IB Biology SLTopic 2 — Carbohydrates & LipidsPaper 1 & 2Structure and function~10 min read
Lipids
Pour oil into water and it sits there in a stubborn blob. That refusal to mix is not a small quirk — it is the reason lipids can be stored in bulk without wrecking a cell, and the reason every membrane you own is built from them. Get why lipids hate water and most of this page explains itself.
📘 What you need to know
Lipids include fats, oils, waxes and steroids. They contain carbon, hydrogen and oxygen — but far less oxygen than a carbohydrate.
They are built from long hydrocarbon regions with non-polar covalent bonds, so they are insoluble in water (hydrophobic).
A triglyceride is one glycerol joined to three fatty acids.
The bond is an ester bond, formed by condensation between a hydroxyl (—OH) and a carboxyl (—COOH) group. This process is esterification.
Three ester bonds form, so three water molecules are released per triglyceride.
Lipids store about twice as much energy per gram as carbohydrates, because of their many C—H bonds.
Respiring lipids also releases a lot of metabolic water — useful if drinking water is scarce.
Why lipids will not dissolve
A water molecule is polar: the oxygen end is slightly negative and the hydrogen ends slightly positive. Water molecules cling to each other, and they happily cling to anything else with charged or polar parts — which is why sugar, with all its hydroxyl groups, dissolves so easily.
A lipid has almost none of that. Its long hydrocarbon chains are made of C—H bonds, which share their electrons fairly evenly, so no part of the chain is charged. Water has nothing to grip. It sticks to itself instead and pushes the lipid out of the way, which is what you see as an oil droplet.
Useful get-around. Cells can make lipids travel through watery blood by attaching them to something polar. Add a carbohydrate and you get a glycolipid; add a protein and you get a lipoprotein. The polar outside faces the water while the lipid hides inside.
Building a triglyceride
Glycerol has three hydroxyl groups. A fatty acid has a carboxyl group at one end and a long hydrocarbon tail. Bring them together and each —OH reacts with a —COOH, water leaves, and an ester bond forms. Do that three times and you have a triglyceride.
Notice there is no repeating monomer here: one glycerol, three fatty acids. That is why a triglyceride is a macromolecule but not a polymer.
Learn the ester bond as “hydroxyl plus carboxyl”. You already know both groups from the carbon page, and this is the third time the same condensation idea has appeared. Nothing new is being asked of you.
Why fat is such a good energy store
Hydrolyse a triglyceride and you get glycerol and fatty acids, both of which feed into respiration. What makes them so valuable is how many C—H bonds they contain. Carbohydrates already carry a lot of oxygen, so they are partly oxidised before you start. Lipids are not, so there is much more energy left to release.
This is why long-term stores are fat and short-term stores are glycogen. If you carried the same energy as glycogen you would be carrying twice the mass, plus the water it holds.
The full list of reasons
Energy dense — about twice the energy per gram compared with carbohydrate.
Insoluble — so it does not affect the water potential of the cell and does not drift out of storage cells.
Releases metabolic water — respiring lipids produces a lot of water, which can be used when drinking water is not available.
Also insulates — a fat layer traps heat, which matters for animals in cold habitats.
🤔 Camels and eggs: metabolic water in real life
A camel’s hump is not a water tank. It is a store of lipid, and when that lipid is respired the reaction produces water as a product. In a desert, water you make yourself is worth having. The same trick keeps a developing chick alive: the yolk of a bird’s egg is rich in lipid, and respiring it supplies both energy and water to the embryo inside a sealed shell.
Where lipids are stored
In animals, in adipose tissue made of adipose cells that hold large fat globules. These cells shrink when the fat is respired.
Subcutaneous fat sits under the skin; visceral fat sits around the internal organs.
Seals and walruses are endotherms with a thick adipose layer called blubber, which traps the heat their respiration generates.
Many plants store lipid in seeds to fuel the growing seedling — olives, sunflowers, nuts, coconuts and oilseed rape are all harvested for their oils.
LIPIDS: THE LONG-TERM STORE
About 37 kJ per gram
Insoluble, stored dry
Slower to mobilise
Also insulates and protects organs
GLYCOGEN: THE SHORT-TERM STORE
About 17 kJ per gram
Insoluble, but stored with water
Highly branched, so hydrolysed fast
Used first when energy is needed quickly
Worked examples
WORKED EXAMPLE
Calculate the number of water molecules released when four triglycerides are formed. [2]
Step 1: bonds in one triglyceride
3 fatty acids join to 1 glycerol, so 3 ester bonds form.
Step 2: one water per bond, then scale up3 × 4 = 1212 water moleculesThe n − 1 chain rule does not apply here. A triglyceride is not a chain.
WORKED EXAMPLE
Using 37 kJ g⁻¹ for lipid and 17 kJ g⁻¹ for carbohydrate, calculate how much more energy is released by respiring 20 g of lipid than 20 g of carbohydrate. [2]
Step 1: energy from eachlipid: 37 × 20 = 740 kJcarbohydrate: 17 × 20 = 340 kJStep 2: find the difference740 − 340 = 400 kJ400 kJ more from the lipidRead the question — “how much more” wants a difference, not both totals.
WORKED EXAMPLE
Explain why lipids are better than carbohydrates for long-term energy storage in animals. [3]
Point 1
Lipids contain more C—H bonds, so they release about twice as much energy per gram.
Point 2
So less mass is carried for the same energy, which matters for an animal that moves.
Point 3
Lipids are insoluble, so they do not affect water potential, and they also insulate the body.
More energy, less mass, no osmotic effectThe mass point is the one students leave out. It is often the third mark.
💡 Exam tip
Say ester bond, not “glycosidic” or “peptide”. Each family has its own bond name and examiners check.
Three ester bonds means three waters. Never two.
“Energy dense” needs a reason: more C—H bonds per gram.
When you say lipids are insoluble, add the consequence: no effect on water potential.
Metabolic water is water made during respiration, not water stored in the fat.
Lipids contain C, H and O — but never nitrogen. That is one way to tell a lipid from a protein in a data question.
⚠ Common mix-up
Calling a triglyceride a polymer. Big, yes. Repeating monomers, no.
Saying a camel’s hump stores water. It stores lipid; the water comes from respiring it.
Writing “lipids are insoluble because they are big”. Size is not the reason — being non-polar is.
Mixing up glycerol and glucose. Glycerol has three carbons and three hydroxyl groups; glucose has six carbons.
Forgetting that fatty acids and glycerol are different building blocks. A triglyceride is not three of the same thing.
Saying lipids give “more energy”. Be precise: more energy per gram.
Up next: Fatty Acids — saturated, monounsaturated and polyunsaturated, and why one double bond is enough to turn a solid fat into a liquid oil.
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