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Another type of lipids are steroid s , complex, ringed structures that are found in cell membranes; some function as hormones. The most common types of steroids are sterol s , which are steroids containing an OH group. These are mainly hydrophobic molecules, but also have hydrophilic hydroxyl group s. The most common sterol found in animal tissues is cholesterol . Its structure consists of four rings with a double bond in one of the rings, and a hydroxyl group at the sterol-defining position. The function of cholesterol is to strengthen cell membranes in eukaryotes and in bacteria without cell walls, such as Mycoplasma . Prokaryotes generally do not produce cholesterol, although bacteria produce similar compounds called hopanoids , which are also multiringed structures that strengthen bacterial membranes ( [link] ). Fungi and some protozoa produce a similar compound called ergosterol , which strengthens the cell membranes of these organisms.
The moisturizing cream prescribed by Penny’s doctor was a topical corticosteroid cream containing hydrocortisone. Hydrocortisone is a synthetic form of cortisol, a corticosteroid hormone produced in the adrenal glands, from cholesterol. When applied directly to the skin, it can reduce inflammation and temporarily relieve minor skin irritations, itching, and rashes by reducing the secretion of histamine, a compound produced by cells of the immune system in response to the presence of pathogens or other foreign substances. Because histamine triggers the body’s inflammatory response, the ability of hydrocortisone to reduce the local production of histamine in the skin effectively suppresses the immune system and helps limit inflammation and accompanying symptoms such as pruritus (itching) and rashes.
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Lipids are a naturally occurring group of substances that are not soluble in water but are freely soluble in organic solvents.
False
Fatty acids having no double bonds are called “unsaturated.”
True
A triglyceride is formed by joining three glycerol molecules to a fatty acid backbone in a dehydration reaction.
False
Waxes contain esters formed from long-chain __________ and saturated __________, and they may also contain substituted hydrocarbons.
alcohols; fatty acids
Cholesterol is the most common member of the __________ group, found in animal tissues; it has a tetracyclic carbon ring system with a __________ bond in one of the rings and one free __________group.
steroid; double; hydroxyl
Describe the structure of a typical phospholipid. Are these molecules polar or nonpolar?
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