Lesson 1.5 · Unit 1 · CED topic 1.5
Lipids
Oil and water do not mix, and that fact runs a surprising share of biology. Fat stores energy, hormones carry signals, and the membrane around every cell keeps the inside separate from the outside, all using molecules that avoid water. These are the lipids. Unlike the other three classes, lipids are not defined by a shared structure but by a shared behavior: they are mostly nonpolar and do not dissolve in water.
Describe the structure and function of lipids. The required statements: lipids are typically nonpolar, hydrophobic molecules whose structure and function come from how their parts are assembled (EK 1.5.A.1); examples are fats, steroids including cholesterol, and phospholipids (EK 1.5.A.2). The CED notes that the molecular structure of specific lipids is beyond the scope of the exam.
Why lipids avoid water
Recall from lesson 1.1 that water dissolves things by attracting their partial charges. A lipid is mostly long chains of carbon and hydrogen, and a carbon–hydrogen bond is nonpolar: the electrons are shared almost equally, so there are no partial charges for water to attract. Water molecules, which prefer to hydrogen-bond with each other, squeeze the nonpolar molecules together. Substances that behave this way are hydrophobic; substances that interact well with water are hydrophilic. Everything that follows is a consequence of that one difference.
Fats: energy storage
A fat (a triglyceride) is built from one glycerol and three fatty acids. A fatty acid is a long hydrocarbon tail with a carboxyl group at one end. Three dehydration synthesis reactions join the fatty acids to glycerol, releasing three waters, and each new bond is an ester linkage. Because the tails are long chains of C–H bonds, a fat is strongly hydrophobic and packed with stored chemical energy. Fat provides about 9 kilocalories per gram, compared with about 4 kcal per gram for carbohydrates and proteins. Fat also insulates (the blubber of marine mammals, the layer under the skin of humans) and cushions organs.
Saturated and unsaturated fatty acids
- Saturated fatty acids have only single bonds between carbon atoms. The chain is straight.
- Unsaturated fatty acids have at least one carbon–carbon double bond, which makes the chain kink (bend).
- The more double bonds in the tails, the more unsaturated the lipid.
- The more unsaturated a lipid is, the more liquid it is at room temperature.
The word “saturated” refers to hydrogen: a saturated chain holds as many hydrogen atoms as its carbons can bond to. Every double bond removes two hydrogens from the chain (carbon always makes four bonds, lesson 1.2). Straight tails can lie side by side and pack tightly, so the molecules attract each other strongly and the fat is solid at room temperature (butter, lard). Kinked tails cannot pack closely, so the molecules slide past one another and the fat is liquid at room temperature (vegetable oils). Most animal fats are mostly saturated and most plant oils are mostly unsaturated.
The table gives approximate melting points of four fatty acids, all with an 18-carbon chain. Describe the trend, explain it, and predict what would happen to a liquid oil if hydrogen were added to its double bonds.
| Fatty acid | Number of C=C double bonds | Melting point (°C) |
|---|---|---|
| Stearic | 0 | 70 |
| Oleic | 1 | 13 |
| Linoleic | 2 | −5 |
| α-Linolenic | 3 | −11 |
Describe. As the number of double bonds increases from 0 to 3, the melting point falls steadily from 70 °C to −11 °C. Only stearic acid is a solid at room temperature (about 25 °C); the other three melt below that temperature, so they are liquids.
Explain. Each double bond puts a kink in the chain. Straight (saturated) chains pack closely and need more energy (a higher temperature) to pull apart. Kinked chains cannot pack tightly, so less energy is needed to separate them and they melt at a lower temperature.
Predict. Adding hydrogen to an oil’s double bonds (hydrogenation) converts C=C into C–C, so the chains become straighter and pack more tightly. The oil would become more solid, which is how liquid oil is turned into margarine.
Phospholipids: the membrane builders
A phospholipid has a glycerol backbone, two fatty acid tails, and a phosphate-containing head in place of the third tail. The head is charged and hydrophilic and the tails are hydrophobic, so one molecule has two personalities. A molecule with both a hydrophilic and a hydrophobic region is amphipathic. In water, phospholipids arrange themselves so that the heads face the water and the tails face each other, away from it. The result is a lipid bilayer: the basic structure of the plasma membrane and of the membranes around organelles (Unit 2).
Steroids and cholesterol
Steroids do not look like fats. Their carbon skeleton is four connected rings, but they are still nonpolar and hydrophobic. Many hormones are steroids (estrogen, testosterone, and cortisol), and steroid hormones support growth and development, energy metabolism, and homeostasis. Cholesterol is a steroid found in the membranes of animal cells. It sits between the phospholipid tails and gives the membrane essential structural stability. Cholesterol is also the starting material for the cell to make other steroids.
| Lipid | Structure in brief | Function (EK 1.5.A.2) |
|---|---|---|
| Fats | Glycerol + 3 fatty acids | Energy storage and cell function; insulation in some mammals |
| Phospholipids | Glycerol + 2 fatty acids + phosphate head (amphipathic) | Form the lipid bilayer of plasma and cell membranes |
| Steroids | Four fused carbon rings | Hormones that support growth, development, energy metabolism, and homeostasis |
| Cholesterol (a steroid) | Four fused rings with a small hydroxyl group | Stabilizes animal cell membranes |
A snack bar label lists 14 g fat, 6 g carbohydrate, and 6 g protein. Use 9 kcal/g for fat and 4 kcal/g for carbohydrate and protein to find the energy in the bar and the percent that comes from fat. Then: a migrating bird must carry 1800 kcal of stored energy. Glycogen provides about 4 kcal/g and holds about 2 g of water for every gram stored. Compare the mass of fat and of hydrated glycogen the bird would need.
The bar. \(14 \times 9 + 6 \times 4 + 6 \times 4 = 126 + 24 + 24 = 174\) kcal. Fat supplies \(126 / 174 = 72.4\) percent.
The bird, as fat. \(1800 / 9 = 200\) g.
The bird, as glycogen. \(1800 / 4 = 450\) g of glycogen, plus \(2 \times 450 = 900\) g of bound water, so \(1350\) g in all, which is \(1350 / 200 = 6.75\) times heavier than the fat.
Per gram of dry mass, fat stores 2.25 times as much energy as glycogen (\(9/4\)), and because fat is hydrophobic it holds no water. That makes it the better fuel for a flying animal.
A few phospholipid molecules are shaken into a beaker of water. Predict what arrangement they form, and justify your prediction.
Prediction. The phospholipids form a bilayer (or, in small droplets, a ball of one layer) with the hydrophilic heads facing the water and the hydrophobic tails facing each other, away from the water.
Justification. The heads are charged or polar, so they form hydrogen bonds and other attractions with water and prefer to stay in contact with it. The tails are nonpolar and cannot interact with water, and the
water's hydrogen bonding squeezes them away from it. The only arrangement that satisfies both is one with the heads outward and the tails buried together. A grader looks for both halves: the arrangement, and the hydrophilic/hydrophobic reason.
- Lipid does not mean fat. Fats, phospholipids, and steroids are all lipids with different structures and functions. Match the function to the right type.
- Saturated means no C=C and a straight tail. Unsaturated means at least one C=C and a kinked tail. Say “kink” and “packing” when you explain why unsaturated fats are liquid.
- A question about a trend (more double bonds, lower melting point) wants all three steps: describe the trend, explain it with molecular packing, and use it to make a prediction.
- Amphipathic molecules (phospholipids) are the reason membranes form: heads toward water, tails away.
- Cholesterol is in animal cell membranes. It is a steroid, not a fat.
- Explain why lipids do not dissolve in water, using what you know about bonds and charge.
Show answer
Lipids consist mainly of carbon–hydrogen bonds, which are nonpolar because the electrons are shared almost equally, so lipids have no partial charges. Water dissolves a substance by attracting its charges. With nothing to attract, water cannot form hydrogen bonds with a lipid, and the water molecules, which hydrogen-bond with each other, push the lipid molecules together. The lipids are hydrophobic and stay separate.
- Describe how a saturated fatty acid differs from an unsaturated one, and explain why butter is solid and olive oil is liquid at room temperature.
Show answer
A saturated fatty acid has only single bonds between carbon atoms and a straight tail. An unsaturated fatty acid has at least one carbon–carbon double bond, which kinks the tail. Butter is rich in saturated fatty acids, whose straight tails pack tightly together, so it is solid at room temperature. Olive oil is rich in unsaturated fatty acids, whose kinked tails cannot pack closely, so the molecules move past one another and it is liquid.
- Identify the lipid described in each case: (a) forms the double layer around a cell; (b) stabilizes the membrane of an animal cell; (c) stored under the skin of a mammal for insulation and energy; (d) a hormone that regulates growth and development.
Show answer
(a) Phospholipids. (b) Cholesterol. (c) Fat (triglycerides). (d) A steroid hormone (such as estrogen or testosterone).
- A cold-water fish has more unsaturated fatty acids in its membranes than a warm-water fish. Predict what would happen to the fish’s membranes if they instead contained mostly saturated fatty acids, and justify your prediction.
Show answer
Prediction: the membranes would become too stiff, nearly solid, in cold water. Justification: straight saturated tails pack tightly, so a membrane made of them gets more rigid as temperature falls. Kinked unsaturated tails cannot pack closely, which keeps the membrane fluid at low temperature. A membrane that is too stiff cannot function, so the fish needs unsaturated fatty acids in cold water.
- A nutrition label shows 5 g fat, 40 g carbohydrate, and 10 g protein. Calculate the total energy and the percent of the energy from fat (9 kcal/g for fat, 4 kcal/g for the others).
Show answer
Energy: \(5 \times 9 + 40 \times 4 + 10 \times 4 = 45 + 160 + 40 = 245\) kcal. Percent from fat: \(45 / 245 = 18.4\) percent.