LIPIDS: Hydrophobic Molecules

Lipids are water-insoluble, oily, or greasy biochemical compounds that can be extracted from cells by nonpolar solvents such as ether, chloroform, or benzene.

ë Unlike carbohydrates, lipids share no common chemical structure. ë Instead, these biochemicals share the physical property of not dissolving in water.

WHAT MAKES A MOLECULE SUCH AS A LIPID INSOLUBLE IN WATER?

A good generalization to remember is “like dissolves like.”

ë That is, to dissolve in water a solute must be “like” water.

ë The solute should be relatively small so that it can move quickly and freely through the solution like a water molecule.

ë Also, the solute should bond to water molecules just as water molecules bond to each other.

In contrast, lipids are “unlike” water molecules and are water insoluble.

➷ Lipids interact weakly with water molecules because they are composed primarily of nonpolar alkyl groups.

Also, lipids tend to be very large compared with water molecules

➷ Thus, lipids are too big and don’t attract water strongly enough to be water soluble.

➷ They are classified as hydrophobic (“water fearing”) to designate their strong tendency to move away from water as if moving away from something fearful.

A. Some Naturally Occurring Fatty Acids

Fatty acid Number of FormulaC atoms

Saturated acids

Butyric acid 4 CH3CH2CH2COOHCaproic acid 6 CH3(CH2)4COOHCaprylic acid 8 CH3(CH2)6COOHCapric acid 10 CH3(CH2)8COOHLauric acid 12 CH3(CH2)10COOHMyristic acid 14 CH3(CH2)12COOHPalmitic acid 16 CH3(CH2)14COOHStearic acid 18 CH3(CH2)16COOHArachidic acid 20 CH3(CH2)18COOH

Unsaturated acids

Palmitoleic acid 16 CH3(CH2)5CH=CH(CH2)7COOHOleic acid 18 CH3(CH2)7CH=CH(CH2)7COOHLinoleic acid 18 CH3(CH2)4CH=CHCH2CH=CH(CH2)7COOHLinolenic acid 18 CH3CH2CH=CHCH2CH=CHCH2CH=CH(CH2)7COOH Arachidonic acid 20 CH3(CH2)4(CH=CHCH2)4CH2CH2COOHEicosapentaenoic acid 20 CH3CH2(CH=CHCH2)5CH2CH2COOHDocosahexaenoic acid 22 CH3CH2(CH=CHCH2)6CH2COOH

B. Classification of Lipids

1. Simple lipids

(a) Fats and oils: esters of fatty acids and glycerol.

(b) Waxes: esters of high-molar-mass fatty acids and high-molar-mass alcohols.

2. Compound lipids

(a) Phospholipids: substances that yield glycerol, phosphoric acid, fatty acids, and a nitrogen-containing base upon hydrolysis.

(b) Sphingolipids: substances that yield an unsaturated amino alcohol (sphingosine), a long-chain fatty acid, and either a carbohydrate or phosphate and a nitrogen base upon hydrolysis.

(c) Glycolipids: substances that yield sphingosine, a fatty acid, and a carbohydrate upon hydrolysis.

The two most important classes of glycolipids are CEREBROSIDES and GANGLIOSIDES. These substances are found mainly in cell membranes of nerve and brain tissue.

3. Steroids

Substances that possess the steroid nucleus, which is a 17-carbon structure consisting of four fused carbocyclic rings. Cholesterol and several hormones are in this class.

i. CHOLESTEROL, the most abundant steroid in the body, which is widely distributed in all cells and serves as a major membrane component

ii. BILE SALTS, which aid in the digestion of fats

iii. ERGOSTEROL, a yeast steroid, which is converted to vitamin D by ultraviolet radiation

iv. Digitalis And Related Substances called CARDIAC GLYCOSIDES, which are potent heart drug

v. ADRENAL CORTEX HORMONES, which are involved in metabolism

vi. SEX HORMONES, which control sexual characteristics and reproduction.

Cholesterol is the parent compound from which the steroid hormones are synthesized.

4. Miscellaneous lipids

ë Substances that do not fit into the preceding classifications;

ë These include the fat-soluble vitamins A, D, E, and K, and lipoproteins.

ë The most abundant lipids are the fats and oils.

ë These substances constitute one of the three important classes of foods.

C. Fats in Metabolism

ë Fats are an important food source for humans and normally account for about 25–50% of our caloric intake.

ë Even small amounts of fat in each meal can represent a large caloric intake.

This is because fats are an especially good source of metabolic energy.

ë To understand the energy-rich nature of fats, it is important to understand that most metabolic energy is derived from carbon oxidation.

Fats are a concentrated carbon source. They are about 75% carbon by mass as compared to about 40% for carbohydrates.

Not only do fats have more carbons to oxidize, but each fat carbon starts out less oxidized than carbons found in most other foods.

Because fat carbons are more reduced, they can undergo more oxidation reactions before carbon dioxide is formed.

When oxidized to carbon dioxide and water, fats supply about 40 kJ per gram (9.5 kcal/g), which is more than twice the amount obtained from carbohydrates or proteins.

. Compared with other foodstuffs, fats are a more concentrated source of carbons that are more reduced; these molecules are indeed a rich source of biochemical energy.

ë Being a rich source of biochemical energy, fats are what our bodies prefer when storing energy reserves.

These reserves are in the form of triacylglycereols in fatty tissue.

On average, this tissue stores about two to three weeks’ worth of energy.

Obese individuals may have energy stores sufficient to subsist without food for about half a year.

Fats make an excellent, concentrated energy storage.

ë Fats are not an energy source for every cell.

For example, the brain cannot use fats for energy.

Even cells that can use fats can do so only in the presence of molecular oxygen (O2). Aerobic metabolism is required when fat is involved.

Skeletal muscle uses fat most of the time but must use carbohydrate during strenuous, anaerobic work.

D. Atherosclerosis

ATHEROSCLEROSIS - is a metabolic disease that leads to deposits of cholesterol and other lipids on the inner walls of the arteries.

The name athere is Greek for “mush” and nicely describes the appearance of these fatty deposits (called plaque).

As plaque accumulates, the arterial passages become progressively narrower.

The walls of the arteries also lose their elasticity and their ability to expand to accommodate the volume of blood pumped by the heart.

Blood pressure increases as the heart works harder to pump sufficient blood through the narrowed passages, which may eventually lead to a heart attack.

The accumulation of plaque also causes the inner walls to have a rough rather than a normal smooth surface, which is a condition that may lead to CORONARY THROMBOSIS (heart attack due to blood clots).

E. Biological Membranes

BIOLOGICAL MEMBRANES - are thin, semi-permeable cellular barriers.

❧ The general function of these barriers is to exclude dangerous chemicals from the cell while allowing nutrients to enter.

❧ Because almost all the dangerous chemicals, nutrients, and special molecules are water soluble, the membranes can act as effective barriers only if they impede the movement of hydrophilic (water-soluble) molecules.

To act as such a barrier, a membrane must have some special properties.

To exclude water and water solutes, the bulk of a membrane must be hydrophobic.

But a membrane necessarily touches water both inside and outside the cell.

Therefore, the surface of a membrane must be hydrophilic.

Thus, a membrane can be visualized as being layered much like a piece of laminated plywood

The hydrophobic interior provides the barrier while the hydrophilic exterior interacts with the aqueous environment

The cell uses lipids to give the membrane its hydrophobic nature.

❧ Membrane lipids naturally aggregate to form lipid bilayers.

A lipid bilayer - is composed of two adjoining layers of lipid molecules aligned so that their hydrophobic portions form the bilayer interior while their hydrophilic portions form the bilayer exterior.

A lipid bilayer has the necessary properties of a membrane—a hydrophobic barrier and a hydrophilic surface.

Lipids give this barrier an oily or fluid appearance.

The more unsaturated fatty acids in the membrane, the more fluid it will be.