Lipids: Fats, Phospholipids, Steroids and Waxes

The camel's secret

A camel's hump isn't a water tank. It is mostly fat. Fat is the best way for an animal to carry stored energy: gram for gram, it holds about twice as much energy as carbohydrate or protein. Lipids also build every cell membrane, waterproof feathers and leaves, and act as hormones that control growth and reproduction. They are the most varied of the four classes of biological molecules.

What makes a lipid a lipid?

Lipids are grouped by a property, not by a monomer: they are mostly non-polar hydrocarbon, so they don't dissolve in water. They are not polymers. They are built from smaller pieces, but not from a repeating chain of one kind of unit.

Triglycerides (fats and oils)

A fatty acid is a long hydrocarbon chain (usually 14 to 22 carbons, an even number) with a carboxyl group at one end. Glycerol is a three-carbon alcohol with three hydroxyl groups.

Each –OH of glycerol reacts with the –COOH of a fatty acid in a condensation reaction, forming an ester bond and releasing water. Three fatty acids give one triglyceride and three water molecules. Digestion by lipase is the reverse: hydrolysis of the ester bonds.

Saturated, unsaturated, cis and trans

• A saturated fatty acid has only single C–C bonds. It is "saturated" with hydrogen. Its tail is straight, so the molecules pack tightly and have strong attractions: fats such as butter and lard are solid at room temperature. Formula pattern: CₙH₂ₙO₂ (stearic acid, C₁₈H₃₆O₂).
• An unsaturated fatty acid has one or more C=C double bonds. Each double bond removes 2 H from the formula (oleic acid, one C=C: C₁₈H₃₄O₂; linoleic acid, two: C₁₈H₃₂O₂).
• In natural unsaturated fats the double bonds are almost always cis: the chain bends at each one. The kinks stop the molecules packing tightly, so plant and fish oils (canola, olive, sunflower, salmon oil) are liquid.
• Trans double bonds keep the chain nearly straight, so trans fats behave more like saturated fats. Most trans fat in processed food came from partial hydrogenation, which adds hydrogen to some double bonds of an oil to make it solid and moves others into the trans form. Trans fats raise "bad" LDL cholesterol and lower "good" HDL cholesterol, so Health Canada banned partially hydrogenated oils in foods sold in Canada from September 2018.
• Omega-3 and omega-6 fatty acids are named for where the first double bond is, counting from the methyl ("omega") end. Two of them (linoleic acid, an omega-6, and α-linolenic acid, an omega-3) are essential: we must get them from food.

Phospholipids: the membrane builders

A phospholipid is glycerol with two fatty acids and, on the third carbon, a phosphate group, usually linked to another small polar group such as choline. (Phosphatidylcholine, also called lecithin, is the phospholipid in egg yolk.)

The phosphate head is charged and hydrophilic; the two tails are hydrophobic. The molecule is amphipathic. Put phospholipids in water and they arrange themselves so that the heads face water and the tails hide from it:

• a bilayer: two layers, tails facing inward. This is the basic structure of every cell membrane;
• or, for molecules with a single tail (such as soaps and free fatty acids), a ball-shaped micelle.

No energy or enzyme is needed for this: it happens spontaneously because of the hydrophobic effect. You'll build on this in The Fluid Mosaic Model.

Steroids and waxes

Steroids look nothing like fats: they are built from four fused carbon rings. Different groups attached to the rings make different steroids.

• Cholesterol sits among the phospholipids of animal cell membranes and helps control their fluidity. It is also the starting material for steroid hormones (testosterone, estrogen, progesterone, cortisol), vitamin D and bile salts. The liver makes most of the body's cholesterol.
• Cholesterol travels in the blood packed in lipoproteins: high levels of LDL are linked to fatty deposits in arteries.

Waxes are esters of a long-chain fatty acid and a long-chain alcohol. They are extremely hydrophobic. Plants coat leaves with a waxy cuticle that reduces water loss, bees build combs of beeswax, and birds spread oils from a gland onto their feathers for waterproofing.