Steroids such as cholesterol, testosterone, and estrogen share a multi‐ring structure formed via a complex biosynthetic process. Their synthesis begins with squalene, a triterpene composed of six isoprene units. Isoprene polymerizes to form terpenes, whose double bonds enable squalene to cyclize into the core structure of steroids.
Terpenes form when isoprene is polymerized. They contain double bonds, which gives the molecule the ability to undergo cyclization. They are classified by their isoprene subunits:
In lipid biochemistry, triacylglycerols (fats) form when three fatty acids esterify with a glycerol molecule. This reaction reverses through saponification, which splits fats back into glycerol and free fatty acids. Before further metabolism, fatty acids are activated via phosphorylation and transesterification. Additionally, fatty acid synthesis follows a mechanism similar to a Claisen condensation, demonstrating a common biochemical assembly pathway.
The endocrine system is a chemical control network that maintains homeostasis at the cell, tissue, and organ level. It operates through hormones released by an endocrine gland—a structure that secretes these signaling molecules directly into surrounding fluids, rather than through ducts. Each hormone travels through the bloodstream to act on specific target cells, regulating their metabolism and function.
In contrast:
The pineal gland secretes melatonin, which helps regulate sleep cycles.
The thyroid produces metabolic thyroid hormones (requiring iodine) and releases calcitonin, lowering blood calcium by depositing it into bone.
In contrast, the parathyroid secretes parathyroid hormone (PTH), raising blood calcium by bone resorption and enhancing calcium uptake.
The thymus secretes factors that support T-cell development.
The adrenal glands make epinephrine and norepinephrine for the fight-or-flight response, mineralocorticoids like aldosterone to boost sodium and water retention, glucocorticoids like cortisol to increase blood sugar during stress, and androgens that include testosterone.
Within the pancreas, glucagon elevates blood sugar (breaking down glycogen) while insulin lowers it (enabling cellular glucose uptake).
The ovary primarily produces estrogen, whereas the testis chiefly secretes testosterone to support male reproductive functions.
The hypothalamus is crucial: it produces and releases regulatory factors that influence the pituitary gland:
The hypothalamus also synthesizes ADH (antidiuretic hormone, also called vasopressin), which increases water reabsorption in the kidneys, and oxytocin, which induces uterine contractions during labor and promotes milk ejection during breastfeeding.
The pituitary gland produces a suite of hormones collectively remembered as FLAT PEG:
The pituitary also stores ADH (antidiuretic hormone) and oxytocin produced by the hypothalamus.
In diabetes, the body either fails to produce sufficient insulin or its insulin receptors do not function properly, preventing glucose from entering cells and resulting in high blood sugar. This energy deficit forces cells to shift toward fatty acid metabolism, leading to the formation of ketone bodies and potentially causing ketoacidosis; excess sugar in urine further contributes to water loss via osmosis.
In hypothyroidism, decreased levels of thyroid hormone slow down the metabolism. When this condition stems from an iodine deficiency, the thyroid may enlarge into a goiter due to a buildup of uniodinated hormone precursors.
Conversely, hyperthyroidism involves an overproduction of thyroid hormones that accelerates metabolic processes.
An excess of Growth Hormone during childhood can cause Gigantism, characterized by well-proportioned excessive growth, while its overproduction later in life leads to Acromegaly, marked by disproportionate growth in regions still responsive to the hormone.
Neuroendocrinology is the study of how neurons interact with the endocrine system to regulate bodily functions. In this field, researchers explore how electrical signals from neurons influence the secretion of hormones by endocrine glands.
A key example is the role of the hypothalamus, which sends neural signals to the pituitary gland to control the release of hormones that manage processes like growth, metabolism, and stress. These hormonal signals, in turn, affect neural activity, creating feedback loops that help maintain homeostasis. Certain molecules known as neuropeptides can act both as neurotransmitters and hormones.
Cellular mechanisms of hormone action involve how signals are transmitted within target cells once a hormone binds its receptor.
Pathways:
In the Phospholipid pathway, a similar sequence occurs, but the activated Phospholipase C splits a membrane phospholipid into DAG and IP3; DAG activates another protein kinase cascade, while IP3 releases from the ER.
The Steroid pathway works differently: the hormone crosses the membrane, binds its receptor in the cytoplasm or nucleus, and forms a transcription factor complex that activates specific genes.
Hormone transport and specificity
Specificity depends on whether cells possess the appropriate receptors, which can be upregulated or downregulated. The nervous system modulates hormones through feedback control, such as sometimes adjusting normal blood glucose set points in response to stress or mood changes caused by low estrogen during menses.
Hormones are regulated through 3 types of stimuli:
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