Lipids: triglycerides, phospholipids and ester bonds

What defines a lipid

Lipids are grouped together by how they behave rather than by one shared structure. They are largely hydrocarbon, so they are non-polar: they do not dissolve in water and they do dissolve in organic solvents such as ethanol. Triglycerides, phospholipids, cholesterol and the waxes on a leaf surface are all lipids, but only the first two are built the same way. Being non-polar, lipids also make waterproof layers: the waxy cuticle of a leaf, and the oily secretions on skin, fur and feathers.

One point of vocabulary needs care. A triglyceride is a large molecule, but it is not a polymer. A polymer is many similar monomers joined in a repeating chain, and a triglyceride is three fatty acids and one glycerol, which are not the same unit repeated. Polysaccharides and proteins are polymers; lipids are not.

Lipid

A biological molecule that is largely non-polar, insoluble in water and soluble in organic solvents such as ethanol.

Triglyceride

One glycerol molecule joined to three fatty acids by three ester bonds.

Ester bond

The covalent bond formed by condensation between a hydroxyl group of glycerol and the carboxyl group of a fatty acid.

Glycerol, fatty acids and the ester bond

Every triglyceride is made from the same alcohol, glycerol, which carries three hydroxyl groups. What varies is the three fatty acids attached to it. A fatty acid is a carboxyl group with a hydrocarbon tail behind it, and the tail may be any length and may or may not contain double bonds.

A hydroxyl group of the glycerol reacts with the carboxyl group of the fatty acid. The OH of the carboxyl group and the H of the glycerol's hydroxyl group leave together as water, and an ester bond, –O–CO–, is left joining the two. That happens three times, once at each hydroxyl of the glycerol, so building one triglyceride forms three ester bonds and releases three molecules of water.

Describe the drawing

At the top, a tall block labelled glycerol with three arms coming off it, each ending in a hydroxyl group, and beside them three fatty acids, each drawn as a carboxyl group written HOOC followed by a long zig-zag hydrocarbon tail. An arrow points down to the product. In the product the glycerol block is joined to all three tails by three thick bonds, labelled ester bonds, and three molecules of water have been released, one for each bond formed.

Saturated and unsaturated fatty acids

In a saturated fatty acid every carbon in the tail holds as many hydrogen atoms as it can, and there is no carbon to carbon double bond. The chain is free to rotate about every bond, so it can lie straight. In an unsaturated fatty acid at least one pair of carbons is joined by a double bond, and the chain cannot rotate there. In the cis arrangement, which is the usual one in biological fatty acids, the chain leaves the double bond on the same side it arrived and picks up a permanent kink; the trans arrangement keeps the chain nearly straight, which is why trans fats behave more like saturated ones.

Describe the drawing

On the left, a saturated fatty acid tail drawn as a straight zig-zag with no double bond in it. On the right, an unsaturated tail of the same length with a marked carbon to carbon double bond about halfway down, below which the chain bends away to one side. Underneath, five straight tails are drawn packed tightly side by side, and four bent tails are drawn with much wider gaps between them, because the bends stop them lying close. The straight ones are labelled solid at room temperature and the bent ones liquid.

A bent molecule cannot pack against its neighbours as closely as a straight one, so fewer of the attractions between neighbouring chains form and less energy is needed to separate them. That is why the melting point falls: triglycerides made largely from saturated fatty acids are solid at room temperature and are called fats, while those with unsaturated fatty acids are liquid and are called oils. Animal lipids are mostly the first, plant and fish lipids mostly the second.

One double bond in the tail makes a fatty acid monounsaturated; more than one makes it polyunsaturated.

Triglycerides as an energy store

A gram of lipid releases roughly twice the energy of a gram of carbohydrate when respired, because the tails are long chains of carbon and hydrogen containing very little oxygen, and the hydrogen supplies the electrons that drive the electron transport chain. That matters most where the constraint is mass carried, as in a migrating bird or a hibernating mammal.

Triglycerides are also insoluble in water, so a large store in an adipose cell has no effect on the water potential of its cytoplasm. A store of glucose of the same energy content would draw water in by osmosis; the same argument explains why starch and glycogen are insoluble stores.

Two further functions follow from the same physical properties. Adipose tissue under the skin is a poor conductor of heat and therefore provides thermal insulation, and around the kidneys it provides physical protection. Respiring lipid also releases a substantial volume of metabolic water, which is significant for a desert animal such as a kangaroo rat that drinks very little.

Phospholipid structure

A phospholipid contains two fatty-acid tails and a phosphate-containing head group, in place of the third fatty acid of a triglyceride. That single substitution changes the molecule from an energy store into the structural component of a membrane.

The two ends differ in polarity. The phosphate group carries a negative charge, so it interacts with water molecules and is hydrophilic. The two hydrocarbon tails are non-polar, are excluded from water, and are hydrophobic. A molecule with both properties at opposite ends is amphipathic, and membrane structure follows from that property.

Describe the drawing

Two molecules drawn side by side. On the left a triglyceride: a glycerol block with three fatty acid tails attached to it by three bonds. On the right a phospholipid: the same glycerol block, but the top tail has been replaced by a round phosphate group, leaving two tails. Labels record that the phosphate group is charged and therefore attracts water while the tails do not. Below a dividing line, many phospholipids are drawn as a bilayer: one row of round heads facing the water above, a second row facing the water below, and all the tails pointing inwards to meet in the middle.

No enzyme assembles the bilayer and no covalent bond joins one phospholipid to the next. Each molecule can move laterally past its neighbours, which makes a membrane fluid. The sheet is held together by weaker collective interactions: the hydrophobic effect, since any other arrangement would place more hydrophobic tails in contact with water, together with van der Waals attractions between tails packed side by side. Next topic: the fluid mosaic model.

Hydrophilic

Attracted to water: charged or polar, so it interacts with water molecules.

Hydrophobic

Not attracted to water: non-polar, so it is excluded from water rather than dissolving in it.

The emulsion test

The test for a lipid uses its defining solubility: a lipid dissolves in ethanol and not in water. Shake the sample with absolute ethanol so that any lipid dissolves, then pour that solution into an equal volume of water in a clean tube. A milky white emulsion forms and means a lipid is present.

State the reason for the result. In ethanol the lipid is in solution. Poured into water, it is no longer in a solvent in which it dissolves, so it comes out of solution as small droplets suspended through the water, and those droplets scatter light. The cloudiness is finely divided lipid.