Hormones function as chemical messengers, coordinating various bodily functions by carrying signals through the bloodstream to organs, skin, muscles, and other tissues. These substances are produced and released by endocrine glands, which are specialized organs that lack ducts and secrete their products directly into the blood. The endocrine system, comprising these glands, regulates processes including metabolism, growth, development, and reproduction. Hormones exert their effects on specific “target cells” that possess the necessary components to receive and interpret their messages.
How Hormones Recognize Target Cells
Hormones circulate throughout the body, yet they only affect specific cells known as target cells. This selectivity occurs because target cells possess unique protein structures called receptors, which act like locks designed to fit a particular hormone’s key. A cell is considered a target cell for a given hormone only if it contains functional receptors for that hormone; cells without these specific receptors cannot be directly influenced by that hormone. The presence or absence of these receptors is the fundamental mechanism determining hormone specificity.
These receptor proteins are not uniformly located; they can be situated either on the surface of the cell membrane or inside the cell, within the cytoplasm or nucleus. The chemical nature of the hormone dictates where its corresponding receptor will be found. This distinction in receptor location sets the stage for the two primary ways hormones alter their target cells.
Altering Cells from Within
Lipid-soluble hormones, such as steroid hormones (like estrogen, testosterone, and cortisol) and thyroid hormones, have a chemical structure that allows them to easily pass through the lipid bilayer of the cell membrane. Once inside the target cell, these hormones bind to specific receptors located in the cytoplasm or directly within the nucleus. This binding forms a hormone-receptor complex, which is the activated form of the hormone.
The hormone-receptor complex then moves into the nucleus, if it wasn’t there already. Inside the nucleus, this complex binds directly to specific regions of DNA. This interaction directly influences gene expression, either increasing or decreasing the transcription of particular genes into messenger RNA (mRNA). The altered mRNA then directs the synthesis of new proteins, which ultimately changes the cell’s function or structure. This mechanism results in slower but often longer-lasting effects on the cell, such as those seen in growth and development or the regulation of metabolic processes.
Altering Cells from the Surface
Water-soluble hormones, including peptide hormones (like insulin and growth hormone) and most amine hormones (such as epinephrine), cannot diffuse through the cell membrane due to their chemical properties. Instead, these hormones bind to specific receptor proteins located on the outer surface of the target cell’s plasma membrane. This binding event acts as a “first messenger” signal.
The hormone-receptor binding triggers a cascade of events inside the cell, often involving specialized proteins known as G proteins. These activated G proteins, in turn, activate enzymes within the cell membrane. A common pathway involves the enzyme adenylyl cyclase, which converts adenosine triphosphate (ATP) into cyclic adenosine monophosphate (cAMP), a crucial “second messenger” molecule. Other second messengers include calcium ions and inositol trisphosphate (IP3).
These second messengers amplify the initial signal and activate or deactivate various enzymes and proteins within the cell. This complex internal signaling leads to rapid cellular responses, such as changes in enzyme activity, the opening or closing of ion channels, or the secretion of other substances. For example, insulin binding to its surface receptor initiates pathways that lead to glucose uptake and storage.
Wide-Ranging Cellular Changes
Hormone action on target cells leads to a diverse array of physiological changes. One common change involves cellular metabolism, such as the regulation of glucose uptake and utilization. For example, insulin promotes the uptake of glucose into muscle and fat cells and stimulates its conversion into glycogen for storage.
Hormones can also influence protein synthesis, leading to the production of new proteins that alter cell structure or function. This is particularly evident in processes like growth and development, where hormones such as growth hormone and sex hormones play significant roles in increasing cell size and number. Additionally, hormones can trigger the secretion of other substances from target cells, like the release of digestive enzymes or other hormones. Changes in membrane permeability, affecting the movement of ions or molecules across the cell membrane, are another outcome. These cellular changes contribute to maintaining the body’s internal balance and coordinating complex physiological processes.