Lipids: Study Guide (Biology 1.5)

What makes a molecule a lipid?

Carbohydrates, proteins and nucleic acids are each built from one family of monomers. Lipids are different. They are a mixed group of molecules that share one property: they do not mix well with water. Fats, oils, phospholipids, steroids and waxes are all lipids.

The reason is their make-up. Lipids are mostly carbon and hydrogen, with only a little oxygen. Carbon and hydrogen have similar electronegativity, so a C–H bond shares its electrons almost equally. It is nonpolar. A molecule made mainly of C–H bonds has no δ⁺ and δ⁻ ends for water to attract (Topic 1.1). Water molecules stay hydrogen-bonded to each other instead, and the lipid is pushed aside. Molecules like this are hydrophobic (“water-fearing”).

Most lipids are not polymers. They are assembled from a few subcomponents, such as glycerol and fatty acids. How those parts are put together decides what the lipid does. That is the main idea of this topic.

Fatty acids: saturated and unsaturated

A fatty acid is a long chain of carbon atoms with hydrogen atoms attached (a hydrocarbon tail), with an acid (carboxyl) group at one end. Most natural fatty acids have between about 12 and 24 carbon atoms.

• In a saturated fatty acid, every carbon–carbon bond is a single bond. Each carbon holds as many hydrogen atoms as it can, so the chain is “saturated” with hydrogen. The tail is straight.
• In an unsaturated fatty acid, there is at least one C=C double bond. Each double bond puts a kink (bend) in the chain.
• One double bond makes a monounsaturated fatty acid. Two or more make a polyunsaturated one. The more double bonds, the more unsaturated the fatty acid.

A useful check: each C=C double bond means two fewer hydrogen atoms. An 18-carbon fatty acid with no double bonds has the formula C₁₈H₃₆O₂. With one double bond it is C₁₈H₃₄O₂, and with two it is C₁₈H₃₂O₂. You do not need to learn these formulas; they show what “less saturated with hydrogen” means.

Why kinks change whether a fat is solid or liquid

Neighbouring hydrocarbon tails attract each other through weak attractions along their length. Straight saturated tails can line up side by side, touching along their whole length, like pencils in a box. Many attractions hold them in place, so it takes a higher temperature to make them move freely. Saturated fats such as butter and lard are solid at room temperature.

Kinked unsaturated tails cannot pack closely. There are gaps between them, fewer points of contact and weaker overall attraction. They slide past each other at lower temperatures. Unsaturated fats such as olive oil and most plant and fish oils are liquid at room temperature.

The rule to remember: the more double bonds, the more kinks, the looser the packing, and the more liquid the lipid is at a given temperature.

Fats (triglycerides): energy storage and insulation

A fat (also called a triglyceride) is made from one glycerol molecule and three fatty acids. Glycerol is a small three-carbon molecule with an –OH group on each carbon. Each fatty acid joins to one of those –OH groups by dehydration synthesis (Topic 1.3): an –OH is removed from the fatty acid and an H from glycerol, which releases one water molecule and forms a covalent ester linkage. So building one fat releases three water molecules. Digesting a fat reverses this by hydrolysis, adding three water molecules back.

Energy storage. Fat tails are packed with C–H bonds, and fats contain very little oxygen. When fats are respired, they release about 37 kJ per gram, compared with about 17 kJ per gram for carbohydrate. So fat stores more than twice as much energy per gram. Because fats are hydrophobic, they are stored as compact droplets in fat cells (adipose tissue) and seeds without drawing in water. This matters for organisms that must carry their energy store with them: animals that migrate or hibernate, and seeds that must be light enough to spread.

Insulation. Fat conducts heat poorly. A thick layer of fat under the skin, such as the blubber of whales and seals, slows the loss of heat to cold water and helps keep mammals warm. Fat layers also cushion some organs.

Supporting cell function. Stored fat is a source of fatty acids, which cells use to build other lipids, including the phospholipids of their membranes.

Phospholipids: the basis of every membrane

A phospholipid looks like a fat with one change. Glycerol carries only two fatty acids. The third position holds a phosphate group, often with another small charged or polar group attached. Phosphorus is one of the elements cells must take in to build these molecules (Topic 1.2).

The phosphate end is charged and polar: it is a hydrophilic head. The two fatty acid tails are nonpolar: they are hydrophobic. A molecule with a hydrophilic part and a hydrophobic part is called amphipathic.

In water, phospholipids arrange themselves so that each part is where it is most stable. The heads face the water. The tails hide from the water by pointing towards each other. The result is a phospholipid bilayer: two layers of phospholipids, tail to tail. This happens on its own, without energy input from the cell, because it lets water molecules keep their hydrogen bonds with each other and with the heads.

The hydrophobic middle of the bilayer is the reason a membrane can act as a barrier. Ions and large polar molecules cannot easily cross it. You will study how membranes control what enters and leaves the cell in Unit 2.

Fatty acid tails matter here too. Kinked unsaturated tails keep the bilayer fluid, so membrane proteins can move and the membrane can bend. If a membrane contained only saturated tails, it would become stiff in the cold.

Steroids and cholesterol

Steroids look nothing like fats. Their carbon skeleton is four rings joined together (fused rings), with different small groups attached. They are still lipids, because they are mostly carbon and hydrogen and are hydrophobic.

• Steroid hormones carry signals around the body. Examples include testosterone, oestrogen and cortisol. They help to control growth and development (for example, at puberty), energy metabolism (for example, cortisol raises blood glucose during stress) and homeostasis. Background: because they are lipids, steroid hormones can pass through the membrane of a target cell; you will meet this in cell signalling (Unit 4).
• Cholesterol is a steroid found in animal cell membranes, tucked between the phospholipids. It gives the membrane structural stability: it helps the membrane keep its shape and stay intact as conditions change. Background: at warm temperatures cholesterol restrains phospholipid movement; at cool temperatures it stops the tails from packing too tightly. Animal cells also use cholesterol to make steroid hormones.

Structure decides function

Lipid	Key structural feature	Function
Saturated fat	straight tails pack tightly	solid store of energy (for example, in many animals)
Unsaturated fat or oil	kinked tails pack loosely	liquid store of energy (for example, in many seeds)
Any fat	many C–H bonds, little oxygen, hydrophobic	stores about 37 kJ g⁻¹; insulates mammals
Phospholipid	hydrophilic phosphate head, two hydrophobic tails	forms the bilayer of plasma and other cell membranes
Steroid hormone	four fused rings, hydrophobic	signal that controls growth, development, metabolism and homeostasis
Cholesterol	four fused rings, sits among phospholipids	structural stability of animal cell membranes

Scope note. You must be able to describe these general structures. You do not need to know or draw the exact molecular structure of any specific lipid.

Worked example 1: why store energy as fat?

Question. Before a long journey, a small animal needs an energy store of 1850 kJ. Calculate the mass of (a) fat and (b) carbohydrate that would store this energy. Take 37 kJ g⁻¹ for fat and 17 kJ g⁻¹ for carbohydrate. (c) Explain why fat is the better store for an animal that must move.

1. Rearrange: energy = mass × energy per gram, so mass = energy ÷ energy per gram.
2. (a) Fat: mass = 1850 kJ ÷ 37 kJ g⁻¹ = 50 g.
3. (b) Carbohydrate: mass = 1850 kJ ÷ 17 kJ g⁻¹ = 108.8 g, so about 110 g (109 g to 3 significant figures).
4. Check the units. kJ ÷ (kJ g⁻¹) leaves g, which is right for a mass.
5. Compare: 108.8 ÷ 50 = 2.18. The carbohydrate store would be about 2.2 times as heavy, an extra 59 g to carry.

(c) Interpretation. Fat packs more energy into each gram because its tails are made of energy-rich C–H bonds, with little oxygen. It is also hydrophobic, so it is stored in compact droplets without drawing in water. A lighter store costs less energy to carry, which matters for an animal that flies, swims or migrates. Carbohydrate (Topic 1.4) is still useful: it can be broken down faster when energy is needed quickly.

Worked example 2: predicting the effect of a change in membrane lipids

Question. In a fictional study, one species of bacterium was grown at four temperatures. The percentage of unsaturated fatty acid tails in its membrane phospholipids was measured.

Growth temperature / °C	10	20	30	40
Unsaturated tails / %	62	48	35	24

(a) Describe the trend, using numbers. (b) Explain how this change helps the bacterium. (c) A mutant strain cannot make the enzyme that adds double bonds to fatty acid tails. Predict how the mutant would grow at 10 °C compared with normal bacteria, and justify your prediction.

(a) Trend.

1. As temperature rises, the percentage of unsaturated tails falls, from 62% at 10 °C to 24% at 40 °C.
2. That is a fall of 62 − 24 = 38 percentage points over 30 °C, an average of about 1.3 percentage points per °C (38 ÷ 30 = 1.27).
3. Each 10 °C step gives a smaller fall: 14, then 13, then 11 percentage points.

(b) Explanation. Cold makes tails move less, so a membrane tends to become stiff. More kinked, unsaturated tails stop the phospholipids packing tightly, which keeps the membrane fluid in the cold. In the heat, tails move more and the membrane can become too fluid and leaky; more straight, saturated tails pack closer and keep it stable. So the bacterium adjusts the lipids it makes to keep its membranes working across temperatures.

(c) Prediction. The mutant can make only saturated tails. At 10 °C its membranes would pack tightly and become too rigid. Membrane proteins could not move and work properly, and transport across the membrane would slow. So the mutant would grow more slowly than normal bacteria at 10 °C, or not at all. At warm temperatures the difference should be much smaller, because normal bacteria make few unsaturated tails there anyway.

How confident? This is a prediction from a model of lipid packing. To test it, you would grow both strains at 10 °C and at a warm temperature, with all other conditions the same, and compare growth rates. The data in the table show a pattern for one species only; they support the idea of adjustment, but they do not show that the change in lipids causes better survival. The mutant experiment would test cause.

Common misconceptions

• “Lipids are polymers made of fatty acid monomers.” No. A fat is one glycerol plus three fatty acids, not a long repeating chain.
• “Unsaturated means it contains no hydrogen.” It means it holds fewer hydrogen atoms than it could, because of C=C double bonds.
• “Unsaturated fats are liquid because they have weaker C–C bonds.” The covalent bonds are not the point. Kinks stop the tails packing closely, so attractions between tails are weaker.
• “All fats are bad; organisms try to avoid them.” Fats store energy, insulate and supply the fatty acids used to build membranes.
• “Phospholipid heads point inward to protect them.” The heads are hydrophilic and face the water on both sides. The hydrophobic tails are inside.
• “Cells use energy to build the bilayer shape.” The bilayer forms on its own in water because of how the hydrophilic and hydrophobic parts interact with water.
• “Steroids are not lipids because they have rings.” They are lipids: they are mostly carbon and hydrogen, and hydrophobic.
• “Plant fats are always unsaturated, animal fats always saturated.” That is a tendency, not a rule. Coconut oil is mostly saturated and turns solid on a cool day; fish oils are mostly unsaturated.

Where this leads

Next, Topic 1.6 covers the last group of macromolecules: nucleic acids. Phospholipids and cholesterol return in Unit 2, where you will build the fluid mosaic model of the membrane, and steroid hormones return in cell signalling. If you need to look back at energy-storing carbohydrates, see the Topic 1.4 study guide. Test yourself with the practice questions, then use the revision notes and the checklist to consolidate.