IB Biology HLCarbohydrates & LipidsPaper 1 & 2~11 min read
Macromolecules
Biology builds almost everything the same way: take a small unit, join thousands of them with one reaction, and take them apart again with the reverse. Learn that pair of reactions once and it covers carbohydrates, proteins and nucleic acids.
📚 What you need to know
Monomers are the smaller units from which larger molecules are made. Polymers are made from many monomers joined in a chain, by polymerisation.
Macromolecules are very large molecules containing 1000 or more atoms, so they have a high molecular mass.
Polymers can be macromolecules, but not all macromolecules are polymers — lipids are not polymers, because they are not made of repeating monomers.
Macromolecules form in condensation reactions: molecules combine, covalent bonds form, and water is removed.
Named bonds: glycosidic (monosaccharides), peptide (amino acids), phosphodiester (nucleotides), ester (glycerol and fatty acids).
Hydrolysis breaks covalent bonds by adding water. The –O and –OH from the water form the functional groups of the products.
Monomers, polymers and macromolecules
Carbon compounds can be large molecules made from many small, repeating subunits.
Monomers are the smaller units from which larger molecules are made.
Polymers are molecules made from a large number of monomers joined together in a chain.
The process by which monomers join to form polymers is polymerisation.
Macromolecules are simply very large molecules. They contain 1000 or more atoms and so have a high molecular mass.
The distinction examiners test. Polymers can be macromolecules, but not all macromolecules are polymers. A polymer must consist of many repeating subunits. Lipids are macromolecules but not polymers, because they are not built from repeating monomers.
Macromolecule
Monomer or subunits
Carbohydrates (polysaccharides)
Monosaccharides
Lipids
Fatty acids, glycerol, phosphate groups
Proteins (polypeptides)
Amino acids
Nucleic acids
Nucleotides
Notice the lipid row is the odd one out: it lists subunits, not one repeating monomer. A triglyceride is glycerol plus three fatty acids and then it stops — there is no chain to extend. That is exactly why lipids are not polymers.
Condensation and hydrolysis
Macromolecules are formed during condensation reactions. Molecules combine together, covalent bonds form, and the result is a polymer or macromolecule. Water is removed as part of the reaction.
The reverse reaction is hydrolysis — from hydro (water) and lyse (to break). Covalent bonds are broken when water is added, and the –O and –OH from the water molecule are used to form the functional groups of the products. This is what happens in digestion, when macromolecules need to be broken down into their monomers.
Count bonds and you can count waters. A triglyceride has three ester bonds, so making one releases exactly three water molecules.
The four named bonds
Only the names change. If you can spot the monomers in an unfamiliar diagram, you can name the bond without recognising the molecule.
Where each bond forms
Polysaccharides. Formed when two hydroxyl (–OH) groups on different monosaccharides interact to form a strong covalent bond called a glycosidic bond.
Polypeptides. Two amino acid monomers interact to form a strong covalent bond called a peptide bond.
Nucleic acids. Separate nucleotides are joined by condensation to form a phosphodiester bond. The reaction occurs between the phosphate group of one nucleotide and the pentose sugar of the next. It is called a phosphodiester bond because it consists of a phosphate group and two ester bonds.
Hydrolysis in digestion
Bond hydrolysed
In
Products
Glycosidic
Disaccharides and polysaccharides
Monosaccharides
Peptide
Polypeptides
Amino acids
Ester
Triglycerides
Three fatty acids and glycerol
Worked examples
WE 1
Polymer or not?
Explain why a triglyceride is described as a macromolecule but not as a polymer. (2 marks)
Point 1: why macromolecule
It is a very large molecule containing a large number of atoms, giving it a high molecular mass.
Point 2: why not a polymer
A polymer must be made of many repeating monomers joined in a chain. A triglyceride is glycerol plus three fatty acids, which are not repeating subunits.
Big, but not built from a repeating unitthis exact distinction is a favourite one-mark trap
WE 2
Counting water molecules
A polypeptide is made from 120 amino acids. Deduce how many water molecules were released during its formation, and explain your answer. (2 marks)
Step 1: count the bonds
120 amino acids joined in a chain need 120 − 1 = 119 peptide bonds.
Step 2: link bonds to water
Each peptide bond is formed by a condensation reaction, and one water molecule is released per bond.
119 water moleculesthe classic error is answering 120 — there is always one fewer bond than monomers in a chain
WE 3
Describing hydrolysis
Describe what happens during the hydrolysis of a disaccharide. (3 marks)
Point 1: what is added
A molecule of water is added to the disaccharide.
Point 2: what breaks
The covalent glycosidic bond between the two monosaccharides is broken.
Point 3: the products
The –O and –OH from the water form the functional groups of the two monosaccharides produced.
Water in, glycosidic bond out, two monosaccharidesnaming the bond is usually worth its own mark
💡 Exam tips
Define monomer, polymer and macromolecule as three separate things.
Always name the bond in a condensation or hydrolysis answer.
Say water is removed in condensation and water is added in hydrolysis — do not just say “the reverse”.
For a chain of n monomers there are n − 1 bonds and n − 1 water molecules.
Remember lipids as the exception: macromolecules, not polymers.
Phosphodiester: the “di” is because there are two ester bonds.
⚠ Common mistakes
Calling every macromolecule a polymer. Lipids are not polymers.
Saying water is used up in condensation. It is produced.
Getting the water count wrong. Bonds, not monomers, is what you count.
Mixing up glycosidic and peptide bonds. Sugars get glycosidic, amino acids get peptide.
Forgetting the bonds are covalent. That is why polymers are stable.
Up next: Carbohydrates: Function & Examples — monosaccharides, the two isomers of glucose, and why starch, glycogen and cellulose behave so differently.
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