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Questions and Answers
Steroids consist of 5 fused rings in their structure.
Steroids consist of 5 fused rings in their structure.
False
Phytosterols are sterols that occur naturally in animals.
Phytosterols are sterols that occur naturally in animals.
False
Cholesterol is the most important zoosterol in the human body.
Cholesterol is the most important zoosterol in the human body.
True
Sterols are synthesized from acetyl coenzyme A through the HMG-CoA reductase pathway.
Sterols are synthesized from acetyl coenzyme A through the HMG-CoA reductase pathway.
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Sterols are exclusively found in prokaryotic organisms.
Sterols are exclusively found in prokaryotic organisms.
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Cortisol functions as a signalling compound in the body.
Cortisol functions as a signalling compound in the body.
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Cholesterol has a molecular formula of C26H45OH.
Cholesterol has a molecular formula of C26H45OH.
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Supplemental plant sterols are recommended by the American Heart Association for all individuals.
Supplemental plant sterols are recommended by the American Heart Association for all individuals.
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Cholesterol is soluble in water.
Cholesterol is soluble in water.
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Phytosterols may help block cholesterol absorption in the human intestine.
Phytosterols may help block cholesterol absorption in the human intestine.
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The melting point of cholesterol is approximately 145 to 148 degrees Celsius.
The melting point of cholesterol is approximately 145 to 148 degrees Celsius.
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Cholesterol occurs in the largest amounts in the brain and nervous tissue.
Cholesterol occurs in the largest amounts in the brain and nervous tissue.
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A hen's egg weighing 2 oz provides approximately 450 mg of cholesterol.
A hen's egg weighing 2 oz provides approximately 450 mg of cholesterol.
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Cholesterol is primarily produced in the skin through UV rays to aid vitamin D formation.
Cholesterol is primarily produced in the skin through UV rays to aid vitamin D formation.
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Norum's disease results from an excess of lecithin in the plasma.
Norum's disease results from an excess of lecithin in the plasma.
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Aging causes an increase in blood cholesterol levels until the age of 55, after which levels start to decrease.
Aging causes an increase in blood cholesterol levels until the age of 55, after which levels start to decrease.
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The normal level of serum total cholesterol in an adult ranges from 100 to 200 mg per cent.
The normal level of serum total cholesterol in an adult ranges from 100 to 200 mg per cent.
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Cholesterol esters are predominantly formed from esterification involving fatty acids at the C3 position.
Cholesterol esters are predominantly formed from esterification involving fatty acids at the C3 position.
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The cholesterol level in women is influenced significantly by hormonal changes during the menstrual cycle.
The cholesterol level in women is influenced significantly by hormonal changes during the menstrual cycle.
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Lecithin cholesterol acyl transferase (LCAT) is not involved in the formation of plasma cholesterol esters.
Lecithin cholesterol acyl transferase (LCAT) is not involved in the formation of plasma cholesterol esters.
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Cholesterol in the adrenal gland is primarily related to vitamin D synthesis.
Cholesterol in the adrenal gland is primarily related to vitamin D synthesis.
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7-Dehydrocholesterol is converted to cholecalciferol in the skin through the action of UV rays.
7-Dehydrocholesterol is converted to cholecalciferol in the skin through the action of UV rays.
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Ergosterol is found primarily in animal tissues.
Ergosterol is found primarily in animal tissues.
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During pregnancy, there is a decrease in free cholesterol levels.
During pregnancy, there is a decrease in free cholesterol levels.
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Sitosterol may help in lowering blood cholesterol levels by decreasing intestinal absorption.
Sitosterol may help in lowering blood cholesterol levels by decreasing intestinal absorption.
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Coprosterol results from the hydrogenation of cholesterol through bacterial action.
Coprosterol results from the hydrogenation of cholesterol through bacterial action.
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Ergosterol can be converted to vitamin D2 through exposure to UV rays.
Ergosterol can be converted to vitamin D2 through exposure to UV rays.
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7-Dehydrocholesterol differs from cholesterol by having an additional hydroxyl group at position C7.
7-Dehydrocholesterol differs from cholesterol by having an additional hydroxyl group at position C7.
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Bile acids are classified as important steroids of biomedical importance.
Bile acids are classified as important steroids of biomedical importance.
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Study Notes
Sterols: Structures and Biological Functions
- Steroids are complex molecules with 4 fused rings.
- Often found in association with fats, but separable after saponification.
- All steroids share a similar cyclic nucleus resembling phenanthrene (rings A, B, and C), with a cyclopentane ring (ring D) attached.
- This structure is called cyclopentanoperhydrophenanthrene, or sterane/gonane ring.
- Sterols are steroid alcohols, where a hydroxyl group replaces a hydrogen atom at position 3.
- Methyl side chains typically occur at positions 10 and 13 (carbons 19 and 18, respectively), plus a side chain at position 17.
- Sterols are amphipathic lipids synthesized from acetyl coenzyme A through the HMG-CoA reductase pathway.
- Found in eukaryotes (plants, animals, fungi).
- Plant sterols are called phytosterols; animal sterols are zoosterols.
- Cholesterol is the most important zoosterol.
- Notable phytosterols include campesterol, sitosterol, and stigmasterol.
- Sterols play crucial roles in eukaryotic physiology.
- Cholesterol is a significant component of animal cell membranes, affecting fluidity and acting as a secondary messenger in developmental signaling.
- Corticosteroids (e.g., cortisol) act as signaling compounds in humans.
- Sterols are components of human skin oils.
- Phytosterols reduce cholesterol absorption in the human intestine, as shown in clinical trials.
- The American Heart Association recommends supplemental plant sterols for hypercholesterolemia (but not for pregnant or nursing women).
- Some phytosterols show preliminary anti-cancer effects.
Cholesterol
- The most important sterol in humans, with molecular formula C27H45OH.
- Biosynthesized by all animal cells and is an essential structural component of animal cell membranes.
- Possesses a cyclopentanoperhydrophenanthrene nucleus.
- Contains an –OH group at C3.
- Has an unsaturated double bond between C5 and C6.
- Has two –CH3 groups at C10 and C13.
- An eight-carbon side chain is attached to C17.
- The name "cholesterol" stems from the Greek word for "solid bile."
- Occurs as a white or faintly yellow, almost odorless, granular solid.
- Insoluble in water but sparingly soluble in alcohol and soluble in ether, chloroform, hot alcohol, ethyl acetate and vegetable oils.
- No saponification occurs.
- Melting point is 147–150 °C.
- Can take up two halogen atoms due to an unsaturated bond.
Cholesterol Sources
- Exogenous: Dietary cholesterol (approximately 0.3 g/day) from foods rich in cholesterol (butter, cream, milk, egg yolks, meat, etc.). A hen's egg (2 oz) provides ~250 mg of cholesterol.
- Endogenous: Synthesized in the body from acetyl CoA (approximately 1 g/day).
- High concentrations in brain/nervous tissue (2% ), liver (0.3%), skin (0.3%), intestinal mucosa (0.2%).
- Also present in endocrine glands (e.g. adrenal cortex, up to 10%) and corpus luteum.
Cholesterol Forms
- Cholesterol exists in free and ester forms.
- Esterified with fatty acids at the –OH group at position C3.
- Ester forms are also referred to as "bound" forms.
- The various fatty acids making up cholesterol esters include linoleic acid (50%), oleic acid (18%), palmitic acid (11%), arachidonic acid (5%), and other fatty acids (16%).
- Most plasma cholesterol esters are produced in the plasma itself by the transfer of an acyl group (mostly unsaturated) from the β-position of lecithin to cholesterol, aided by the enzyme lecithin-cholesterol acyltransferase (LCAT).
Norum's Disease
- A genetic deficiency of LCAT.
- Results in the failure of cholesterol esterification, impacting lecithin.
- Characterized by increased free cholesterol and lecithin in plasma, and reduced cholesterol esters and lysolecithin.
Normal Cholesterol Levels
- Serum total cholesterol levels in adults typically vary between 150 and 250 mg/dL.
- About 40-50 mg% is free cholesterol.
- 110-200 mg% is in cholesterol ester form.
- Cholesterol levels vary with age, sex (slightly higher in women before and a little lower after menstruation), and pregnancy (increased during pregnancy).
Other Sterols
-
7-Dehydrocholesterol: An important sterol in skin, differing from cholesterol by a second double bond between C7 and C8.
- A precursor to vitamin D3. UV radiation converts 7-dehydrocholesterol to vitamin D3 (cholecalciferol), which is crucial for preventing rickets.
- Ergosterol: A plant sterol, precursor to vitamin D2. UV irradiation converts ergosterol to vitamin D2. Over-irradiation can cause toxic products
- Stigmasterol and Sitosterol: Plant sterols with no nutritional value for humans. They reduce intestinal absorption of cholesterol.
- Coprosterol: Formed from cholesterol in the gut by bacterial action, where the C5-C6 double bond is saturated.
- Other important steroids include bile acids, adrenocortical hormones, gonadal hormones, D vitamins, cardiac glycosides, and some alkaloids.
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Description
Explore the structures and biological functions of sterols in this quiz. Learn about their unique cyclic structures, differences between phytosterols and zoosterols, and the pathways involved in their synthesis. Test your knowledge on the crucial roles sterols play in eukaryotic organisms.
