Lipids

A lipid is a hydrophobic organic molecule, usually composed only of carbon, hydrogen, and oxygen, with a high ratio of hydrogen to oxygen. A fat called tristearin (tri-STEE-uh-rin), for example, has the molecular formula C57H110O6—more than 18 hydrogens for every oxygen. Lipids are less oxidized than carbohydrates, and thus have more calories per gram. Beyond these criteria, it is difficult to generalize about lipids; they are much more variable in structure than the other macromolecules we are considering. We consider the five primary types of lipids in humans—fatty acids, triglycerides, phospho-lipids, eicosanoids, and steroids (table 2.7).

A fatty acid is a chain of usually 4 to 24 carbon atoms with a carboxyl group at one end and a methyl group at the other. Fatty acids and the fats made from them are classified as saturated or unsaturated. A saturated fatty acid such as palmitic acid has as much hydrogen as it can carry. No more could be added without exceeding four covalent bonds per carbon atom; thus it is "saturated" with hydrogen. In unsaturated fatty acids such as linoleic acid, however, some carbon atoms are joined by double covalent bonds (fig. 2.19). Each of these could potentially share one pair of electrons with another hydrogen atom instead of the adjacent carbon, so hydrogen could be added to this molecule. Polyunsaturated fatty acids are those with many C=C bonds. Most fatty acids can be synthesized by the human body, but a few, called essential fatty acids, must be obtained from the diet because we cannot synthesize them (see chapter 26).

A triglyceride (try-GLISS-ur-ide) is a molecule consisting of three fatty acids covalently bonded to a three-carbon alcohol called glycerol; triglycerides are more correctly, although less widely, also known as triacyl-glycerols. Each bond between a fatty acid and glycerol is formed by dehydration synthesis (see fig. 2.19). Once joined to glycerol, a fatty acid can no longer donate a proton to solution and is therefore no longer an acid. For

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76 Part One Organization of the Body

Glycerol

Palmitic acid (saturated)

CH3(CH2)14COOH

Stearic acid (saturated)

CH3(CH2)16COOH

HH HH

Linoleic acid (unsaturated)

CH3(CH)4CH=CHCH2CH=CH(CH2)7COOH

H—C—O—C—C—C— C—C—C—C—C—C—C—C—C—C—C—C—C—H

Unsaturated fat

Figure 2.19 Triglyceride (fat) Synthesis. Note the difference between saturated and unsaturated fatty acids and the production of 3 H2O as a by-product of this dehydration synthesis reaction.

this reason, triglycerides are also called neutral fats. Triglycerides are broken down by hydrolysis reactions, which split each of these bonds apart by the addition of water.

Triglycerides that are liquid at room temperature are also called oils, but the difference between a fat and oil is fairly arbitrary. Coconut oil, for example, is solid at room temperature. Animal fats are usually made of saturated fatty acids, so they are called saturated fats. They are solid at room or body temperature. Most plant triglycerides are polyunsaturated fats, which generally remain liquid at room temperature. Examples include peanut, olive, corn, and linseed oils. Saturated fats contribute more to cardiovascular disease than unsaturated fats, and for this reason it is healthier to cook with vegetable oils than with lard or bacon fat.

Saladin: Anatomy & Physiology: The Unity of Form and Function, Third Edition

2. The Chemistry of Life

Text

Chapter 2 The Chemistry of Life 77

CH3-N+-

ch2

ch2

O

-O-P=

O

ch2 —

-CH-CH2

O

O

O=C

c=o

(CH2)5

(CH2)12

CH

CH3

CH

(CH2)5

CH3

Nitrogen" containing group (choline)

  • Phosphate group
  • Glycerol
  • Fatty acids

Nitrogen" containing group (choline)

  • Phosphate group
  • Glycerol
  • Fatty acids

Figure 2.20 Lecithin, a Representative Phospholipid. (a) Structural formula, (b) a space-filling model that gives some idea of the actual shape of the molecule, and (c) a simplified representation of the phospholipid molecule used in diagrams of cell membranes.

Hydrophilic region

- Hydrophobic region (c)

Figure 2.20 Lecithin, a Representative Phospholipid. (a) Structural formula, (b) a space-filling model that gives some idea of the actual shape of the molecule, and (c) a simplified representation of the phospholipid molecule used in diagrams of cell membranes.

The primary function of fat is energy storage, but when concentrated in adipose tissue, it also provides thermal insulation and acts as a shock-absorbing cushion for vital organs (see chapter 5).

Phospholipids are similar to neutral fats except that, in place of one fatty acid, they have a phosphate group which, in turn, is linked to other functional groups. Lecithin is a common phospholipid in which the phosphate is bonded to a nitrogenous group called choline (COleen) (fig. 2.20). Phospholipids have a dual nature. The two fatty acid "tails" of the molecule are hydrophobic, but the phosphate "head" is hydrophilic. Thus, phospho lipids are said to be amphiphilic21 (AM-fih-FIL-ic). Together, the head and the two tails of a phospholipid give it a shape like a clothespin. The most important function of phospholipids is to serve as the structural foundation of cell membranes (see chapter 3).

Eicosanoids22 (eye-CO-sah-noyds) are 20-carbon compounds derived from a fatty acid called arachidonic (ah-RACK-ih-DON-ic) acid. They function primarily as

1amphi = both + philic = loving

2eicosa = 20

Saladin: Anatomy & I 2. The Chemistry of Life I Text I I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

78 Part One Organization of the Body hormonelike chemical signals between cells. The most functionally diverse eicosanoids are the prostaglandins, in which five of the carbon atoms are arranged in a ring (fig. 2.21). They were originally found in the secretions of bovine prostate glands, hence their name, but they are now known to be produced in almost all tissues. They play a variety of signaling roles in inflammation, blood clotting, hormone action, labor contractions, control of blood vessel diameter, and other processes (see chapter 17).

A steroid is a lipid with 17 of its carbon atoms arranged in four rings (fig. 2.22). Cholesterol is the "parent" steroid from which the other steroids are synthesized. The others include cortisol, progesterone, estrogens, testosterone, and bile acids. These differ from each other in the location of C=C bonds within the rings and in the functional groups attached to the rings.

Cholesterol is synthesized only by animals (especially in liver cells) and is not present in vegetable oils or other plant products. The average adult contains over 200 g (half a pound) of cholesterol. Cholesterol has a bad reputation as a factor in cardiovascular disease (see insight 2.3), and it is true that hereditary and dietary factors can elevate blood cholesterol to dangerously high levels. Nevertheless,

Figure 2.21 A Prostaglandin. This is a modified fatty acid with five of its carbon atoms arranged in a ring.

cholesterol is a natural product of the body. Only about 15% of our cholesterol comes from the diet; the other 85% is internally synthesized. In addition to being the precursor of the other steroids, cholesterol is an important component of cell membranes and is required for proper nervous system function.

The primary lipids and their functions are summarized in table 2.7.

Insight 2.3 Clinical Application

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