Lipids

What Are Lipids?

  • Lipids are hydrophobic (nonpolar) biological molecules.

  • Mostly made of carbon (C) and hydrogen (H) with very little oxygen.

  • Do not dissolve in water.

  • Not true polymers, but still considered macromolecules.

  • Main functions:

    • Long-term energy storage

    • Cell membrane structure

    • Insulation and protection

    • Chemical signaling (hormones)

Basic Structure

  • Most lipids are built from:

    • Glycerol (3-carbon alcohol)

    • Fatty acids (long hydrocarbon chains)

  • Fatty acids attach to glycerol via ester linkages (covalent bonds).

Types of Lipids

Triglycerides (Fats and Oils)

  • Structure:

    • 1 glycerol + 3 fatty acids

  • Function:

    • Long-term energy storage

    • Insulation and cushioning

  • Two main types:

    • Saturated fats

      • No double bonds between carbons

      • Straight chains, pack tightly

      • Usually solid at room temperature

    • Unsaturated fats

      • One or more double bonds

      • Bent chains, pack loosely

      • Usually liquid at room temperature

Phospholipids

  • Structure:

    • Glycerol + 2 fatty acids + phosphate group

  • Properties:

    • Hydrophilic head (phosphate)

    • Hydrophobic tails (fatty acids)

  • Function:

    • Main component of cell membranes

  • Form phospholipid bilayers in water

Steroids

  • Structure:

    • Four fused carbon rings

  • Examples:

    • Cholesterol

    • Estrogen

    • Testosterone

  • Functions:

    • Hormones (chemical signaling)

    • Membrane fluidity (cholesterol)

Waxes

  • Made of fatty acids linked to long-chain alcohols

  • Function:

    • Waterproofing

    • Protection

  • Examples:

    • Plant cuticle

    • Beeswax

Dehydration Synthesis and Hydrolysis in Lipids

  • Fatty acids attach to glycerol via dehydration synthesis

    • –H and –OH removed to form water

  • Broken apart by hydrolysis

    • Water added to break ester bonds

  • Occurs during fat digestion

Why Lipids Store More Energy

  • Many C–H bonds

  • Release large amounts of energy when broken

  • More energy-dense than carbohydrates

Structure Determines Function

  • Nonpolar structure → hydrophobic behavior

  • Amphipathic phospholipids → membrane formation

  • Ring structure of steroids → hormone signaling ability