1.5 Lipids

Lipids are grouped by behavior, not structure: they are mostly nonpolar hydrocarbons, so they're hydrophobic. They store energy, form membranes and act as signals (steroid hormones).

A fat = glycerol + three fatty acids, joined by ester linkages via dehydration synthesis. Fats store about twice the energy per gram of carbohydrates because of their many C–H bonds.

Saturated fatty acids have no C=C double bonds — carbons are 'saturated' with hydrogen. The chains are straight and pack tightly → solid at room temperature (butter, lard).

Unsaturated fatty acids have one or more C=C double bonds. A cis double bond puts a kink in the chain, so they can't pack tightly → liquid at room temperature (olive oil). Trans fats are artificially straightened and linked to heart disease.

A phospholipid swaps one fatty acid for a phosphate group. The phosphate head is hydrophilic; the two fatty-acid tails are hydrophobic. A molecule that is both is amphipathic.

In water, phospholipids spontaneously form a bilayer: heads face the watery inside and outside of the cell, tails hide in the middle. This is the basis of every cell membrane (more in Unit 2).

Steroids have four fused carbon rings. Cholesterol sits in animal membranes and is the starting molecule for hormones like testosterone and estrogen.

Saturated = straight = solid. Unsaturated = kinked = liquid.

Membrane fluidity link: more unsaturated tails → more fluid membrane. Organisms in cold climates increase unsaturated fatty acids so membranes don't stiffen — an AP favorite.