in which hormones work
Hormones
A hormone is a chemical released by a cell or a gland in one part of the body that sends out messages that affect cells in other parts of the organism. Only a small amount of hormone is required to alter cell metabolism. In essence, it is a chemical messenger that transports a signal from one cell to another. All multicellular organisms produce hormones; plant hormones are also called phytohormones. Hormones in animals are often transported in the blood.
How Hormones Work
Hormones mediate changes in target cells by binding to specific hormone receptors. In this way, even though hormones circulate throughout the body and come into contact with many different cell types, they only affect cells that possess the necessary receptors. Receptors for a specific hormone may be found on many different cells or may be limited to a small number of specialized cells. For example, thyroid hormones act on many different tissue types, stimulating metabolic activity throughout the body. Cells can have many receptors for the same hormone, but often also possess receptors for different types of hormones. The number of receptors that respond to a hormone determines the cell’s sensitivity to that hormone and the resulting cellular response. Additionally, the number of receptors that respond to a hormone can change over time, resulting in increased or decreased cell sensitivity. In up-regulation, the number of receptors increases in response to rising hormone levels, making the cell more sensitive to the hormone, allowing for more cellular activity. When the number of receptors decreases in response to rising hormone levels, called down-regulation, cellular activity is reduced.
Cells respond to a hormone when they express a specific receptor for that hormone. The hormone binds to the receptor protein, resulting in the activation of a signal transduction mechanism that ultimately leads to cell type-specific responses. Receptor binding alters cellular activity, resulting in an increase or decrease in normal body processes. Depending on the location of the protein receptor on the target cell and the chemical structure of the hormone, hormones can mediate changes directly by binding to intracellular hormone receptors and modulating gene transcription, or indirectly by binding to cell surface receptors and stimulating signaling pathways.
Key Points
- Hormones can only affect cells that display receptors that are specific to them; cells can display receptors for many different hormones at once.
- The more receptors for a particular hormone that a cell displays, the more sensitive to that hormone it will be.
- When a cell displays more receptors in response to a hormone, this is called up-regulation, but when a cell reduces its number of receptors for a particular hormone, this is called down-regulation.
- A hormone can make changes directly to a cell by changing what genes are activated, or make changes indirectly to a cell by stimulating particular signaling pathways inside the cell that affect other processes.
Key Terms
- phytohormone : a plant hormone
- hormone : any substance produced by one tissue and conveyed by the bloodstream to another to affect physiological activity
- receptor : a protein on a cell wall that binds with specific molecules so that they can be absorbed into the cell in order to control certain functions