Lipid Notes

LIPIDS

Objectives

  • Classify lipids

  • Describe fatty acids and classify them

  • Enlist functions of lipids

  • Describe cholesterol and its importance

Lipids

  • Lipids are a heterogeneous group of compounds including fats, oils, steroids, waxes, and related compounds.

  • Defined more by physical properties than chemical properties.

  • Insoluble in water and soluble in nonpolar solvents.

  • Important in biological systems:

    • Form the cell membrane, a mechanical barrier that divides from external environment.

    • Provide energy.

    • Several essential vitamins are lipids.

Biological Roles of Lipids

  • Relatively insoluble in water and soluble in nonpolar solvents like ether and chloroform.

  • Important dietary constituents:

    • High energy value.

    • Source of fat-soluble vitamins and essential fatty acids.

  • Fat is Stored in adipose tissue:

    • served as Thermal insulator in subcutaneous tissues and around organs.

  • Lipoproteins (lipid and protein combinations) are important cellular constituents:

    • Occur in the cell membrane and mitochondria.

    • Transport lipids in the blood.

  • Knowledge of lipid biochemistry is necessary for understanding biomedical areas:

    • Obesity

    • Diabetes mellitus

    • Atherosclerosis

    • Role of polyunsaturated fatty acids in nutrition and health.

Classifications of Lipids

Simple Lipids

  • Esters of fatty acids with various alcohols (fats, oils, and waxes)

    • Fats: Esters of fatty acids with glycerol.

      • Oils are fats in the liquid state.

    • Waxes: Esters of fatty acids with higher molecular weight monohydric alcohols.

Complex Lipids

  • Esters of fatty acids containing groups in addition to an alcohol and a fatty acid. (additional components like phospate)

    • Phospholipids: Lipids containing fatty acids, an alcohol, and a phosphoric acid residue.

      • Often have nitrogen-containing bases and other substituents.

      • Glycerophospholipids: Alcohol is glycerol.

      • Sphingophospholipids: Alcohol is sphingosine.

    • Glycolipids (Glycosphingolipids): Lipids containing a fatty acid, sphingosine, and carbohydrate.

    • Other complex lipids: Sulfolipids and aminolipids.

    • Lipoproteins may also be placed in this category.

Precursor and Derived Lipids

  • Include fatty acids, glycerol, steroids, other alcohols, fatty aldehydes, ketone bodies, hydrocarbons, lipid-soluble vitamins, and hormones.

  • Neutral lipids: uncharged acylglycerols (glycerides), cholesterol, and cholesteryl esters.

Fatty Acid

  • A molecule characterized by a carboxyl group attached to a long hydrocarbon chain.

  • Formula: R–COOH where R is a hydrocarbon chain.

  • Carboxylic acids.

Saturated Fatty Acid

  • Do not have any double bonds.

  • Every carbon atom in the hydrocarbon chain is bonded to as many hydrogen atoms as possible.

  • Are solids at room temperature.

  • Animal fats are a source of saturated fatty acids.

  • Pack easily and form rigid structures (e.g., in membranes).

Unsaturated Fatty Acids

  • Can have one or more double bonds along its hydrocarbon chain.

  • Monounsaturated: One double bond.

  • Polyunsaturated: Two or more double bonds.

  • Melting point is influenced by the number of double bonds and the length of the hydrocarbon tail.

    • More double bonds: lower melting point.

    • Longer tail: higher melting point.

  • Plants are the source of unsaturated fatty acids.

  • –CH = CH – CH = CH –

    • Unsaturated fatty acid chain

  • –CH – CH – CH –

    • Saturated fatty acid chain

Essential Fatty Acids

  • If a fatty acid can only be obtained from the diet (for humans), it is an essential fatty acid.

  • Two fatty acids cannot be synthesized in the human body:

    • Linoleic acid

    • Linolenic acid

    • Both are unsaturated.

  • Nonessential fatty acids can be made by the human body.

    • Made from carbohydrates, proteins, or other fatty acids.

  • Fatty acids are an important source of energy.