Lipids: Fats, Oils, Phospholipids and Steroids

Butter, olive oil, the wax on a leaf, the membranes around every cell, and hormones like testosterone all look completely different, yet chemists group them together as lipids. The family isn’t defined by one structure, as carbohydrates and proteins are. Instead, lipids are defined by a shared behaviour: they don’t dissolve in water, but they do dissolve in non-polar solvents such as ethanol or hexane. This article explains why, and introduces the main kinds of lipid and what they do.

Why lipids avoid water

Most of a lipid molecule is made of long hydrocarbon chains or rings: carbon and hydrogen atoms bonded together. Carbon and hydrogen have very similar electronegativities, so C–H bonds are almost non-polar (see polar vs non-polar molecules).

Water molecules are strongly polar and attract each other through hydrogen bonds. A non-polar chain can’t form hydrogen bonds, so pushing it into water forces the water around it into a more ordered arrangement, which is thermodynamically unfavourable. The result is that non-polar molecules are squeezed together and away from water. This is the hydrophobic effect, and it drives everything from oil floating on vinegar to the formation of cell membranes.

Lipids contain relatively little oxygen compared with carbohydrates. That’s also why they’re so rich in energy: their carbon atoms are less oxidised to begin with, so more energy is released when they’re fully oxidised to carbon dioxide and water.

1. Triglycerides: fats and oils

The most common lipids in food and in the body are triglycerides (also called triacylglycerols). Each molecule is built from:

• one molecule of glycerol, a small alcohol with three –OH groups, and
• three fatty acids, long hydrocarbon chains with a carboxylic acid group (–COOH) at one end.

Each fatty acid joins to one –OH of glycerol through an ester bond, formed in a condensation reaction that releases water. Three fatty acids, three ester bonds, three water molecules released.

Fats vs oils. Both are triglycerides. The difference is their state at room temperature, which depends mainly on the fatty acids:

• Fats (butter, lard) are solid. They’re rich in saturated fatty acids, whose straight chains pack tightly together.
• Oils (olive, sunflower, fish oil) are liquid. They’re rich in unsaturated fatty acids, whose C=C double bonds put kinks in the chains, stopping them packing closely. Weaker intermolecular forces mean lower melting points (see intermolecular forces).

More detail on saturated, unsaturated, trans and essential fatty acids is in fatty acids.

Functions of triglycerides:

• Energy storage. Fat provides about 37 kJ per gram, more than twice as much as carbohydrate or protein (about 17 kJ per gram). It’s also stored without water, unlike glycogen, making it a very compact fuel. A typical adult stores far more energy as fat than as glycogen.
• Insulation. Fat under the skin reduces heat loss; whales and seals rely on thick blubber.
• Protection. Fat cushions organs such as the kidneys.
• Buoyancy in aquatic animals.

2. Phospholipids: building membranes

A phospholipid is like a triglyceride in which one fatty acid has been replaced by a phosphate group (often linked to another small polar group, such as choline).

This gives the molecule two very different ends:

• a hydrophilic head (the charged phosphate group), which attracts water;
• two hydrophobic tails (the fatty acid chains), which avoid it.

Molecules with both kinds of region are called amphipathic. In water, phospholipids arrange themselves automatically into a bilayer: two layers with the tails pointing inwards, away from water, and the heads facing the water on both sides. This bilayer is the basic structure of every cell membrane. It lets small non-polar molecules through but blocks ions and large polar molecules, which must pass through protein channels and pumps instead.

The full story is in phospholipids and cell membranes.

3. Steroids: four fused rings

Steroids look nothing like triglycerides. Their core is four fused carbon rings (three six-membered and one five-membered). They’re classed as lipids because they’re mostly hydrocarbon and don’t dissolve in water.

Important steroids include:

• Cholesterol, a component of animal cell membranes and the starting material for other steroids (see cholesterol chemistry).
• Steroid hormones, such as testosterone, oestrogen, progesterone and cortisol, which pass through cell membranes and regulate genes.
• Vitamin D, made in the skin from a cholesterol derivative when exposed to sunlight.
• Bile salts, made in the liver, which help digest fats.

4. Waxes

Waxes are esters formed from a long-chain fatty acid and a long-chain alcohol. They’re very hydrophobic and fairly solid at room temperature, which makes them ideal waterproofing:

• the waxy cuticle on leaves reduces water loss;
• beeswax builds honeycomb;
• earwax and the natural oils in skin and feathers repel water.

Digesting and using fats

Because fats don’t dissolve in water, digesting them needs a special trick.

1. Emulsification. In the small intestine, bile salts (made from cholesterol) break large fat droplets into tiny ones. They act like detergents: part of each bile salt molecule is attracted to fat and part to water. Smaller droplets have a much larger surface area.
2. Hydrolysis. The enzyme lipase hydrolyses the ester bonds, releasing fatty acids and monoglycerides.
3. Absorption. These products enter the cells lining the intestine, where they’re reassembled into triglycerides and packaged with proteins into particles called chylomicrons, which travel through the lymph into the blood.
4. Use or storage. Cells either break fatty acids down for energy (mainly in mitochondria) or store them as triglycerides in fat tissue.

Testing for lipids: the emulsion test

A simple school test:

1. Shake a small food sample with about 2 cm³ of ethanol to dissolve any lipid. (Ethanol is flammable; keep it away from flames.)
2. Pour the ethanol into a test tube of water.
3. A cloudy white emulsion means lipid is present. The lipid, no longer dissolved, forms tiny droplets that scatter light.

A clear result means no lipid was detected.

Why lipids matter in everyday life

• Soap and detergents work on the same principle as phospholipids: molecules with a charged head and a long non-polar tail surround grease and carry it into water.
• Margarine was traditionally made by hydrogenating vegetable oils to raise their melting point (a process that could create trans fats; modern methods largely avoid this).
• Rancidity happens when unsaturated fats react with oxygen or are hydrolysed, producing unpleasant-smelling compounds. Antioxidants, cool storage and airtight packaging slow it down.

Common misconceptions

• “All fat is bad for you.” Some fats are essential in the diet, and all cells need lipids for their membranes. Health depends on types and amounts.
• “Oils have less energy than fats.” Oils and fats have almost the same energy per gram; they just differ in melting point.
• “Cholesterol is a type of fat.” Cholesterol is a steroid lipid, not a triglyceride.
• “Lipids are polymers.” Triglycerides are large molecules but are not built from long repeating chains of identical units.

Key takeaways

• Lipids are defined by being insoluble in water and soluble in non-polar solvents, because they’re mostly non-polar hydrocarbon.
• Triglycerides (glycerol + 3 fatty acids, joined by ester bonds) are fats and oils, storing about 37 kJ g⁻¹.
• Phospholipids have hydrophilic heads and hydrophobic tails and form the bilayers of cell membranes.
• Steroids (cholesterol and hormones) have four fused rings; waxes waterproof surfaces.
• Fats are digested by bile emulsification and lipase hydrolysis. See how lipids fit with other biomolecules in the four major biomolecules.