Lipids Overview

Lipids Overview

• Lipids are molecules produced by living organisms.
• Major organic biomolecules include lipids, nucleic acids, carbohydrates, and proteins.
• Minerals and vitamins are also considered biomolecules.
• Structure and function of biomolecules are fundamentally related and influenced by environmental factors.
• Lipids are a diverse group of organic compounds related to fatty acids.
• Fats, oils, waxes, and other compounds are included in this group.
• Lipids are oily and greasy, and are not soluble in water.
• Polar solvents dissolve lipids.
• The predominant ingredient in lipids is the hydrocarbon chain.
• Lipids are not polymer structures made up of monomers.
• Lipids make up 18-25% of an adult's body mass.
• Lipids are primarily found in plasma, biological membranes, and adipose tissue.
• Adipose tissue: loose connective tissue designed to store fat in adipocytes (fat cells) found under the skin, around the kidneys, and in the abdomen and breasts.
• Adipose tissue functions: reserve fuel, insulation against heat loss, and support/protection of organs.

Biological Importance of Lipids

• Lipids act as structural components of biological membranes and nerves.
• They include glycolipids, phospholipids, sphingomyelins, cholesterol, and lipoproteins.
• Lipids maintain membrane fluidity.
• Lipids are a valuable energy reservoir.
• Stored lipids are the chief form of energy in the body and are more palatable and storable compared to carbohydrates.
• Lipid oxidation produces twice the energy produced from carbohydrates.
• Lipids insulate the body from heat loss by acting as a thermal insulator in subcutaneous tissues.
• Lipids act as a protective cushion for internal organs and are a protective coat for many organisms.
• Lipids are involved in cell-cell adhesion, cell recognition, and tissue immunity (glycolipids).
• Lipids such as bile acids and lecithins aid in the digestion and absorption of dietary fats.
• Lipids act as crucial cellular metabolic regulators, modulators of intracellular messengers, and intracellular messengers (steroid hormones, prostaglandins, inositol triphosphate (IP3)).
• Lipids provide the body with fat-soluble vitamins (A, D, E, and K) and essential fatty acids.
• Lipids are components of the electron transport chain (ubiquinone: coenzyme Q).

Classification of Lipids

• Saponification Classification System: Based on a chemical reaction called saponification (lipid undergoes ester hydrolysis in a basic condition (KOH)).
  • Saponifiable: triglycerides, waxes, phospholipids, glycolipids
  • Non-saponifiable: steroids, terpenes, vitamins, eicosanoids
• Bloor's Classification System: Based on the chemical composition of lipids.
  • Simple lipids: fats and oils, waxes
  • Compound lipids: phospholipids, glycolipids, lipoproteins, glycerophospholipids, sphingophospholipids, glyceroglycolipids, glycosphingolipids,
  • Derived lipids: steroids, fatty acids, terpenes, carotenoids

Fatty Acids (FAs)

• FAs are aliphatic monocarboxylic organic acids.

• FAs contain a hydrocarbon chain with one carboxylic group at one end.

• FAs are amphipathic, possessing both hydrophilic (water-soluble) and hydrophobic (water-insoluble) regions.

• They have a carboxyl group (-COOH) and a hydrocarbon chain (R group).

• More than 100 different FAs occur naturally.

• Most FAs occur as esters (not free).

• Most FAs are commonly synthesized from the condensation of 2-carbon units (acetate).

• In most biological systems, FAs have an even number of carbon atoms (4-36); odd numbers occur less frequently.

• FAs are typically straight and unbranched, but some have branched or heterocyclic chains.

• They are classified as short, medium, or long chain depending on the number of carbon atoms in their hydrocarbon chain.

• Saturated FAs: have no double bonds within their hydrocarbon chains.

• Unsaturated FAs: contain one or more double bonds within their hydrocarbon chains.

• The presence or absence of double bonds determines the degree of saturation.

• Nomenclature: Fatty acids can be numbered from the carboxyl end or methyl end.

Structure and Properties of Saturated Fatty Acids

• Short-chain FAs (≤10 carbons) are water-soluble and volatile.
• Long-chain FAs (>10 carbons) are water-insoluble, non-volatile, and solid at room temperature.
• Saturated FAs are common in animals and plants.
• Examples of saturated FAs include palmitic acid (16:0) and stearic acid (18:0).
• Longer chains result in higher melting points.
• The higher melting point is a result of the increased energy required to disrupt hydrophobic interactions between saturated fatty acid molecules.
• Saturated FA sources include butter, cheese, chocolate, coconut, egg yolk, lard, meat, milk, palm oil, poultry, vegetable shortening, margarine, and salad dressings.