Lipids

Lipids

1. Lipids are water-insoluble substances

   • Lipids are classified into four groups: fats (triglycerides); complex lipids; steroids; and prostaglandins, thromboxanes, and leukotrienes

2. Triglycerides structure: Fats consist of fatty acids and glycerol. In saturated fatty acids, the hydrocarbon chains have only single bonds; unstaurated fatty acids have hydrocarbon chains with one or more double bonds, all in the cis configuration

3. Triglyceride properties: Solid fast contain mostly saturated fatty acids, whereas oils contain substantial amounts of unsaturated fatty acids. - The alkali slats of fatty acids are called soaps

4. Complex lipids: can be classified into two groups: pholipids and glycolipids.

   • Phosphoplipids are made of central alchohol (glycerol or sphingosine), fatty acids, and a nitrogen-ccontaining phosphate ester such as phosphorylcholine or inositol phosphate
     • Glycolipids- contain sphingosine and a fatty acid, collectively known as ceramide portion of the molecule, and a carbohydrate portion.

5. Structure of membranes: many phospholipids and glycolipids are important components of cell memebranes. Membranes are made of a lipid bilayer in which the hydrophobic parts of phospholipids (fatty acid residues) point toward the middle of the bilayer and the hydrophillic parts point toward the inner and outer surfaces of the membrane.

6. Glycerophospholipids: are complex lipids that consist of a central glycerol moiety to which two fatty acids are esterified. The third alchohol group of the glycerol is esterified to a nitrogen-containing phosphate ester.

7. Sphinogolipids are complex lipids that consist of the long chain alcohol sphingosine esterified to a fatty acid (the ceramide moiety). Nitrogen containg phosphate esters are also bonded to the sphingosine moiety.

8. Steroids: The third major groupd of lipds comprises the steroids. The characteristic feature of the steroid strucutre is the four- ring nucleus.

   • The most common steriod, cholesterol, serves as a starting ,material for synthesis of other steroids, such as bile salts, sex and other hormones. Cholesterol is also an intergral part of membranes, occupying the hydrophobic region of the lipid bilayer. Because of their low solubilty in water, cholesterol deposits are implcated in the foramation of gallstones and the plaque like depositis of atherosclerosis
     • Cholesterol is transported in the blood plasma mainly by two kinds: HDL and LDL. LDL delivers cholesterol to the cells to be used mostly as a membrane component. HDL delivers cholesteryl esters mainly to the liver to be used and synthesis of bile acids and steroid hormones.
       • High levels of LDL and low levels of HDL are symptoms of faulty cholesterol transport, indicating greater risk of atherosclerosis

9. Physiological roles of steroid hormones: - An oxidation product of cholesterol is progesterone a sex hormone . It also gives rise to the synthesis of other sex hormones, such as testosterone and estradiol.

   • Progesterone is also a precursor of the adrenocorticoid hormones. Within this group, cortisol and cortisone are best known for their anti-inflammatory action.

10. Bile salts: are oxidation products of cholesterol. They emulsify all kinds of lipids, including cholesterol, and are essential in the digestion of fats.

11. Prostaglandins, thromboxanes and leuotriens are derived from arachionic acid. They have a wide variety of effects on body chemistry. Among other things, they can lower or raise blood pressure, cause inflammation and bood clotting, induce labour. In genral, they mediate hormone action.