OIA1003 LIPIDS I - STRUCTURE

Lipids

Organic compounds that are hydrophobic or amphipathic; key roles in energy, structure, and signaling.

Biological roles of lipids

Energy storage, cell membrane structure, signal transduction, digestion (bile), vitamin absorption, and antioxidant functions.

Energy yield of lipids

Lipids provide 9 kcal/g, more than carbohydrates (4 kcal/g), and are stored anhydrously for efficiency.

Simple lipids

Esters of fatty acids with alcohols: e.g., triglycerides (TAG), waxes.

Complex lipids

Lipids with additional groups: phospholipids, glycolipids, lipoproteins.

Derived lipids

Substances derived from hydrolysis: fatty acids, cholesterol, ketone bodies, fat-soluble vitamins.

Common plasma lipids

Fatty acids, cholesterol (free & esterified), TAGs, phospholipids, glycolipids.

Fatty acid structure

Hydrophobic hydrocarbon chain with a hydrophilic carboxyl group (–COOH).

Amphipathic property

Hydrophilic head + hydrophobic tail allows interaction with both water and lipids.

Transport of free fatty acids

In blood, they are bound to albumin; most are in esterified form in lipoproteins.

Saturated fatty acids

Contain no double bonds.

Monounsaturated fatty acids

Contain one cis double bond, causing a bend in the chain.

Polyunsaturated fatty acids

Contain two or more double bonds; usually cis configuration.

Nomenclature system

e.g. 18:2(9,12) → 18 carbon atoms, 2 double bonds at positions 9 and 12.

ω-carbon notation

Carbon from methyl end is ω-1; used to classify fatty acids like ω-3, ω-6.

Essential fatty acids

Linoleic acid (ω-6) and α-linolenic acid (ω-3); must be obtained from the diet.

TAG structure

Three fatty acids esterified to glycerol; also known as natural fat.

Simple TAG

All three fatty acids are the same.

Mixed TAG

Different fatty acids attached to the glycerol backbone.

TAG function

Efficient energy storage; forms lipid droplets in cytosol.

Phospholipid structure

Glycerol backbone + 2 fatty acids + phosphate group + alcohol (e.g., choline).

Amphipathic nature

Hydrophilic phosphate head and hydrophobic fatty acid tails.

Main roles (phospholipid)

Major membrane component, signal transduction, lung surfactant, bile solubilization.

Examples of phospholipids

Phosphatidylcholine, phosphatidylethanolamine, phosphatidylinositol.

Sphingomyelin

Major sphingophospholipid with a sphingosine backbone instead of glycerol.

Ceramide

Precursor to sphingomyelin; formed by attaching a fatty acid to sphingosine.

Sphingomyelin role

Important in myelin sheaths surrounding nerves (insulation).

Glycolipid definition

Lipids containing carbohydrate moieties; also called glycosphingolipids.

Backbone of glycolipids

Based on ceramide; similar to sphingolipids but with sugar groups instead of phosphate.

Main site (glycolipids)

Found most abundantly in nerve tissues.

Functions (glycolipids)

Cell signaling, recognition, and development.

Cholesterol structure

Four fused hydrocarbon rings (steroid nucleus), hydroxyl group at C3, and hydrocarbon tail at C17.

Sterol classification

Steroids with hydroxyl at C3 and 8–10 carbon side chain at C17.

Cholesteryl ester

Fatty acid esterified to the OH group at C3 → more hydrophobic than free cholesterol.

Cholesterol roles

Precursor of steroid hormones, bile salts, and vitamin D.

Vitamin A

Retinol — essential for vision and epithelial health.

Vitamin D

1,25-dihydroxycholecalciferol — calcium and bone metabolism.

Vitamin E

Tocopherol — antioxidant.

Vitamin K

Phylloquinone — necessary for coagulation factor synthesis.

Key digestion challenges

Lipids are water-insoluble and tend to aggregate, making enzyme access difficult.

Digestion requires emulsification by bile salts, mechanical mixing, and micellisation to increase surface area and enhance absorption by enterocytes.