Unit 3 Chemistry – large Biomolecule #1 – Lipids

·  lipids are fats, oils, waxes or steroids, non-polar, insoluble organic compounds

·  in our diet, lipids supply and store energy [37 kJg-1] as well as a protective layer around organs

Formation of Fats and Oils

·  molecules in fats and oils are formed by a condensation reaction between a glycerol [a tri-alkanol] molecule and 3 molecules of a fatty acid [high Mr carboxylic acid] …

glycerol
/ stearic acid CH3(CH2)16COOH
+
+
+

condensation reaction [esterification]
H2C-O-CO-(CH2)16CH3
l
HC-O-CO-(CH2)16CH3 + 3 H2O
ester linkage l water molecules produced
H2C-O-CO-(CH2)16CH3
tristearin – a triglyceride ester
[ molecule in fat ]

·  the condensation reaction [producing water molecules] is also an ester forming reaction, the tristearin being a triglyceride ester formed between glycerol [a tri-alkanol] and carboxylic acid [fatty acid].

·  in the body, this condensation reaction is enzyme controlled

·  the –O-C- is an “ester linkage” or ester functional group

ll

O

Types of Fat

·  fats can be classified according to the fatty acids from which they are made

·  saturated fats are made from fatty acids having only single C-C bonds ..

eg. CH3(CH2)14COOH palmitic acid

·  mono-unsaturated fats are made from fatty acids having one C=C double bond

eg. CH3(CH2)7CH=CH(CH2)7COOH oleic acid

·  polyunsaturated fats are made from fatty acids having >1 C=C double bond

eg. CH3(CH2)4CH=CHCH2CH=CH(CH2)7COOH linoleic acid

·  saturated fats have a higher melting point, are more waxy and cause cardiovascular problems; unsaturated fats tend to be flowing oils

·  animal fats tend to have a higher proportion of saturated fat than vegetable sources

Reactions of Fats in the Body

1. Condensation

·  enzyme catalysed esterification between glycerol and fatty acid

·  reaction in aqueous solution [assisted by emulsifiers]

·  occurs when excess fatty acids in body are converted into fat in adipose tissue

OH O-CO-R

OH + 3 R-COOH(aq) O-CO-R + 3H2O(l)

fatty acid water

OH O-CO-R

glycerol triglyceride

ester

2. Hydrolysis

·  enzyme catalysed hydrolysis [reaction with water] of lipid molecule

·  hydrolysis is the reverse reaction of the condensation reaction

·  occurs during digestion of fats in the body

O-CO-R OH

O-CO-R + 3H2O(l) OH + 3 R-COOH(aq)

O-CO-R OH

3. Oxidation

·  enzyme controlled exothermic redox reaction of fatty acid and oxygen

CH3(CH2)14COOH(aq) + 23O2(g) ® 16CO2(g) + 16H2O(l)