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Lipids: Structure, Types, and Biological Functions

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Lipids: Structure and Classification

Overview of Lipid Classes

Lipids are a diverse group of hydrophobic biomolecules essential for cellular structure and energy storage. The four main classes of lipids are:

  • Triglycerides (Triacylglycerols): Commonly known as fats and oils, composed of three fatty acids and one glycerol molecule.

  • Phospholipids (Phosphoglycerides): Key components of cell membranes, consisting of two fatty acids, one phosphate group, and one glycerol.

  • Steroids: Includes cholesterol and sex hormones (e.g., estrogen, progesterone, testosterone), characterized by a four-ring structure.

  • Waxes: Formed from one saturated fatty acid and one alcohol (not glycerol).

Steroids: Structure and Examples

Steroids are lipids with a characteristic four-ring structure. They play vital roles in cell signaling and membrane structure.

  • Basic Structure: Four fused carbon rings.

  • Examples: Cholesterol, estrogen, progesterone, testosterone.

Steroid four-ring structure Structure of cholesterol Structure of progesterone Structure of testosterone Structure of estrogen

Triglycerides: Structure and Properties

Composition and Formation

Triglycerides are formed by the esterification of three fatty acids with one glycerol molecule.

  • Glycerol: A three-carbon alcohol with hydroxyl groups.

  • Fatty Acids: Organic acids with a carboxyl group (-COOH); can be saturated or unsaturated.

  • Formation: Dehydration synthesis links fatty acids to glycerol, forming ester bonds and releasing water.

Structure of glycerol Triglyceride formation: 3 fatty acids + 1 glycerol Dehydration synthesis Ester and phosphoester linkages 3 H2O released in triglyceride formation

Fatty Acids: Saturated vs. Unsaturated

Fatty acids are classified based on the presence of double bonds.

  • Saturated Fatty Acids: Lack C=C bonds; solid at room temperature.

  • Unsaturated Fatty Acids: Have at least one C=C bond; usually liquid at room temperature.

Saturated and unsaturated fatty acid structures Table comparing saturated and unsaturated fatty acids

General Formula and Examples

The general formula for fatty acids is:

  • Saturated:

  • Monounsaturated:

  • Polyunsaturated: (where n = number of double bonds)

General formula for fatty acids

Specific Fatty Acids

  • Lauric Acid: C12 saturated fatty acid (12:0),

  • Stearic Acid: C18 saturated fatty acid (18:0),

  • Palmitic Acid: C16 saturated fatty acid (16:0),

  • Myristic Acid: C14 saturated fatty acid (14:0),

  • Palmitoleic Acid: C16 monounsaturated fatty acid (16:1-7),

  • Oleic Acid: C18 monounsaturated fatty acid (18:1-9),

  • Linoleic Acid: C18 di-unsaturated fatty acid (18:2-6,9),

Lauric acid structure Palmitic acid structure Myristic acid structure Stearic acid structure

Phospholipids: Structure and Function

Composition and Amphipathic Nature

Phospholipids are major components of cell membranes, consisting of two fatty acids, one phosphate group, and one glycerol.

  • Amphipathic: Possess both hydrophilic (phosphate head) and hydrophobic (fatty acid tails) regions.

  • Arrangement: Form bilayers in aqueous environments, with tails facing inward and heads facing outward.

Phospholipid structure Phospholipid structure with R-group Phospholipid tail and head Phosphate head and fatty acid tails Phospholipid bilayer (plasma membrane) Phospholipid bilayer cross-section Phospholipid bilayer cross-section

Phospholipid Linkages

  • Ester Linkages: Connect fatty acids to glycerol.

  • Phosphoester Linkage: Connects phosphate group to glycerol.

  • Comparison: Triglycerides have three ester linkages; phospholipids have two ester and one phosphoester linkage.

Ester and phosphoester linkages in phospholipids

Waxes: Structure and Function

Composition and Examples

Waxes are esters formed from one saturated fatty acid and one long-chain alcohol.

  • Structure: Not based on glycerol; contain a fatty acid and an alcohol.

  • Examples: Melissyl palmitate, other wax esters.

Wax structure example Wax structure example Wax synthesis reaction

Short-Chain Fatty Acids: Acetic Acid

Properties and Biological Role

Acetic acid (CH3COOH) is the simplest short-chain fatty acid, highly water-soluble due to its small size and polar carboxyl group.

  • Produced by: Microbial fermentation in the gut.

  • Role: Key in energy metabolism; does not typically form triglycerides.

Summary Table: Saturated vs. Unsaturated Fatty Acids

Saturated

Unsaturated

Lack C=C bonds Solid at room temperature (25°C)

Have at least one C=C bond

Table comparing saturated and unsaturated fatty acids

Example Problems

Triglyceride Molecular Formula Calculation

  • Problem 1: What is the molecular formula of a triglyceride with lauric acid, stearic acid, and palmitoleic acid? Answer: C49H92O6

  • Problem 2: A triglyceride has a molecular formula of C51H96O6. Two monomers are palmitoleic acid and myristic acid. What is the third fatty acid? Answer: Stearic acid

Key Terms and Definitions

  • Hydrophilic: Water-loving; polar.

  • Hydrophobic: Water-fearing; nonpolar.

  • Lipophilic: Fat-loving; nonpolar.

  • Lipophobic: Fat-fearing; polar.

  • Amphipathic: Molecule with both hydrophilic and hydrophobic regions.

  • Ester Linkage: Bond formed between an acid and an alcohol via dehydration synthesis.

  • Phosphoester Linkage: Bond between a phosphate group and an alcohol.

Additional info:

  • Acetic acid is highly water-soluble due to its small size and polar carboxyl group, making it distinct from longer-chain fatty acids.

  • Phospholipids are fundamental to cell membrane structure, forming bilayers that separate internal and external environments.

  • Steroids, unlike other lipids, do not contain fatty acids or glycerol and lack ester or phosphoester linkages.

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