Saturated Fatty Acids: These contain only single bonds between carbon atoms in the hydrocarbon chain, allowing the chain to be straight and pack closely together.
Unsaturated Fatty Acids: These contain one or more double bonds (). A monounsaturated fatty acid has one double bond, while polyunsaturated fatty acids have multiple.
Cis vs. Trans: In 'cis' configurations, hydrogen atoms are on the same side of the double bond, causing a 'kink' in the chain. In 'trans' configurations, hydrogens are on opposite sides, resulting in a straighter chain often associated with health risks.
| Feature | Triglyceride | Phospholipid |
|---|---|---|
| Structure | 1 Glycerol + 3 Fatty Acids | 1 Glycerol + 2 Fatty Acids + 1 Phosphate |
| Polarity | Entirely Non-polar (Hydrophobic) | Amphipathic (Polar head, Non-polar tails) |
| Main Function | Energy storage and insulation | Structural component of cell membranes |
| Water Released | 3 molecules during formation | 3 molecules during formation |
Identify the Bond: In diagrams, look for the linkage to identify the ester bond. This is distinct from the glycosidic bonds found in carbohydrates.
Count the Waters: Remember that the number of water molecules released (or required for hydrolysis) always equals the number of ester bonds present.
Solubility Logic: If asked why lipids are good for storage, emphasize their hydrophobicity. Because they don't dissolve, they do not affect the osmotic potential of the cell.
Energy Density: Lipids contain more carbon-hydrogen bonds per gram than carbohydrates, which is why they yield significantly more energy ( vs ) when oxidized.
The Polymer Myth: Students often mistakenly call lipids polymers. Clarify that while they are large molecules, they do not consist of a continuous chain of identical repeating monomers.
Hydrolysis vs. Condensation: Ensure you can distinguish between the two. Hydrolysis adds water to break the ester bond (e.g., during digestion), while condensation removes water to form it.
Phosphate Polarity: Do not assume the whole phospholipid is hydrophobic. The phosphate group is highly polar and hydrophilic, which is essential for forming the bilayer.
