IB Biology HLCarbohydrates & LipidsPaper 1 & 2~12 min read
Lipids
A camel’s hump is not full of water. It is full of fat — and the water comes out later, when the fat is respired. That one fact tells you almost everything about why organisms store energy as lipid rather than as sugar.
📚 What you need to know
Lipids include fats, oils, waxes and steroids, and contain carbon, hydrogen and oxygen.
Lipids contain hydrocarbon chains with many non-polar covalent bonds, so they are insoluble in water and other polar solvents.
A triglyceride is three fatty acids joined to one glycerol molecule.
They form by esterification: an ester bond forms between the hydroxyl (–OH) group of glycerol and the carboxyl (–COOH) group of a fatty acid. This is a condensation reaction, so three water molecules are released per triglyceride.
Lipids are energy dense because of their high number of C–H bonds — about twice the energy per gram of carbohydrate.
Respiring lipids produces a lot of metabolic water, useful when drinking water is unavailable.
In animals lipids are stored in adipose tissue, which also acts as a thermal insulator; in plants, many seeds store fats.
What lipids are, and why they will not dissolve
Examples of lipids in living organisms are fats, oils, waxes and steroids. Lipid macromolecules contain carbon, hydrogen and oxygen atoms, and are composed of a glycerol molecule with fatty acid hydrocarbon chains attached.
Their structure controls their solubility. Lipids contain hydrocarbon chains, which contain many non-polar covalent bonds. Because water is polar and lipids are not, lipids are insoluble in water or other polar solvents.
The workaround. Living organisms improve lipid solubility by combining lipid molecules with other molecules, for example glycolipids and lipoproteins. That is how fats are moved through the blood despite being insoluble.
Making a triglyceride
Some lipids are categorised as triglycerides: three fatty acids joined to one glycerol molecule. Fatty acids contain hydrocarbon chains that can be either saturated (only single carbon–carbon bonds) or unsaturated (one or more double bonds) — covered in detail on the next page.
Triglycerides form by a process called esterification. An ester bond forms when the hydroxyl (–OH) group of the glycerol bonds with the carboxyl (–COOH) group of a fatty acid. Forming an ester bond is a condensation reaction, so a water molecule is released for each bond — and since a triglyceride has three ester bonds, three water molecules are released.
The orange blocks are the ester bonds. Add water back to them and the whole thing comes apart into glycerol and three fatty acids again.
Lipids as an energy store
The hydrolysis of triglycerides releases glycerol and fatty acids, which are useful respiratory substrates. Several features make lipids ideal for long-term energy storage.
Twice the energy for the same mass is the headline, but insolubility is just as important — a sugar store that large would wreck the cell’s water balance.
Energy dense. Lipids have a high number of C–H bonds compared with carbohydrates, and contain about twice as much energy per gram.
Insoluble. They are not transported around the body easily and remain in their storage cells.
Metabolic water. When lipids are respired, a lot of water is produced compared with the respiration of carbohydrates. This metabolic water can be used as a dietary water source when drinking water is unavailable.
Example
What it shows
A camel’s hump
Not filled with water, but a lipid-rich storage organ that yields metabolic water in a dry desert habitat
A bird’s egg
Lipid-rich yolk provides both energy and metabolic water to the growing chick
Where lipids are stored
In animals
Lipids are stored in adipose tissue. Subcutaneous fats are stored below the skin; visceral fats are stored around the major internal organs.
Fat is stored in adipose cells, which are specialised to contain large globules of fat. Adipose cells shrink when the fat is respired to generate energy.
Adipose tissue can also act as a thermal insulator in animals living in cold environments. Seals and walruses are endotherms with thick adipose tissue called blubber, which helps trap the heat generated by respiration.
In plants
In many plants, seeds have evolved to store fats to provide energy for the growing seedling. Olives, sunflowers, nuts, coconuts and oilseed rape are examples of crops whose oils are harvested for edible oil production.
Notice the pattern: lipids are used where mass matters. A seed has to be carried by the wind or an animal; a migrating bird has to fly with its fuel on board. Anywhere weight is a problem, evolution reaches for fat rather than carbohydrate.
Worked examples
WE 1
Describing triglyceride formation
Describe how a triglyceride is formed. (3 marks)
Point 1: the components
Three fatty acids join to one glycerol molecule.
Point 2: the bond
An ester bond forms between the hydroxyl group of the glycerol and the carboxyl group of each fatty acid. The process is called esterification.
Point 3: the reaction type
Each bond forms by condensation, so three water molecules are released in total.
3 fatty acids + 1 glycerol → 3 ester bonds + 3H2Onaming both functional groups involved is usually worth a mark on its own
WE 2
Lipid or carbohydrate for storage?
Explain why lipids are better suited than carbohydrates to long-term energy storage. (4 marks)
Point 1: energy density
Lipids have a high number of C–H bonds, so they release about twice as much energy per gram as carbohydrates.
Point 2: mass
Less mass is needed to store the same energy, which matters for animals that must move.
Point 3: insolubility
Lipids are insoluble, so they do not affect the water potential of the cell and stay in their storage cells.
Point 4: bonus product
Respiring lipids also releases a large amount of metabolic water.
Twice the energy, no osmotic effect, and water as a by-product“long-term” is the clue that they want more than just the energy figure
WE 3
The camel’s hump
A student says a camel’s hump is full of water. Correct this statement. (2 marks)
Point 1: what it actually contains
The hump is a lipid-rich storage organ, not a water store.
Point 2: where the water comes from
Respiring those lipids produces a large amount of metabolic water, which the camel can use in its dry desert habitat.
Fat first, water second — as a product of respirationthe same argument applies to the lipid-rich yolk of a bird’s egg
💡 Exam tips
Learn the four lipid examples: fats, oils, waxes, steroids.
Explain insolubility through non-polar hydrocarbon chains, not just “lipids do not dissolve”.
Name esterification and the ester bond, and say which functional groups react.
Always give three water molecules per triglyceride.
Attribute the energy density to C–H bonds.
Have one animal example (blubber, camel) and one plant example (seeds, oilseed rape) ready.
⚠ Common mistakes
Calling a triglyceride a polymer. It is a macromolecule, but not a polymer.
Saying lipids contain nitrogen. They contain carbon, hydrogen and oxygen only.
Releasing one water molecule instead of three. One per ester bond, and there are three.
Saying the camel’s hump stores water. It stores lipid; the water is metabolic.
Confusing the glycerol and fatty acid functional groups. Glycerol supplies –OH, the fatty acid supplies –COOH.
Up next: Fatty Acids — saturated, monounsaturated and polyunsaturated, and why one double bond decides whether a fat is butter or olive oil.
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