HO - 02.3 Lipids

Lipids

• Definition and Characteristics

  • Lipids are mainly hydrophobic (mostly non-polar).

  • They do not form polymers.

  • Examples of non-polar solvents include chloroform and ether.

• Types of Lipids

  • Cholesterol, free fatty acids, triglycerides, phospholipids.

Fatty Acids

• Structure

  • Consist of a carboxylic acid attached to an aliphatic chain.

  • Chain length varies highly from less than 5 to over 20.

  • Main components of fats and oils.

• Examples

  • Palmitic acid (16-carbon fatty acid) described as 16:0.

  • Unsaturated fatty acids like palmitoleic acid (16:1 n-7); features a double bond, making it unsaturated.

Glycerol

• Description

  • Simple 3-carbon polyol (many -OH groups).

  • Also known as glycerin.

• Role in Lipid Formation

  • Glycerol acts as the backbone for attaching fatty acid molecules.

  • Combines with three fatty acids via an esterification process (dehydration reaction).

Triglycerides

• Formation

  • All three fatty acids on glycerol can vary in length and saturation.

• Energy Storage

  • Triglycerides (properly referred to as triacylglycerides) serve as energy storage in most organisms.

  • Medium for some metabolites of fat-soluble substances.

Differences Between Fats and Oils

• Fats

  • Solid at room temperature.

  • Composed of saturated fatty acids; higher melting point (e.g., butter).

• Oils

  • Liquid at room temperature.

  • Composed of at least one unsaturated fatty acid; lower melting point.

Fatty Acid Isomers

• Cis and Trans Fatty Acids

  • Cis fatty acids are 'bent'; trans fatty acids are 'straight.'

  • Example: Oleic acid (cis) has a melting point of 14 ºC; Elaidic acid (trans) has a melting point of 45 ºC.

  • Cis means 'the same'; trans means 'different' in relation to the position of hydrogen atoms around double bonds.

Retinal and Isomerization

• Retinal Function

  • 11-cis retinal and all-trans retinal play roles in light energy capture for vision.

  • Light causes isomerization of retinal, impacting protein interactions and image formation in the brain.

Phospholipids

• Structure

  • Phosphate group attached to one of the glycerol carbons.

  • Amphipathic: possessing both hydrophilic (polar) and hydrophobic (non-polar) properties.

  • Glycerol backbone with fatty acids; one often being unsaturated.

• Role in Cell Membranes

  • Form bilayers spontaneously in water; crucial to cell membrane structure.

Cholesterol and Steroids

• Steroid Signaling

  • Class of lipids including cholesterol, human hormones (e.g., estradiol, testosterone, cortisol).

  • Cholesterol serves as a precursor for various steroids.

Waxes and Other Lipid Classifications

• Waxes

  • Composed of lipid chains and alkanes (e.g., carnauba wax).

• Glycolipids

  • Complex lipid-saccharide molecules that contribute to the cell membrane and immune response.

  • Include lipopolysaccharides, significant in bacterial cell walls (e.g., E. coli).

Energy Storage in Humans

• Long-Term Energy Storage

  • Uses fats more efficiently compared to carbohydrates: 9 Cal/g for lipids compared to 4 Cal/g for carbohydrates.

• Population Differences

  • Inuit populations tend to store more subcutaneous fat vs. European populations with more visceral adipose tissue.

Summary of Carbohydrates and Lipids

• Carbohydrates

  • Monosaccharides and polysaccharides, with multiple -OH and =O groups.

  • Used for structural components and quick energy storage.

• Lipids

  • C-chains with terminal -COOH (carboxyl) groups, hydrophobic, amphipathic when combined with polar molecules.

  • Important for energy storage and do not form polymers.