STPM Biology - chapter 1: Revision essay question 9

9. What are triglycerides?

Triglycerides are the main constituents of vegetable oils and animal fats. Triglycerides have lower densities than water (they float on water), and at normal room temperatures may be solid or liquid. When solid, they are called "fats" or "butters" and when liquid they are called "oils". A triglyceride, also called triacylglycerol (TAG), is a chemical compound formed from one molecule of glycerol and three fatty acids.

Oleic Acid Glycerol or Glycerin

Glycerol is a trihydric alcohol (containing three -OH hydroxyl groups) that can combine with up to three fatty acids to form monoglycerides, diglycerides, and triglycerides. Fatty acids may combine with any of the three hydroxyl groups to create a wide diversity of compounds. Monoglycerides, diglycerides, and triglycerides are classified as esters which are compounds created by the reaction between acids and alcohols that release water (H₂O) as a by-product.

Triglycerides

The triglyceride structural formula on the left is typical of olive oil. It consists of two radicals of oleic acid and one of palmitic acid attached to glycerol (the vertical carbon chain). The small squares represent the fatty acid components of the glyceride molecules. The picture on the right shows the three-dimensional molecular structure of tristearin, a triglyceride with three stearic acid radicals. Oxygen atoms are shown in red, carbon atoms as dark gray, and hydrogen atoms as blue. Tristearin is found as a minor component in many natural fats.

Soap is made traditionally by heating an alkali like sodium hydroxide (NaOH) with animal fat. The chemical reaction (hydrolysis) produces glycerol and soap, which consists of the sodium salts of the fatty acids, e.g., sodium stearate (CH₃(CH₂)₁₆C(O)O^- Na⁺).

A diglyceride, or diacylglycerol (DAG), has two fatty acid radicals and exists in the 1,2 form and the 1,3 form depending on how the fatty acids are attached to the glycerol molecule. A monoglyceride, or monoacylglycerol (MAG), has only one fatty acid radical per molecule of glycerol. The fatty acid may be attached to carbon 1 or 2 of the glycerol molecule.

All esters of glycerol and fatty acids are metabolized in the same way. Monoglycerides, diglycerides, and triglycerides all have 9 Calories per gram, but some nutrition labels hide the calories of mono- and diglycerides under the contention that "fat" consists only of triglycerides.

Structure of Olestra. R represents fatty acids.

 Assignment submitted by L.Y., Kong,  S. H., Lim, S. C., Soo, C. W. Tan and C.C.,Teh 2011/2012 (IBM)