Lipids- Coleman Flashcards
Lipids
Lipids - a collection of organic molecules united by solubility in nonpolar solvents (such as ether and chloroform)
Lipids are highly reduced carbon-rich substances that are insoluble in water
Varying chemical composition
Four main groups
Main Groups
- Fatty acids
Saturated and unsaturated - Glycerides
Contain glycerol (HOCH2CHOHCH2OH) - Nonglycerides
Steroids, waxes - Membrane Lipids
Functions
As a result of difference in their structure, lipids serve many different functions in the human body:-
Energy source
Energy storage
Cell membrane structural components
Hormones
Vitamins
Vitamin absorption
Protection
Insulation
Fatty Acids
Fatty acids are carboxylic acids with hydrocarbon chains ranging from 4 to 36 carbons in length.
Some fatty acids are “saturated,” they contain no double bonds between two consecutive carbon atoms of the hydrocarbon chain.
Some fatty acids are “monounsaturated,” they contain one double bond between two consecutive carbon atoms of the hydrocarbon chain.
Some fatty acids are “polyunsaturated,” they contain more than one double bond in the hydrocarbon chain.
unsaturated fatty acid
has one or more carbon-carbon double bonds in the chain. The first double bond is usually at the ninth carbon. The double bonds are not conjugated and are usually cis.
Fatty Acid Nomenclature
When naming fatty acids, all carbons count: the carboxyl group as well as the alkyl chain
Some of these fatty acids have double bonds. Be familiar with the convention for specifying the number and position of the double bond(s)
In nature, almost all fatty acids have an even number of carbons. This is due to the fact that fatty acids are synthesized stepwise from acetyl building blocks
Saturated Fatty acid Nomenclature
The general formula for a saturated fatty acid is CH3(CH2)nCOOH in which n in biological systems is an even number.
Recall that –COOH is a representation of the carboxyl group. If n=16, the result is an 18-carbon saturated fatty acid, stearic acid, having the following structural formula…
Unsaturated fatty acid Nomenclature
An unsaturated fatty acid is one that contains at least one carbon-to-carbon double bond.
Oleic acid, an 18-carbon unsaturated fatty acid, has the following structural formula….
Physical Properties of Fatty Acids
The physical properties of fatty acids are largely determined by the length and degree of saturation of the hydrocarbon chain.
Fatty acid chain length
1) The longer the fatty acid chain length, the poorer the solubility in water.
2) Because the carboxylic acid of the fatty acid is polar, it accounts for the moderate solubility of short-chain (less than 10 carbons) fatty acids in water.
3) The longer the fatty acid chain (assuming the degree of unsaturation remains the same), the higher the melting point.
The degree of unsaturation also is very important in determining the physical properties of fatty acids.
The more double bonds in a fatty acid (assuming that the length of the fatty acid remains the same), the greater the solubility of the fatty acid in water.
The more double bonds in a fatty acid, the lower the melting point of the fatty acid. This assumes that the length of the fatty acid remains the same, and that the double bonds present are all in the cis configuration.
In saturated lipids the most stable arrangement is very close packing of the side chains of the lipids, which is due to van der Waals interactions. The packing is such that the lipids assume an almost crystalline array.
Because of the kink that results from cis double bonds, tight packing of unsaturated fatty acid chains cannot take place.
Since the interactions between these arrays are less extensive, it takes less energy to disrupt them, resulting in a lower melting point.
fatty acid reactions
esterification = fatty acid + alcohol <> ester + water
acid hydrolysis = ester + water <> fatty acid and alcohol
saponification = ester + base <> salt + alcohol
hydrogenation
Industrial uses
Saponification: ionized salt formed is a soap which form micelles that dissolve oil and dirt particles. Dirt is emulsified and rinsed away.
Hydrogenation: Used in food industry to convert polyunsaturated vegetable oils into saturated solid fats. Partial hydrogenation is also useful in industry – extent of hydrogenation is carefully controlled
Essential Fatty Acids
Carbohydrate can be converted and stored as fat so how can fats be essential?
A fat-free diet was fed to rats over extended time
They developed scaly skin and retarded growth
Added fat to the diet:
Butter and coconut oil—no effect
Olive oil—no effect
Corn and linseed oil—very effective
Analysis of the oils showed that corn and linseed oil contained specific fatty acids:
Linoleic (18:2), Linolenic (18:3), and Arachidonic (20:4)
Arachidonic acid (precursor of a class of hormone like molecules called eicosanoids) can be synthesized from dietary linoleic acid
Reduce the risk of cardiovascular disease by decreasing blood clot formation, blood triglyceride levels.
Glycerides
Lipid esters that contain the glycerol molecule and fatty acids.
Neutral glycerides are nonionic and nonpolar
Phosphoglyceride molecules have a polar region, the phosphoryl group, in addition to the nonpolar fatty acid tails.
The structure of each of these types of glycerides are critical to their function.
Neutral glycerides
The esterification of glycerol with a fatty acid produces a neutral glyceride.
Esterification may occur at one, two, or all three positions, producing monoglycerides, diglycerides or triglycerides.
Glycerides: Triacylglycerols
When all three alcohol groups of glycerol form esters with fatty acids a neutral triacylglycerol (triglyceride) is formed.
Triglycerides serve as energy storage in adipose cells.
Neutral glycerides are non-ionic & nonpolar
Adipose cell, also called adipocyte or fat cell, connective-tissue cell specialized to synthesize and contain large globules of fat. There are two types of adipose cells: white adipose cells contain large fat droplets, only a small amount of cytoplasm, and flattened, noncentrally located nuclei; and brown adipose cells contain fat droplets of differing size, a large amount of cytoplasm, numerous mitochondria, and round, centrally located nuclei. The chief chemical constituents of adipose cell fat are triglycerides, which are esters made up of a glycerol and one or more fatty acids, such as stearic, oleic, or palmitic acids. Enzymes contained in adipose cells specialize in the hydrolysis of triglycerides in order to generate fatty acids and glycerol for physiological processes. The fat stored in these cells in part comes directly from the fats eaten and in part is manufactured within the body from fats and carbohydrates in the food and sometimes from protein. The main reservoir of fat in the body is the adipose tissue beneath the skin, called the panniculus adiposus. There are also deposits of fat between the muscles, among the intestines and in their mesentery, around the heart, and elsewhere. One function of these deposits is to act as soft, elastic padding between the various organs.
