Triglyceride Triacylglycerol

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Triglyceride is the subcategory of lipids that includes fat and oil. These molecules are formed by the condensation of one molecule of glycerol (a three-carbon alcohol) with three molecules of fatty acids. Because of this structure, chemists currently prefer the name triacylglycerol, although the name triglyceride is still in wide use.

Each fatty acid molecule consists of a nonpolar hydrocarbon chain with a carboxyl group (abbreviated COOH) on one end. If the carbon atoms within the hydrocarbon chain are joined by single covalent bonds so that each carbon atom can also bond with two hydrogen atoms, the fatty acid is said to be saturated. If there are a number of double covalent bonds within the hydrocarbon chain so that each carbon atom can bond with only one hydrogen atom, the fatty acid is said to be unsaturated. Triglycerides contain combinations of different saturated and unsaturated fatty acids. Those with mostly saturated fatty acids are called saturated fats; those with mostly unsaturated fatty acids are called unsaturated fats (fig. 2.17).

gjC^nl The saturated fat content (expressed as a percentage ! of total fat) for some food items is as follows: canola, ^ ¡7 or rapeseed, oil (6%); olive oil (14%); margarine (17%); chicken fat (31%); palm oil (51%); beef fat (52%); butter fat (66%); and coconut oil (77%). Health authorities recommend that a person's total fat intake not exceed 30% of the total energy intake per day, and that saturated fat contribute less than 10% of the daily energy intake. This is because saturated fat in the diet may contribute to high blood cholesterol, which is a significant risk factor in heart disease and stroke (see chapter 13). Animal fats, which are solid at room temperature, are generally more saturated than vegetable oils because the hardness of the triglyceride is determined partly by the degree of saturation. Palm and coconut oil, however, are notable exceptions. Though very saturated, they nonetheless remain liquid at room temperature because they have short fatty acid chains.

Chemical Composition of the Body 35

HHHHHHHHHHHHHHH n

H 1

H H H 1 1 1

H H I I

H 1

H 1

H I

— C I

I I

—C I

—C 1

—c —

H H H

1 1 H H

H

H

H

Palmitic acid, a saturated fatty acid

(a) n — ——o—^—o—o — o—vj w o w ^ w w ^ \

HHHHHHHHHHHHHH

Palmitic acid, a saturated fatty acid

HHHHHHHHHHHHHHHHH _

H H I I

H I

H I

I I

—C I

—C 1

—c—-

1 1 H H

H

H

H

Linolenic acid, an unsaturated fatty acid

■ Figure 2.17 Structural formulas for fatty acids. (a) The formula for saturated fatty acids and (b) the formula for unsaturated fatty acids. Double bonds, which are points of unsaturation, are highlighted in yellow.

Fatty acid Triglyceride

Carboxylic R Ester Hydrocarbon

Glycerol acid Hydrocarbon chain Glycerol bond chain

1

1

1

1

1

C 1

C 1

C 1

C 1

— C---1

H

H

H

H

H

1

1

1

1

1

C 1

C 1

C 1

C 1

— C---1

H

H

H

H

H

1

1

1

1

1

C 1

C 1

C 1

C 1

— C---1

H

H

H

H

H

3H2O

■ Figure 2.18 The formation of a triglyceride (triacylglycerol) molecule from glycerol and three fatty acids by dehydration synthesis reactions. A

molecule of water is produced as an ester bond forms between each fatty acid and the glycerol. Sawtooth lines represent hydrocarbon chains, which are symbolized by an R.

Within the adipose cells of the body, triglycerides are formed as the carboxyl ends of fatty acid molecules condense with the hydroxyl groups of a glycerol molecule (fig. 2.18). Since the hydrogen atoms from the carboxyl ends of fatty acids form water molecules during dehydration synthesis, fatty acids that are combined with glycerol can no longer release H+ and function as acids. For this reason, triglycerides are described as neutral fats.

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