ECA
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Study Notes

Unit 6: Lipids

Types of Lipids

Lipids are largely non-polar (hydrophobic) molecules made mostly of carbon and hydrogen. This unit covers three kinds.

  • Fats (triglycerides): a glycerol backbone with three fatty acid tails. Used for long-term energy storage. Contain C, H, O
  • Steroids: four fused carbon rings plus a side chain. Serve primarily as signaling molecules (hormones). Contain C, H, O
  • Phospholipids: make up the cell membrane. Contain C, H, O, P

Phospholipids & the Cell Membrane

A phospholipid has both hydrophilic and hydrophobic parts. The polar head group (containing phosphate) is oriented toward the cellular environment because it favorably interacts with the water-based environment inside and outside the cell. The non-polar fatty acid tails face inward, away from water.

  • Polar head (phosphate): hydrophilic, faces the watery surroundings
  • Non-polar tails: hydrophobic, tucked into the middle of the membrane
  • Only small non-polar molecules can diffuse passively across the cell membrane

Saturated vs. Unsaturated Fatty Acids

Saturated fatty acids have only single bonds between carbons, so they hold the maximum number of hydrogen atoms.

Unsaturated fatty acids have one or more double bonds between carbons and therefore fewer hydrogen atoms.

  • Cis-unsaturated: hydrogens removed from the SAME side of the chain, creating a kink/bend
  • Trans-unsaturated: hydrogens removed from OPPOSITE sides of the chain, so the chain stays straight
  • Cis-unsaturated fatty acids generally have lower melting points (liquid at room temperature) because the kinks prevent tight packing
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