Lipid Structure and Function

Lipid Characteristics

• Lipids are carbon-containing compounds characterized by insolubility in water.

• This insolubility arises from a high proportion of nonpolar $C-C$ and $C-H$ bonds, relative to polar functional groups.

• They dissolve in nonpolar organic solvents like benzene ($C<em>6H</em>6$).

• Molecules with only carbon and hydrogen are hydrocarbons, which are nonpolar and hydrophobic due to equal electron sharing in $C-H$ bonds.

Hydrocarbon Structure and Saturation

• Isoprene is a five-carbon hydrocarbon subunit that can form branched chains called isoprenoids.

• Fatty acids are simple lipids consisting of a hydrocarbon chain bonded to a polar carboxyl group ($-COOH$).

• Saturated hydrocarbons contain only single bonds between carbons, allowing maximum hydrogen atoms and straight chains.

• Unsaturated hydrocarbons contain one or more carbon-carbon double bonds ($=C=C$), reducing hydrogen atoms and causing kinks (due to cis bonds) in the chain.

• Physical Properties:

  • Straight, saturated chains pack tightly, forming solids (e.g., butter, waxes) with higher melting points (e.g., $\approx 20-22^{\circ}C$).

  • Bent, unsaturated chains (e.g., polyunsaturated lipids) pack loosely, forming liquids called oils with lower melting points.

  • Hydrogenation breaks double bonds and adds hydrogen, converting unsaturated lipids to saturated ones.

Types of Lipids

• Steroids:

  • Distinguished by a bulky, four-ring hydrocarbon structure.

  • Vary by attached functional groups or side groups.

  • Examples: Cholesterol (membrane component, has polar hydroxyl group and nonpolar isoprenoid tail), estrogens, testosterone (hormones).

• Fats (Triacylglycerols/Triglycerides):

  • Nonpolar molecules composed of glycerol linked to three fatty acids via ester linkages.

  • Primary role: energy storage due to high $C-H$ to $C-O$ bond ratio, storing twice as much chemical energy per gram as carbohydrates.

  • Not polymers, as fatty acids are not linked into chains.

• Phospholipids:

  • Consist of glycerol linked to a phosphate group and two hydrocarbon chains (fatty acids in Bacteria/Eukarya, isoprenoids in Archaea).

  • The phosphate group is also bonded to a small charged or polar organic molecule.

  • Amphipathic: Possess both a hydrophilic (polar/charged phosphate head) and hydrophobic (nonpolar hydrocarbon tails) region.

  • Crucial components of plasma membranes due to their amphipathic nature, allowing spontaneous formation of lipid bilayers.