Your body does not build a starch molecule the way you would build a wall, brick by brick with cement. It builds it by squeezing a water molecule out from between two sugars. That one reaction — and its reverse — explains how every giant molecule in you is made and broken. Learn it once and it works for sugars, proteins, DNA and fats.
📘 What you need to know
A monomer is a small unit. A polymer is many monomers joined in a chain. The joining is called polymerisation.
A macromolecule is simply a very large molecule — 1000 or more atoms, so a high molecular mass.
Polymers can be macromolecules, but not all macromolecules are polymers. Lipids are the classic exception.
Condensation joins monomers: a covalent bond forms and a water molecule is removed.
Hydrolysis is the reverse: water is added and the covalent bond is broken. This is what happens in digestion.
Four bonds to name: glycosidic (sugars), peptide (amino acids), phosphodiester (nucleotides), ester (fats).
Joining n monomers into one chain releases n − 1 water molecules.
Monomer, polymer, macromolecule
These three words get muddled constantly, so pin them down now.
Monomer — the repeating unit. Glucose, an amino acid, a nucleotide.
Polymer — a chain built from many of those repeating units. Starch, a polypeptide, DNA.
Macromolecule — any molecule that is simply huge. It does not have to repeat.
🤔 Why lipids break the pattern
A triglyceride is big, so it counts as a macromolecule. But look at what it is made from: one glycerol and three fatty acids. Those are three different things stuck onto a fourth, not the same unit repeating over and over. No repeating unit means no polymer. That is the whole reason lipids sit outside the polymer club, and it is a favourite one-mark question.
Macromolecule
Monomer (or building blocks)
Polymer?
Carbohydrates (polysaccharides)
Monosaccharides
Yes
Proteins (polypeptides)
Amino acids
Yes
Nucleic acids (DNA, RNA)
Nucleotides
Yes
Lipids (e.g. triglycerides)
Glycerol, fatty acids, phosphate groups
No — no repeating unit
Condensation: build by removing water
Two monomers come together. One offers an —OH group; the other offers an —H, or another —OH. Those spare atoms leave together as H2O, and the two monomers are left bonded straight to each other by a strong covalent bond.
The name makes sense once you see it. Condensation — water condenses out. You are not adding glue, you are removing a bit of both molecules so they have no choice but to join.
The reaction you must be able to write
monomer + monomer → dimer + H2O
The two arrows are the same reaction read forwards and backwards. Your cells build with the green arrow and digest with the blue one.
Hydrolysis: break by adding water
Split the word: hydro means water, lysis means splitting. Water is added across the bond, the bond snaps, and the —H and —OH from that water molecule end up on the two pieces, rebuilding their original functional groups.
This is exactly what happens in your small intestine. A slice of bread is mostly starch, which is far too big to be absorbed. Enzymes hydrolyse the glycosidic bonds one by one until you are left with single glucose molecules small enough to cross into your blood.
Glycosidic bonds in starch or maltose are hydrolysed to give monosaccharides.
Peptide bonds in a polypeptide are hydrolysed to give amino acids.
Ester bonds in a triglyceride are hydrolysed to give three fatty acids and glycerol.
If you can only remember one thing from this page: condensation loses water, hydrolysis gains water. Examiners ask you to name the reaction far more often than they ask you to draw it.
Four bonds, four families
The bond gets a different name depending on what is being joined, but the reaction is condensation every time.
A phosphodiester bond gets its name honestly: one phosphate group holding two ester bonds, linking the sugar of one nucleotide to the phosphate of the next.
🧠 A way to remember it
Think of a wedding photo. To join two people you take one thing away from each — their surnames sit side by side and something drops out. To split them up again, you bring water in. Silly, but nobody who has pictured it once forgets which way round condensation and hydrolysis go.
Counting the water molecules
This is the calculation that shows up in Paper 1. Every bond formed removes exactly one water. So a chain of n monomers has n − 1 bonds and released n − 1 waters.
Worth memorising
waters released = number of bonds formed = n − 1
The triglyceride catch. A triglyceride is not a chain, so this rule does not apply to it. Three fatty acids attach to one glycerol, making three ester bonds and releasing three waters — not two.
Worked examples
WORKED EXAMPLE
A polypeptide is made from 120 amino acids. How many water molecules were released during its formation? [2]
Step 1: count the bonds
A chain of 120 units has one fewer bond than units.
120 − 1 = 119 peptide bondsStep 2: one water per bond
Each peptide bond formed by condensation released 1 water.
119 water moleculesThe classic error is writing 120. Count the gaps, not the beads.
WORKED EXAMPLE
Glycine has a relative molecular mass of 75 and alanine has a relative molecular mass of 89. Calculate the relative molecular mass of the dipeptide they form. [2]
Step 1: add the two monomers75 + 89 = 164Step 2: subtract the water that left
One condensation reaction, so one H₂O of mass 18 is removed.
164 − 18 = 146Relative molecular mass = 146Forget the −18 and you lose the second mark every time.
WORKED EXAMPLE
Explain why lipids are classed as macromolecules but not as polymers. [2]
Point 1
They are very large molecules, so they count as macromolecules.
Point 2
A polymer must be built from many identical repeating monomers.
Point 3
A triglyceride is one glycerol plus three fatty acids — no repeating unit, so it is not a polymer.
Big, yes. Repeating, no.The word “repeating” is doing all the work here. Use it.
💡 Exam tip
If a question shows water on the product side, it is condensation. If water is on the reactant side, it is hydrolysis.
Name the bond precisely. “A covalent bond” scores less than “a glycosidic bond” when the question names the monomers.
For n monomers in a chain, waters = n − 1. For a triglyceride, waters = 3.
Digestion questions are hydrolysis questions in disguise. Say “water is added and the bond is broken”.
Mass of a polymer = sum of monomer masses − (18 × number of bonds).
Say “monomers joined together“, not “monomers mixed” or “stuck”. Covalent bonds are formed.
⚠ Common mix-up
Condensation and hydrolysis get swapped constantly. Hydro = water going in. Condensation = water coming out.
Polymer is not a synonym for macromolecule. Every polymer here is a macromolecule, but a triglyceride is a macromolecule that is not a polymer.
Counting monomers instead of bonds. 10 monomers make 9 bonds, so 9 waters.
Thinking water is added from outside during condensation. The water is made from the monomers, an —H from one and an —OH from the other.
Saying “starch is a monomer”. Starch is the polymer; glucose is the monomer.
Forgetting that lipids still use condensation. They are not polymers, but ester bonds still form by condensation and still release water.
Up next: Carbohydrates: Function & Examples — where you meet glucose properly, and find out why starch, glycogen and cellulose are made of nearly the same thing but do completely different jobs.
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