Lipids Overview and Classification

LIPIDS

• Lipids are a heterogeneous group of organic compounds
• They are relatively insoluble in water and other polar solvents, but soluble in nonpolar solvents
• Lipids are esters of fatty acids and alcohols/substances capable of forming esters
• They are not polymers, but small molecules
• Lipids are widely distributed in nature, found in both plants and animals
• In the body, lipids are primarily found in plasma, adipose tissue, and biological membranes

Intended Learning Outcomes

• What are lipids?
• Biological importance of lipids
• Classification of lipids
  • Fatty acids
  • Simple lipids
    • Neutral fats
    • Waxes
  • Conjugated lipids
    • Phospholipids
    • Glycolipids
    • Lipoproteins -Derived lipids
      • Steroids
      • Terpenes

What are Lipids?

• The word "lipid" comes from the Greek word "lipos," meaning fat
• Lipids are a diverse group of organic compounds
• They are relatively insoluble in water but soluble in nonpolar (organic) solvents

Biological Importance of Lipids

• Energy reservoir: Lipids store energy, providing about 25% of daily caloric needs
• Thermal insulation: Lipids in subcutaneous tissue protect against temperature changes
• Structural components of biological membranes and nerves (e.g., glycolipids, phospholipids, sphingomyelins, cholesterol, lipoproteins)
• Cellular recognition and tissue immunity: Lipids play a role in cell signaling
• Fat digestion and absorption: Lipids are involved in the breakdown and absorption of fats
• Cellular metabolic regulators and modulators: Lipids act as hormones and prostaglandins
• Provide body with fat-soluble vitamins (A, D, E, and K), and essential fatty acids
• Components of the electron transport chain
• Fixation and protection of internal organs

Classification of Lipids

• Simple lipids: Fats, oils, and waxes
• Compound lipids: Phospholipids, glycolipids, and lipoproteins
• Derived lipids: Steroids, and terpenes

Fatty Acids (FAs)

• Aliphatic monocarboxylic organic acids (R-COOH)
• Often obtained from natural fats and oils through hydrolysis
• Fatty acids are amphipathic molecules (both hydrophilic and hydrophobic)
• General formula: CH3(CH2)n-2COOH; most have an even number of carbon atoms (4-36), synthesized from condensation of 2C acetate units
• Commonly occurring fatty acids in animals have an even number of carbon atoms (12-24)
• Classified by chain length (short, medium, and long) and degree of saturation (saturated or unsaturated)

Fatty Acids Nomenclature

• Systematic names are derived from the parent hydrocarbon
• Numbering of fatty acid carbon atoms begins from the COOH group (C-system) or the terminal methyl group (ω-system)

Saturated Fatty Acids (SFAs)

• Straight-chain hydrocarbon chains (no double bonds)
• Have high melting points
• Examples include palmitic acid (16:0) and stearic acid (18:0)
• Commonly found in animal fats (butter, cheese, chocolate, etc.).

Unsaturated Fatty Acids (USFAs)

• Contain at least one double bond in their hydrocarbon chain
• Have lower melting points, often liquid at room temperature
• Double bonds (db) are usually in cis-configuration, causing a bend in the chain
• Important types of USFAs include linoleic and linolenic acids, and arachidonic acid

Classification of USFAs

• Mono-unsaturated fatty acids (MUFAs)
• Poly-unsaturated fatty acids (PUFAs)

Essential Fatty Acids (EFAs)

• Those that the body cannot synthesize and must be obtained from the diet
• Examples include linoleic and α-linolenic acids
• Arachidonic acid is a semi-essential fatty acid

Physical Properties of Fatty Acids

• Length and degree of unsaturation influence the physical properties of fatty acids, including solubility
• Short-chain fatty acids are slightly soluble due to the polarity of the -COO- group
• Solubility decreases with increasing chain length of the nonpolar hydrocarbon chain
• Fewer double bonds result in higher melting points

Simple Lipids: Neutral Fats (Triglycerides, TG)

• Esters of three fatty acids with glycerol
• Glycerol is a polyhydric alcohol with three OH groups
• Natural fats are mixtures of mixed TGs with a small amount of simple TGs
• Oils are liquid at room temperature due to high unsaturated fatty acid content
• Solid fats are solid at room temperature due to high saturated fatty acid content
• Oil hydrogenation converts unsaturated double bonds into single bonds, increasing the melting point

Simple Lipids: Waxes

• Esters of one long-chain fatty acid and a long-chain alcohol
• Functions include providing water repellency and protection against parasites
• Found in leaves, waterfowl coverings, and various other applications

Compound Lipids: Phospholipids

• Esters of fatty acids, alcohols, and phosphoric acid with additional polar/charged groups
• In water, they form micelles or bilayers
• Major structural components of biological membranes
• Types include glycerophospholipids and sphingophospholipids

Compound Lipids: Glycolipids

• Lipids containing carbohydrates
• Examples include galactolipids and sphingoglycolipids
• Abundant in thylakoid membranes of chloroplasts

Compound Lipids: Lipoproteins

• Complexes of lipids and proteins that transport lipids through blood
• Four major groups differ in density and composition (VLDL, LDL, HDL, chylomicrons)
• Different types carry different types of lipids

Other Lipid Functions

• Lipids are crucial for energy storage, insulation, and cell signaling
• Some serve as precursors for hormones and other molecules
• They're involved in various biological processes, including transport and structural support