Each gland makes one or more hormones. Hormones are important for almost all cells in the body to work. They influence the metabolism , growth and many other functions. Some hormones influence almost all cells in the body. Others only influence a small number of cells in specific organs.
For example, the pituitary gland releases hormones that tell the thyroid gland, the adrenal glands, the ovaries and the testes to release hormones. When a hormone is released from a gland, it travels in the bloodstream through the body. It passes by most cells, but eventually reaches its target. When it reaches its target, it attaches to a particular type of cell, known as a receptor cell.
The hormone then tells the receptor cell to do something. It can make cells grow faster, release another hormone, absorb sugar from the blood, withhold water from the kidneys, or one of many other important functions in the body. Growth hormone : This is released from the pituitary gland.
It is essential for normal physical growth in children and for some functions in adults, such as fat and muscle mass. Thyroxine : The thyroid gland in the neck converts iodine from the diet into thyroxine.
This controls many functions of the metabolism, including temperature, heart rate and growth. Cortisol : This is released from the adrenal glands just above the kidneys. It is important for controlling blood pressure and for dealing with the body's response to stress. Insulin : This is produced by islet cells in the pancreas. It controls blood sugar levels. Oestrogen and progesterone : These hormones, released from the ovaries, are responsible for female body characteristics and for storing and releasing eggs.
Testosterone : This is released from the testes. These hormones are stored in endocrine cells until needed. They act by binding to protein receptors on the outside surface of the cell. The binding alerts a second messenger molecule inside the cell that activates enzymes and other cellular proteins or influences gene expression. Insulin, growth hormone, prolactin and other water-soluble polypeptide hormones consist of long chains of amino acids , from several to amino acids long.
They are stored in endocrine cells until needed to regulate such processes as metabolism, lactation, growth and reproduction.
Fat Soluble Most water-soluble hormones, like the amino acid derivatives and peptides, can travel freely in the blood because they "like" water. However, they are repelled by lipid or fatty structures such as the membranes that surround the cell and nucleus. Because of this, these hormones generally bind to receptor sites on the outside of the cell and signal from there.
Fat-soluble hormones, like the sex hormone steroids estrogens and androgens, are fat soluble and water repellent. That is, they "like" lipid or fatty structures such as those surrounding cells but are generally repelled by watery areas. Such periods of hormonal change are also associated with fluctuations in mood.
When chemical levels shift, they may cause changes in mood, as well. But research shows that our behavior is collectively shaped by a variety of influences, including the brain and its neurotransmitters, our hormones, and various social factors. The primary function of the endocrine system is to regulate our bodily processes, not control us. Sometimes disease, stress, and even diet can disrupt that regulatory function, however, altering the quantity of hormones that glands secrete or changing the way that cells respond.
Diabetes is one of the most common hormonal disorders, occurring when the pancreas secretes too little insulin, a hormone that manages blood sugar levels. And hypo- and hyperthyroidism occur when the thyroid gland makes too little or too much thyroid hormone. But most of the time, the endocrine system manages to keep our bodies in a state of balance.
Lipid insoluble hormones bind to receptors on the outer surface of the plasma membrane, via plasma membrane hormone receptors. Unlike steroid hormones, lipid insoluble hormones do not directly affect the target cell because they cannot enter the cell and act directly on DNA.
Binding of these hormones to a cell surface receptor results in activation of a signaling pathway; this triggers intracellular activity and carries out the specific effects associated with the hormone.
In this way, nothing passes through the cell membrane; the hormone that binds at the surface remains at the surface of the cell while the intracellular product remains inside the cell. The hormone that initiates the signaling pathway is called a first messenger , which activates a second messenger in the cytoplasm, as illustrated in Figure When a hormone binds to its membrane receptor, a G-protein that is associated with the receptor is activated; G-proteins are proteins separate from receptors that are found in the cell membrane.
When a hormone is not bound to the receptor, the G-protein is inactive and is bound to guanosine diphosphate, or GDP. The activated G-protein in turn activates a membrane-bound enzyme called adenylyl cyclase. The phosphorylation of a substrate molecule changes its structural orientation, thereby activating it. These activated molecules can then mediate changes in cellular processes.
The effect of a hormone is amplified as the signaling pathway progresses. The binding of a hormone at a single receptor causes the activation of many G-proteins, which activates adenylyl cyclase. Each molecule of adenylyl cyclase then triggers the formation of many molecules of cAMP. Further amplification occurs as protein kinases, once activated by cAMP, can catalyze many reactions.
In this way, a small amount of hormone can trigger the formation of a large amount of cellular product. PDE is always present in the cell and breaks down cAMP to control hormone activity, preventing overproduction of cellular products.
The specific response of a cell to a lipid insoluble hormone depends on the type of receptors that are present on the cell membrane and the substrate molecules present in the cell cytoplasm. Cellular responses to hormone binding of a receptor include altering membrane permeability and metabolic pathways, stimulating synthesis of proteins and enzymes, and activating hormone release.
Hormones cause cellular changes by binding to receptors on target cells. The number of receptors on a target cell can increase or decrease in response to hormone activity.
0コメント