See also types of hormones.

Organisms contain many other kinds of lipids besides fats. Terpenes are long-chain lipids that are components of many biologically important pigments, such as chlrophyll and the visual pigment retinal. Rubber is a terpene. Steroids, another class of lipids are composed of four carbon rings. Many animal cell membranes contain the steroid cholesterol. Other steroids, such as testosterone and estrogen, function as hormones in multicellular animals.

Hormones are chemical substances, produced in small, often minute quantities in one part of an organism and then transported to another part where they bring about physiological or developmental responses. In animals, hormones are usually produced at definite sites, most commonly in organs such as glands. In plants, hormones are not produced in specialized tissues.

A variety of hormones are secreted by organs that are not exluively endocrine glands. The thymus, which is located in the upper chest in front of the heart, is the site of T cell production in many vertebrates and T cell maturation in mammals. It also secretes a number of hormones that function inthe regulation of the immune system.

Endocrine Glands Involved in the Secretion of Hormones:

Hypothalamus: controls the production and secretion of its hormones. Neurons in the hyopthalamus secrete two tyeps of neurohormones; releasing hormones and inhibiting hormones that diffuse into blood capillaries at the base of the hyopthalamus. These capillaries drain into small veins that run within the stalk of the pituitary to a seocnd bed of capillaries in the anterio pituitary.

Releasing hormones are peptide neurohormones that stimulate release o other hormones; specifically, TRH stimulates the release of TSH, CRH stimulates the release of ACTH and GnRH stimulates the release of FSH and LH.

For example, the hyopthalamus secretes TRH into the hypothalamochypophyseal portal system, which stimulates the anterior pituitary to secrete TSH which in turn casues the thryoid gland to release thyroxine Thyroxine and other thyroid hormones affect metabolic rate. Among thryoxine’s many target organs are the hypothalamus and the anterio pituitary themselves to provide negative feedback which is essential for homeostasis . The hormone thryoxine contains the element iodine; without idoine, the hygroid gland cannot produce throxine. Individuals living in iodine poor areas lack sufficient iodine to manufacture throxine so the hypothalamus and anterior pituitary receive far less negative feedback inhibition than normal. This reduced inhibition resutls in elevated secretion of TRH and SH. High levels of TSH stimulate the thryoid gland whose cells enlarge in a futile attempt to manufacture more thryoxine. Because they cannot do so without iodine, they throid gland keeps getting larger, a condition known as a goiter.

Pituitary gland (hypophysis): hangs by a stalk form the hypothalamus at the base of the braine posterio to the optic chiasm. The hypothalamus is a part of the CNS that has a major role in regulating body processes.

The posterior pituitary appears fibrous because it contains axons that originate in cell bodies within the hypothalmus and that extend along the stalk of the pituitary as a tract of fibers.

In the 1950s investigators isolated a peptide form the posterior pituitary, antidiuretic hormone (ADH) also called vasopressin. ADH sitmulates water reabsorption by the kidneys and in doing so inhibits diuresis (urine production). When ADH is missing, the kidneys do not reabsorb as much water and excessive quantitites of urine are products. This is why the consumption of alcohol, which inhibits ADH secretion, leads to frequent urination.

The posterior pituitary also secretes oxytocin, a peptide neurohormone that, like ADH, is composed of nine amino acids. In mammals oxytocin stimulates the milk ejection reflex.

ADH and oxytocin are actually produced by neuron cell bodies lcoated in the hypothalamus. These two neuro hormones are transported along the axon tract that runs form the hypothalamus to the posterior pituitary, where they are stored. In response to the appropriate stimulation, increased blood plasma osmolality in the case of ADH or the suckling of a baby in the case of oxytocin, the neurohormones are rleased by the posterio pituitary into the blood.

The anterior pituitary, unlike the posterior pituitary, does not develop form growth of the brain; instead it develops from a pouch of epithelial tissue that pinches off from the roof of the embryo’s mouth. Because it forms from epithelial tissue, it is an independent endocrine gland. It produces a least seven essential hormones, many of which stimulate growth of their target organs, as well as production and secretion of toher hormones form additional endocrine glands.

The importance of the anterio pituitary is shown by a condition known as gigantisms, characterized by excessive grwoth. This is casued by the excessive secretion of GH in a growing child. GH stimulates the production of insulin-like grwoth factors, which liver and bone produce in response to stimulation by GH. These factors then stimulate cell division int he epiphyseal grwoth plates, adn thus the elongation of the bones.

Thyroid gland: varies in shape in different vertebrate species. In humans it is shaped like a bow tie and lies just below the Adam’s apple in the front of the neck. It secretes hormones such as thyroxine and calcitonin (a peptide hormone that plays a role in maintaining proper leves of calcium Ca2+). . Thyroid hormones are unique in being the only molecules in the body containing iodine.

Thyroid hormones regulate enzymes controlling carbohydrate and lipid metabolism in most cells, promoting the appropriate use of these fuels for maintaining the body’s basal metabolic rate. Thyroid hormones often funciton cooperatively with other hormones, promoting the activity of grwoth hormone, epinephrine and reproductive sterodis. Throid hormones direct metamorphosis of tadpoles into frogs.

Parathyroid Glands: are four small glands attached to the thryoid. Parathyroid gland secrete a peptide called parathyroid hormone (PTH) which is syntehsized and released in response to falling levels of Ca2+ in the blood. This decline cannot be allowed to continue uncorrected becasue a significant fall in the blood Ca2+ level can cause severe musle spasms. PTH stimulates the oxteoclasts bone cells in bone to dissolve the calcium phosphate crystals of the bone matrix and release Ca2+. PTH is also encessary as a finals tep in the activation of vitamin D, which is an essential vitamin.

Adrenal Gland: is located just above each kidney. Each bland is composed of an inner portion, the adrenal medulla and an otter layer, the adrenal cortex.

–The adrenal medulla receives neural input form axons of the sympathetic division of the autonomic nervous system and it secretes the catecholamines epinephrine and norepineprhine in response to sitmulation by these axons.

–The adrenal cortex: secretes steroids which are are reffered to as corticosteroids. In mammals, these hormones are referred to as glucocorticoids and their secretion is primarily regualted by ACTH from the anterior pituitary. The glucocortidoids stimulate the break down of musle prtoein into amino acids, which are carried by the blood to the liver. They also sitmulate the liver to produce the enzymes needed for gluconeogenesis, which can convert amino acids into glucose. Glucose syntehsis form protein is particulalry important druing very long periods of fasting or exercise, when blood glucose levels might otherwise become dangerously low.

Pancreas: is located adjacent to the stomach and is connected to the duodenum of the small intestine by the pancreatic duct. It secretes bicarbonate ions and a variety of digestive enzymes into the small intestine through this duct. Clusters of cells called islets of langerhans or pancreatic islets are scatered throughout the pancreas. Surgical removal of the pancrease cause glucose to appear in the urine, the hallmark of the disease diabetes mellitus because these cells produce insulin. Insulin is secreted by the beta cells of the islets. People with type I, or inslin-dependent diabetes mellitus, lack the insulin secreting beta cells and consequently produce no insulin. Treatment requires insulin injections (becaseu insulin is a peptide hormone, it would be digested if taken orally and must instead be injected subcutaneously). Most diabetic patients have type II or non-insulin dependent, diabetes. They generally have normal or even above normal levels of insulin in their blood, but theri cells have a reduced sensitivity to insulin. These people may not require insulin injections and can often control their diabetes through diet and exercise.

The cells produce another hormone called glucagon which acts antagonistically to insulin. When a person eats carbohydrates, the blood glucose concentration rises which directly activates the secretion of insulin by the beta cells and inhibits the secretion of glucagon by the alpha cells. Insulin promotes the cellular uptake of glucose into the liver, muscle and fat cells. It also activates the storage of glucose as glycogen in liver and muscle or as fat in fat cells. Between meals, when the concentration of blood glucose falls, insulin secretion decreases and glucagon secretion increases. glucagon promotes the hydrolysis of stored glycogen in the liver and fat in adipose tissue. As a result, glucose and fatty acids are relased into the blod and can be taken up by cells and used for energy.

Ovaries and Testes: in vertebrates are important endocrine glands producing the sex steorid hormones, including estrogens, progesterone and testosterone. Estrogen and progesterone are the primary feamle sex steroids and testosterone is the primary male sex steroids or androgens. Both types of hromone can be found in both sexes, hwoever. During embryonic development, testosterone production in the male embryo is critical for the development of male sex organs. In mammals, sex steroids are responsible for the development of seconary sexual characteristics at puberty such as body ahir and increased muscle mass in males.

Signaling Pathways for Hormones:

Some steroid hormones bind to theri receptors in the cyoplasm, an then move as a hormone-receptor complex into the nucleus. Other steroids and the thyroid hormones travel directly into the nucleus before encountering their receptor proteins. The hormone-receptor complex can bind to specific DNA sequences in the genome, called hormone response elements (HRE) which are located in the promoters of genes that respond to the hormone. This can activate, or in some cases repress. transcription.

Hydrophilic hormones, such as peptide, protein and glycoprotein hormones, as well as the catecholamine hormones, cannot cross the plasma membranes of their target cells. These hormones act through membrane receptors with an extracellular domain that binds the hormone, and an intracellular domain that interacts with cytoplasmic proteins. This includes RTKs. The frist response form a hormone binding to an RTK is usually to associate together as a dimer, which causes the activated receptor to phosphorylate itself. This autophosphorylation initiates signal transduction pathways through cellular proteins that bind phosphotyrosine.

For example, when the peptide hormone insulin binds its receptor, the response results in glucose transport proteins being inserted into the plasma membrane, and stimulation of enzyme to convert the increased intracellular glucose to glycogen. Growth hormone works through similar mechanisms, except that the phosphotyrosine binding proteins initiate a kinase cascade.

GPCRs function by activating membrane associated G proteins that in turn stimulate effector proteins, which often are enzymes that generate second messengers. When a hormone binds to a GPCR, the G protein is activated by exchanging GDP for GTP. The activated G protein shuttles form the receptor to the second messenger generating enzyme. The activated enzyme then produces second messenger molecules, which can propagate the signal through the cell.

In the case of epinephrine, the G protein activates an enzyme called adenylyl cyclase, which catalyzes the fomraiton of the seocnd messenger cAMP from ATP. The second messenger formed at the inner surface of the plasma membrane then diffuses within the cytoplasm, where it binds to and activates protein kinases.

The proteins that are phosphorylated by the protein kinases depend on the cell type and include enzymes, membrane transport proteins and transcription factors. This diversity provides hormones with distinct actions in different tissues. In live cells, for example, cAMP dependent protein kinases activate enzymes that convert glycogen into glucose. In cardiac muscle cells cAMP causes an increase in the rate and force of cardiac muscle contraction.

Hormonal Regulation of the Gastrointestinal Tract:

The activities of the gastroinestinal tract are coordinated by the nervous system and the endocrine system. The nervous system, for example, stimulates salivary and gastric secretions in response to sigght, smell and consumption of food. When food arrives in the stomach, proteins in the food stimulate the secretion of a stomach hormone gastrin, which in turn stimulates the secretion of pepsingoen and HCL from teh gastric glands. The secretion HCl lowers the pH of the gastric juice, which acts to inhibit additional secretion of gastrin in a negative feedback loop.

The duedenum (the first 25 cm of the small intestine) secretes three hormones; cholecystockinin (CCK) which stimulates contraction of the gallbladder and secretion of pancreatic enzyme, secretin, which stimulates secretion of pancreatic bicarbonate and gastric inhibitory peptide (GIP), which inhibits stomach emptying. The hormones GIP and CCK have receptors in the hypothalamus and seem to send the same kinda of inhibitory signals to the brain as leptin and insulin. The gut homrone ghrelin has the opposite effect of these appetite suppressing hormones Ghrelin also has receptors in the hypothalamus, but ghrelin appears to stimulate food intake. Gastric bypass surgery leads to reduced lelvels of ghrelin and has been sugested that this is one of the reasons for the suppression of appetite seen after this surgery.

The brain neuropeptides neuropeptide Y (NPY) and alphamelanocyte-stimulating hormone (alpha-MSH) are antagonstic, with NPY inducing feeding activity and alpha-MSH suppressing it. The production and release of alpha-MSH has been shown to be stimulated by leptin and the administration of alpha-MSH suppresses feeding. Loss of funciton for the alpha-MSH receptor also leads to obesity. Incontrast, the expression of NPY is negatively regulated by leptin and adminsitraiton of NPY stimulates feeding behavior.

Gastrin: is secreted by the mucosa of the stomach. It stimulates the secretion of HCl and pepsinogen (which is converted into pepsin).

Insulin: has been implicated in signaling satiety and insulin levels fall with fasting and rise with obesity. Insulin and glucagon are produced in the islets of Langerhans, clusters of endocrine cells scattered throughout the pancreas. After a carbohydrate rich meal, the liver and skeletal muscles remove excess glucose from the blood and stores it as the polysaccharide glycogen. This process is stimulated by the hormone insulin, secreted by the beta cells in the pancreatic islets of Langerhans. When blood glucose levels decrease, as they do between meals, during periods of fasting, and during exercise, the liver secretes glucose into the blood. This glucose is obtained in part from a breakdown of liver glycogen to glucose 6-phosphate, a process called glycogenolysis. The phosphate group is then removed, and free glucose is secreted into the blood. Skeletal muscles lack the enzyme needed to remove the phosphate group, and so, even though they have glycogen stores, they cannot secrete glucose into the blood. However, muscle cells can use this glucose directly for energy metabolism because glucose 6-phosphate is actually the product of the first reaction in glycolysis. The breakdown of liver glycogen is stimulated by another hormone, glucagon, which is created by the alpha cells of the islets of Langerhans in the pancreas. If fasting or exercise continues, the liver begins to convert other molecules, such as amino acids and lactic acid, into glucose. This process is called gluconeogenesis. The amino acids used for glucogenogenesis are obtained from muscle protein, which explains the severe muscle wasting that occurs during prolonged fasting.

Leptin: is a peptide hormone. which is encoded by the ob (for obese) gene. Leptin is thought to be the main signaling molecule of the control circuit for energy sensing, food intake, and energy expenditure. Leptin is produced by adipose tissue in response to feeding and leptin levels correlate with feeding behavior and amount of body fat. Dietary restriction reduces leptin levels, signaling the brain that food intake is necessary, whereas refeeding after fasting leads to rapid increase in leptin levels and a loss of appetite. Recent studies indicate, however, that most cases of human obesity in humans may result from a reduced sensitivity to the actions of leptin in the brain, rather than from reduced leptin production by adipose cells.

Plant Hormones/Responses:

Eight major kinds of plant hormones ahve been identified: auxin, cytokinins, strigolactones, gibberellins, brassinosteroids, oligosaccharins, ethylene and absisic acid.

Auxin allows elongation and organizes the body plan.

Cytokinins stimulate cell division and differentiation.

Ethylene induces fruit ripening and aids plant defenses. First, auxin, which is produced in significant amounts in pollinated flowers and developing fruits, stimulates eyylene production, which, in term, speeds fruit ripening. Complex carbohydrates are broken down into simple sugars, chlorophylls are broken down, cell wals become soft, and the folatile compoudns assocaited with flavor and scent in ripe fruits are produced. Tomatoes are often picked green and artificially ripened later by the application of ethylene. Ehylene is widely used to speed the ripening of lemons and oranges as well.

Oligosacharins act as defense signaling molecuels. Oligosaccharins can be released form the cell wall by enzymes secreted by pathogens.

Plant responses to flooding:

Plants can receive too much water, in which case they ultimately drown becasue flooding depletes available oxygen in teh soil and interferes with the transport of minerals and carbohydrates in the roots. Hormone levels change in flooded plants; ethylene, a hormone associated with suppression of root elongation, increases, while gibberellins and cytokinins, which enhance growth of new roots, usually decreases.

Plnts can respond to flooded conditions by forming alrger lenticels which facilitate gas exchange and adventitious roots that reach above flood level for gas excahnge. Some plants have adapted to living in freshwater. The most frequent adaptions is the formation of aerenchyma, loose parenchymal tissue with large air spaces in it. Aerechyma is very prominent in water lilies and other aquatic plants. Oxygen may be transproted form the parts of the plant above water to those below by way of passages in the acrenchyma.