2.4 Lipid Structure
Key Takeaways
Lipids are grouped by hydrophobicity, not by being polymers of one identical monomer.
A triglyceride is glycerol plus three fatty acids joined by ester linkages; building one releases three water molecules.
Saturated fatty acids have no carbon-carbon double bonds and pack tightly, while unsaturated fatty acids have one or more cis double bonds that kink the chain.
Phospholipids are amphipathic, with a hydrophilic head and two hydrophobic tails, and they form bilayers; they are not triglycerides.
Steroids share a four-ring skeleton and are lipids; cholesterol is a membrane steroid and a precursor of steroid hormones, while waxes are protective hydrophobic coatings.
2.4 Lipid Structure
Lipids are grouped because they are hydrophobic: they mix poorly with water. They are not a polymer family the way starch is. Some lipids are built from smaller pieces, but those pieces are not a long chain of one identical monomer. Fatty acids differ in length and in double bonds, and steroids are not fatty-acid chains at all. The family test is poor solubility in water.
Triglycerides and ester linkages
A triglyceride, also called a triacylglycerol, is glycerol plus three fatty acids. Glycerol is a three-carbon molecule with a hydroxyl on each carbon. Each fatty acid is a hydrocarbon chain with a carboxyl group at one end. Dehydration synthesis joins each carboxyl to one hydroxyl and releases one water molecule, so one triglyceride releases three water molecules as it forms. Each covalent join is an ester linkage. Hydrolysis breaks those esters and frees the fatty acids and glycerol. Three esters formed means three waters released. Three esters broken means three waters consumed.
Triglycerides store energy, cushion organs, and insulate the body. They yield about 9 kilocalories per gram, compared with about 4 kilocalories per gram for carbohydrate, so fat stores more than twice as much energy per gram. Those jobs belong to storage fat. A cell membrane is a different lipid structure. A drawing with three fatty acids and no phosphate head is a triglyceride, not the bilayer lipid.
A saturated fatty acid has no carbon-carbon double bonds. The tail is relatively straight, so neighboring chains pack tightly, contact between chains is extensive, and the melting temperature is higher. Fats rich in saturated tails are often solid at room temperature.
An unsaturated fatty acid has one or more carbon-carbon double bonds. In the usual biological case the double bond is cis, which kinks the chain. Kinked chains pack poorly, contact is weaker, and the melting temperature is lower, so these fats are often liquid at room temperature. One double bond means monounsaturated. Two or more means polyunsaturated. A trans double bond is straighter than a cis double bond and packs more like a saturated tail. The kink to remember is the cis kink.
Butter and olive oil
Picture two triglycerides at room temperature. In butter, many fatty acids are saturated and relatively straight. They pack closely, and the fat is solid. In olive oil, many fatty acids are unsaturated, with cis double bonds that bend the tails. Packing is poorer, and the fat is liquid. Both are triglycerides with glycerol and ester linkages. The difference in state is how well the tails pack. The liquid has not stopped being a lipid, and the solid has not become a steroid.
Phospholipids form bilayers
A phospholipid is not a triglyceride. In the common phospholipid, glycerol is the backbone, but only two fatty acids attach. The third position carries a phosphate group, often linked to one more small polar or charged group. The phosphate end associates with water and is hydrophilic. The two tails avoid water and are hydrophobic. A molecule with both characters is amphipathic.
In water, heads face water and tails hide from it. Two-tailed phospholipids form a bilayer, tails inward and heads outward on both faces. That bilayer is the basic fabric of cell membranes. The hydrophobic effect and weak contacts among tails hold the sheet together. Covalent bonds do not tie one phospholipid to the next in an ordinary membrane. A triglyceride, with three tails and no phosphate head, forms droplets instead of this bilayer.
The tails can be saturated or unsaturated, and that mix changes fluidity. Kinked unsaturated tails pack less tightly and keep a membrane more fluid. Straight saturated tails pack more tightly. The head-and-two-tails structure is still what identifies a phospholipid.
Steroids and waxes
Steroids share a skeleton of four fused rings. They are lipids because that skeleton is hydrophobic, even though they are not long fatty-acid chains. Cholesterol is the steroid to place first. In animal membranes it sits among phospholipid tails and modulates fluidity: at warm temperatures it restrains movement, and at cool temperatures it keeps tails from packing into a solid gel. Cholesterol is also a precursor of steroid hormones, including sex hormones and cortisol. A four-ring hydrophobic molecule belongs with the lipids.
Waxes are protective hydrophobic coatings, such as a leaf surface, a feather, or an insect covering that sheds water. A wax is not the storage triglyceride and not the membrane bilayer. Waterproof coating points to wax. Energy per gram points to triglyceride. A two-tailed sheet around a cell points to phospholipid. Four fused rings, or a hormone made from cholesterol, points to steroid.
| Lipid | Built from | Structural fact | Role |
|---|---|---|---|
| Triglyceride | Glycerol + three fatty acids | Three ester linkages | Energy storage, cushioning, insulation |
| Saturated fatty acid | No carbon-carbon double bonds | Straight tail; packs tightly | Favors solid fat at room temperature |
| Unsaturated fatty acid | One or more cis double bonds | Kinked tail; packs poorly | Favors liquid oil at room temperature |
| Phospholipid | Glycerol + two fatty acids + phosphate | Hydrophilic head, two hydrophobic tails | Forms bilayers |
| Steroid | Four fused rings | Hydrophobic, not a fatty-acid chain | Cholesterol in membranes; hormone precursor |
| Wax | Long hydrophobic chains | Coating | Protective water barrier |
Note
Phospholipids are not triglycerides: a phospholipid has a hydrophilic head and two hydrophobic tails and forms bilayers. Steroids are lipids even though they are four-ring molecules rather than fatty-acid chains. Lipids are grouped by hydrophobicity, not by identical repeating monomers.
One triglyceride is solid at room temperature and another is liquid. Both contain glycerol and three fatty acids. Which structural reason best explains the difference?
The liquid oil is a phospholipid bilayer, and the solid fat is a free phosphate group with no carbon chains.
The liquid oil contains only saturated fatty acids whose cis double bonds pack into a tight crystal, while the solid fat has no ester linkages.
The solid fat is richer in saturated fatty acids that pack tightly, while the liquid oil has cis double bonds that kink unsaturated chains.
The solid fat is a steroid with four fused rings, while the liquid is a wax coating on a leaf.
Which description of membrane lipids is accurate?
A phospholipid is a triglyceride with three fatty acids and no hydrophilic region, so the whole molecule dissolves uniformly in water.
Phospholipids are hydrophobic at every atom, lack partial charges on the head, and cannot organize into bilayers.
Cholesterol is a storage polysaccharide of beta glucose and is excluded from the lipid category because it dissolves readily in water.
A phospholipid is amphipathic, with a hydrophilic head and two hydrophobic tails, and phospholipids form bilayers.
A review sheet says lipids are polymers of one identical monomer and that steroids do not count as lipids because they lack fatty-acid chains. Which correction is right?
Every lipid is a polymer of glucose, and steroids are polysaccharides stored in muscle glycogen granules.
Lipids are grouped by hydrophobicity rather than by a repeating identical monomer, and steroids such as cholesterol are lipids with a four-ring skeleton.
Waxes are the only lipids, because only waxes contain peptide bonds between amino acids.
Triglycerides form when hydrolysis adds three water molecules, and saturated tails are defined by several cis double bonds.
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