Introduction to Macromolecules: Study Guide (Biology 1.3)

Big molecules from small parts

A cell needs some very large molecules. Starch stores energy, proteins speed up reactions and DNA stores information. Some of these molecules contain thousands of atoms. Cells do not build each one from scratch. Instead, they join small, similar units together, like beads on a string.

• A monomer is a small molecule that can join to others like it.
• A polymer is a large molecule made of many monomers joined by covalent bonds.
• Polymerization is the joining of many monomers, one at a time, into a polymer.
• Macromolecule means “large molecule”. The four main groups in living things are carbohydrates, proteins, nucleic acids and lipids.

In Topic 1.2 you saw which elements these molecules contain. This topic is about the two reactions that build them up and break them down. The details of each group come in Topics 1.4 to 1.7.

Background (you meet the details later):

Group	Monomer	Example polymer
Carbohydrates	monosaccharides (simple sugars, such as glucose)	starch, glycogen, cellulose
Proteins	amino acids	polypeptides
Nucleic acids	nucleotides	DNA and RNA
Lipids	not true polymers	a fat is built from glycerol and three fatty acids

Lipids are the odd one out. A fat is not a long chain of repeating units. But the same reaction joins its parts, so everything below still applies to it.

Dehydration synthesis: joining monomers

To join two monomers, a cell makes a new covalent bond between them. The reaction works like this:

1. One monomer loses a hydrogen atom (H).
2. The other monomer loses a hydroxyl group (–OH).
3. The H and the –OH combine to form a water molecule (H₂O), which is released.
4. The two monomers are now joined by a covalent bond where the H and –OH used to be.

Some texts, including the course description, call the H that is removed (or added in hydrolysis) a hydrogen ion. For this topic, treat “H” and “hydrogen ion” as the same thing: it is the part that combines with –OH to make water.

“Dehydration” means losing water. “Synthesis” means making something. So dehydration synthesis means “making a bigger molecule by removing water”. Many books call it a condensation reaction.

When a cell builds a polymer, it repeats this reaction again and again. Each new monomer is added with one new covalent bond, and each new bond releases one water molecule.

Hydrolysis: breaking polymers apart

Hydrolysis is the reverse reaction. “Hydro” means water and “lysis” means splitting. Water is used to cleave (split) the covalent bond between two monomers:

1. A water molecule reacts at the bond between two monomers.
2. The bond breaks.
3. A hydrogen (H) from the water joins one monomer.
4. The hydroxyl group (–OH) from the water joins the other monomer.

Each monomer gets back the atom or group it lost when the bond was made. So hydrolysis turns a polymer into smaller molecules: shorter chains, then single monomers.

You use hydrolysis every time you digest food. Enzymes in your gut hydrolyse starch, proteins and fats into small molecules that can be absorbed into the blood. Your cells then use dehydration synthesis to build their own polymers from those parts.

Comparing the two reactions

	Dehydration synthesis	Hydrolysis
What happens to the polymer	built up (gets larger)	broken down (gets smaller)
Covalent bond between monomers	formed	broken
Water	released (one per bond formed)	used up (one per bond broken)
Where H and –OH go	removed from the two monomers, leave as H₂O	added from H₂O: H to one monomer, –OH to the other
Everyday example	a liver cell joining glucose into glycogen	digesting starch into sugars

Background: energy and enzymes. Building a polymer needs an input of energy, and breaking one by hydrolysis releases energy. In cells, both reactions are speeded up by enzymes, and the energy for building usually comes from other reactions. You study this in Unit 3. For this topic, focus on bonds, water and direction.

Counting monomers, bonds and water

Every bond between two monomers is made by one dehydration synthesis and broken by one hydrolysis. That gives a simple counting rule.

For a chain of n monomers:

• number of covalent bonds between monomers = n − 1
• water molecules released when it is built = n − 1
• water molecules used to hydrolyse it completely into monomers = n − 1

The rule also works for a branched chain, such as glycogen, as long as the chain does not close into a ring. Every monomer after the first joins with exactly one new bond, wherever it attaches.

Because water leaves when each bond forms, a polymer has less mass than the monomers it was made from. Its mass is:

mass of polymer = (n × mass of one monomer) − ((n − 1) × mass of water)

Reading diagrams of these reactions

Many exam questions show a diagram and ask you to describe it. Use this routine:

1. Find the arrow. Is it pointing towards the bigger molecule (building) or towards the smaller ones (breaking)?
2. Find the water. Water on the product side means dehydration synthesis. Water on the reactant side means hydrolysis.
3. Count. Count the monomers and the bonds. Check that water molecules equal bonds formed or broken.
4. Track the atoms. In hydrolysis, look for the H joining one monomer and the –OH joining the other. In synthesis, look for which monomer lost H and which lost –OH.
5. Name what is shown, and what is not. Simple diagrams leave out enzymes and energy. Say so if the question asks what the model misses.

Worked example 1: water and mass in a glucose chain

Question. A short, straight chain is built from 8 glucose monomers. Use glucose = 180.2 g mol⁻¹ and water = 18.0 g mol⁻¹.

(a) How many covalent bonds join the monomers, and how many water molecules are released while it is built? (b) Calculate the molar mass of the chain. (c) The cell adds the same 8 glucose units as a branched chain instead. Does your answer to (a) change? (d) Another glucose chain has a molar mass of 2613.2 g mol⁻¹. How many glucose units does it contain?

(a) n = 8, so bonds = n − 1 = 7. One water molecule is released per bond, so 7 water molecules are released.

(b)

1. Mass of 8 separate glucose molecules: 8 × 180.2 = 1441.6 g mol⁻¹.
2. Mass of the water lost: 7 × 18.0 = 126.0 g mol⁻¹.
3. Molar mass of the chain: 1441.6 − 126.0 = 1315.6 g mol⁻¹.

(c) No. Each glucose after the first still joins with one new bond. A branch changes where a monomer attaches, not how many bonds form. Still 7 bonds and 7 water molecules.

(d) Rearrange the mass rule. Each glucose inside the chain contributes 180.2 − 18.0 = 162.2, and one extra water’s worth of mass (18.0) remains at the chain’s two ends:

1. mass = 162.2n + 18.0
2. 162.2n = 2613.2 − 18.0 = 2595.2
3. n = 2595.2 ÷ 162.2 = 16 glucose units.

Check. 16 × 180.2 − 15 × 18.0 = 2883.2 − 270.0 = 2613.2 g mol⁻¹. ✓

Interpretation. Every bond in the chain “cost” one water molecule to make. If the chain is digested, those 15 water molecules are put back, and the products weigh more than the polymer did.

Worked example 2: following the atoms with a labelled diagram

Question. A model dimer X–Y is made of two different monomers. A diagram of its synthesis shows that X loses its –OH group and Y loses an H atom.

(a) The –OH group on monomer X contains the heavy oxygen isotope ¹⁸O, which can be detected. Monomers X and Y are joined by dehydration synthesis. Where is the ¹⁸O after the reaction? (b) A sample of unlabelled X–Y is now hydrolysed in water in which every oxygen atom is ¹⁸O. Where does the ¹⁸O end up? (c) Describe the diagram you would draw for (b).

(a)

1. Read the diagram: X gives up its whole –OH group, including its oxygen.
2. That –OH combines with the H from Y to form water.
3. So the ¹⁸O is in the released water molecule, not in the dimer.

(b)

1. In hydrolysis, water splits into H and –OH.
2. The H joins one monomer; the –OH, which contains the oxygen, joins the other.
3. So the ¹⁸O ends up in only one of the two products, in its new –OH group. It is not shared between both, and it does not stay in the water.

(c) Arrow pointing from X–Y + H₂O towards X + Y. Water shown on the reactant side. One bond broken, one water used. An H drawn joining one monomer and an –OH joining the other.

Why this matters. Tracking a labelled atom is a way to test a model with evidence. If ¹⁸O had appeared in both products, the model “H to one monomer, –OH to the other” would be wrong.

Common misconceptions

• “Hydrolysis means dissolving in water.” No. Dissolving separates molecules or ions from each other. Hydrolysis breaks covalent bonds inside a molecule and uses water as a reactant.
• “Dehydration synthesis needs dry conditions.” No. It happens in the watery cytoplasm. “Dehydration” refers to the water molecule removed from the two monomers.
• “Both monomers lose a whole water molecule.” Only one water molecule forms per bond: H from one monomer, –OH from the other.
• “The water in hydrolysis goes to one product.” The water is split: H to one monomer, –OH to the other.
• “A chain of n monomers has n bonds.” A straight or branched chain has n − 1. Draw it and count.
• “The polymer weighs the same as its monomers added up.” It weighs less, by the mass of the water released.
• “Hydrogen bonds join monomers in a polymer.” The links between monomers are covalent bonds. Hydrogen bonds (Topic 1.1) are weaker attractions, for example between DNA strands.
• “All four groups of macromolecules are polymers.” Lipids are not built from long chains of repeating units, though their parts are joined by dehydration synthesis.

Where this leads

Next, apply these two reactions to each group in turn, starting with Topic 1.4, Carbohydrates, then lipids, nucleic acids and proteins. If you need to revisit the elements these molecules contain, see Topic 1.2, Elements of Life. Test yourself with the practice questions, then use the revision notes and the checklist to consolidate.