Lipids

Fatty acids are long hydrocarbon tails attached to a carboxyl group. If every carbon in the tail is bonded to as many hydrogens as possible, the fatty acid is saturated: the tail is straight, like a straight stick. These straight tails line up against each other closely, the same way straight sticks stack neatly in a box. Close packing means strong attractions between neighboring molecules, so saturated fats like butter stay solid at room temperature. An unsaturated fatty acid has at least one carbon-carbon double bond, and in the cis form that double bond forces a permanent bend in the tail. Bent tails can't stack neatly, so the molecules stay loosely packed with weaker attractions between them, which is why oils like olive oil stay liquid.

A triglyceride is one glycerol molecule joined to three fatty acid tails by ester bonds, built the same way proteins and starch are built, by removing water to form covalent bonds. But it stops there: three tails, one glycerol, done. It never becomes a long repeating chain, so it doesn't count as a true polymer even though the bond-forming chemistry matches.

Phospholipids rearrange this same toolkit for a different job. Swap one fatty acid tail for a phosphate group and the molecule becomes two-faced: the phosphate head is polar and hydrophilic, the two remaining tails stay nonpolar and hydrophobic. Dropped in water, phospholipids spontaneously turn their heads outward and tails inward, forming the bilayer that is the cell membrane. More unsaturated kinks among those tails mean looser packing, so the membrane stays more fluid and flexible even at lower temperatures, which is exactly the saturated-versus-unsaturated logic from fats applied to membranes.