Monomers and polymers
Monomers and polymers are examined on Paper 1 and Paper 3, and the two reactions in this section, condensation and hydrolysis, turn up inside Paper 2 questions on respiration, photosynthesis, digestion and gene technology.
Learn them precisely here and you will use them in almost every unit of the course.
Start with the problem a cell faces.
It needs very large molecules: long chains to store glucose without upsetting its water balance, fibres strong enough to hold a plant upright, chains that fold into enzymes, and molecules that carry genetic information.
Building each of these from scratch, with its own chemistry, would need thousands of different reactions.
Cells solve this with a building-kit approach: a small number of small units are joined, one at a time, by the same kind of reaction, and taken apart by its reverse.
- Every question apart from the Paper 3 essay is point-marked: you earn one mark for each correct point you make, up to the marks shown.
- "3 max" (or any "N max") means there are more correct points than marks; any three of them earn full marks.
- Model answers carry Accept, Reject, Ignore and Do not accept notes. A rejected word costs the mark it stands for; an ignored word neither gains nor loses.
- The list rule: give more answers than the question asks for and a wrong one cancels a right one. Give exactly the number asked.
- A bold and in a model-answer point needs both halves for the mark. A Yes or No verdict earns nothing; the reasons earn the marks.
- A correct calculation answer scores full marks even with no working shown; working earns part marks when the final answer is wrong. The essay is the only item marked by levels.
One chemistry for all life
You need to know that the variety of life, both past and present, is extensive, but the biochemical basis of life is similar for all living things.
A bacterium, an oak tree, a fungus and a human look nothing alike, yet their cells are built from the same few groups of carbon-based molecules: carbohydrates, lipids, proteins and nucleic acids.
They use the same twenty amino acids, the same four bases in DNA, the same energy carrier (ATP) and very similar enzymes for respiration.
Why does this matter?
A shared chemistry is what you would expect if all organisms descended from a common ancestor, so it is indirect evidence for evolution.
It is also practical: a gene from one organism can be read by the cells of another, which is why a human gene put into a bacterium can make a human protein.
You will meet that idea again in gene technology.
Why it matters. Because all organisms share the same monomers and the same reactions to join them, a nutrient from any food source can be hydrolysed to monomers that the consumer's cells can rebuild into its own polymers; this is what lets energy and matter pass along food chains.
Monomers and polymers
A monomer is a small unit; a polymer is a long chain of many of these units joined together.
The three examples you must know are monosaccharides (the monomers of polysaccharides), amino acids (the monomers of polypeptides and proteins) and nucleotides (the monomers of DNA and RNA).
Monomer
Polymer made from it
Bond formed between monomers
Where you meet it
monosaccharide (for example α-glucose, β-glucose)
polysaccharide (starch, glycogen, cellulose)
glycosidic bond
this unit
amino acid
polypeptide, protein
peptide bond
3-1b
nucleotide
polynucleotide (DNA, RNA)
phosphodiester bond
3-1b
"A large number" matters in the definition of a polymer.
Two monosaccharides joined make a disaccharide, two amino acids a dipeptide: these are not polymers.
A triglyceride is made from four molecules (glycerol and three fatty acids), but glycerol and fatty acids are not repeating monomers and there are only four of them, so a triglyceride is not a polymer either.
Condensation and hydrolysis
Figure 1 shows the two reactions as a pair.
Figure 1
In a condensation reaction, two molecules are joined by a new chemical bond and a molecule of water is removed.
Typically an OH group on one molecule and an H atom on the other leave together as H₂O, and the two atoms left behind bond to each other.
Each new bond in a growing chain releases one more water molecule.
In a hydrolysis reaction the reverse happens: a molecule of water is used to break the bond.
The water splits; its H goes to one side of the broken bond and its OH to the other, so each monomer gets back the groups it lost.
"Hydro" means water and "lysis" means splitting.
The two reactions are the cell's way of building up and taking apart.
Condensation builds polymers (anabolism, which needs energy, usually from ATP); hydrolysis releases monomers (in digestion, and when stores such as glycogen or starch are used).
Both are catalysed by enzymes, and the enzymes are specific to the bond: an enzyme that hydrolyses a glycosidic bond cannot hydrolyse a peptide bond.
Why it matters. Because the same reaction joins any two monomers, a cell can build a polymer of any length with one type of enzyme, and because hydrolysis is the exact reverse, the stored monomers can be released when they are needed, for example glucose from glycogen in liver cells between meals.
Which molecules are made by condensation without being polymers?
Recent papers have asked which molecules are formed by condensation but are not polymers.
Condensation is about the reaction; polymer is about the size and repeating structure of the product.
Every polymer is made by condensation, but many condensation products are not polymers.
Molecule
Made by condensation?
A polymer?
Why
maltose, sucrose, lactose
yes
no
only two monosaccharides
a dipeptide
yes
no
only two amino acids
a triglyceride
yes
no
glycerol plus three fatty acids; no repeating monomer
a phospholipid
yes
no
glycerol, two fatty acids and a phosphate-containing group
ATP
yes
no
made from ADP and inorganic phosphate (3-1b)
starch, glycogen, cellulose
yes
yes
many glucose monomers
a polypeptide, DNA, RNA
yes
yes
many amino acids or nucleotides
Not every reaction is condensation or hydrolysis
It is tempting, once you know these two words, to call every reaction in biology one or the other.
That is an error, and it costs marks.
Hydrolysis breaks a covalent bond between two parts of a molecule using water: glycosidic, peptide, ester and phosphodiester bonds, and the bond between phosphate groups in ATP.
Hydrogen bonds are not hydrolysed, and water itself is not hydrolysed (in photosynthesis water is split by light, which is photolysis).
Some reactions you will learn later are neither type: forming a disulfide bridge, adding a phosphate to glucose at the start of glycolysis, reducing GP to triose phosphate, regenerating RuBP, making cyclic AMP from ATP, and adding methyl groups to DNA or acetyl groups to histones.
When you name a reaction type, name the bond made or broken, and the molecules on each side.
Used again in: 3-1b (peptide bonds, phosphodiester bonds and ATP), 3-3 (digestion is the hydrolysis of polymers to monomers), 3-5 (ATP in photosynthesis and respiration) and 3-6b (glycogen in the control of blood glucose).
- Monomers: Monomers are the smaller units from which larger molecules are made.
- Polymers: Polymers are molecules made from a large number of monomers joined together.
- Examples of monomers: Monosaccharides, amino acids and nucleotides are examples of monomers.
- Condensation reaction: A condensation reaction joins two molecules together with the formation of a chemical bond and involves the elimination of a molecule of water.
- Hydrolysis reaction: A hydrolysis reaction breaks a chemical bond between two molecules and involves the use of a water molecule.
Describe how condensation and hydrolysis reactions are involved in forming and breaking down polysaccharides. [4]
One fact per sentence, in the order the reactions happen.
1. Monosaccharides (glucose) are joined by condensation reactions [1]
2. in which a molecule of water is removed and a glycosidic bond forms between the monomers [1].
3. In hydrolysis, a molecule of water is used [1]
4. to break the glycosidic bond, releasing the monosaccharides [1].
Weaker answer: "Glucose joins together and water is lost.
To break it down you add water."
This scores 1 at most.
It never names the glycosidic bond, "lost" is acceptable but "break it down" is not the hydrolysis of a named bond, and it does not say what is released.
Write "hydrolysis of the glycosidic bond", never "broken down".
A student says, "A triglyceride is a polymer because it is made by condensation reactions."
Explain why the student is wrong. [2]
1. A polymer is made from a large number of (repeating) monomers [1]
2. a triglyceride is made from only one glycerol and three fatty acids, which are not repeating monomers [1].
Weaker answer: "It is a lipid, not a carbohydrate."
This scores 0: lipids are not excluded because of their group, but because of the number and kind of units.
Proteins are polymers and are not carbohydrates either.
Four α-glucose molecules are joined in an unbranched chain.
Calculate the number of water molecules released. [1]
Glycosidic bonds = 4 − 1 = 3, and each condensation releases one water molecule
Answer 3 [1]
4 scores 0: the two end glucose molecules each have one free end.
A glycogen molecule contains 1000 glucose units and 50 branch points.
At each branch point one 1,6-glycosidic bond joins a side chain to the main chain; all other bonds are 1,4-glycosidic bonds.
Calculate the number of 1,4-glycosidic bonds. [2]
Every glucose unit except the first is joined to the molecule by exactly one bond, so total glycosidic bonds = 1000 − 1 = 999
1,6-glycosidic bonds = 50 (one per branch point)
1,4-glycosidic bonds = 999 − 50 = 949
Answer 949 [2]
950 (1000 − 50) earns 1 mark: the branch bonds were subtracted but the "one fewer bond than units" step was missed.
Branching does not change the total number of bonds, only which carbon atoms they join.
A short amylose chain of six α-glucose units is completely hydrolysed to glucose.
The relative molecular mass of glucose is 180 and of water is 18.
5.1 Calculate the relative molecular mass of the chain. [1]
Six glucose = 6 × 180 = 1080
Water removed in making the chain = (6 − 1) × 18 = 90
Relative molecular mass = 1080 − 90 = 990
Answer 990 [1]
5.2 Calculate the percentage increase in mass when the chain is completely hydrolysed.
Give your answer to 2 significant figures. [2]
Increase in mass = water used = 5 × 18 = 90
Percentage change = change ÷ start value × 100.
You need to know this formula; no formula sheet is given.
Percentage increase = 90 ÷ 990 × 100 = 9.09...%
Answer 9.1% [2]
8.3% (90 ÷ 1080 × 100, dividing by the end value) earns 1 mark at most; percentage change is always found from the start value.
9.09% earns 1 mark: right working, but not to 2 significant figures as asked.
When a question asks you to name a reaction, write "condensation" or "hydrolysis" and then name the bond ("hydrolysis of glycosidic bonds").
"Broken down", "digested" and "dehydration" do not earn the mark, and "hydrolysis" alone may lose a second mark that needs the bond.
- "Breaks down" for hydrolyses. It is the everyday GCSE phrase and feels like enough, but to score you need hydrolysis of a named bond, with water used.
- Calling any condensation product a polymer. Disaccharides, triglycerides, phospholipids and ATP are made by condensation but are not polymers; a polymer needs a large number of monomers.
- "Water is hydrolysed" or "hydrogen bonds are hydrolysed". Hydrolysis breaks covalent bonds using water. Hydrogen bonds break with heat or are separated by enzymes such as DNA helicase; water is split by photolysis. Both errors were counted as significant in essays.
- Saying water is "added" in condensation. Water is removed (eliminated) in condensation and used in hydrolysis; students swap them because "condense" sounds like water appearing.
- Counting one water molecule per monomer. A chain of n monomers has n − 1 bonds, so n − 1 water molecules.
Topic area: 3.1.1 Monomers and polymers.
Themes it serves: condensation and hydrolysis (a pair of opposite reaction types); molecules and their uses.
Many examples of condensation from one subsection count as one topic area, so pair this with other subsections where the same reactions act (3.1.2 carbohydrates, 3.1.4.1 proteins, 3.3.3 digestion).
Model importance sentence (condensation and hydrolysis): "Hydrolysis of glycosidic and peptide bonds in the gut releases monosaccharides and amino acids small enough to be absorbed, which is important because cells can then use the same monomers to build their own polymers by condensation."
Check your understanding
1. Table 1 lists four molecules. Complete Table 1 by putting a tick (✓) in every box where the statement applies to that molecule. [3]
Table 1
Statement
cellulose
sucrose
phospholipid
polypeptide
contains glycosidic bonds
formed by condensation reactions
is a polymer
2. Give one example of a monomer that is not a monosaccharide. [1]
3. Explain why the similarity of biological molecules in all living things is evidence for evolution. [2]
4. Pectinase is an enzyme that hydrolyses pectin, a polymer in plant cell walls. Fruit-juice makers add it to crushed fruit. 4.1 Describe what happens in a hydrolysis reaction. [2] 4.2 Suggest why adding pectinase increases the volume of juice released from crushed fruit. [2]
5. A branched polysaccharide contains 640 glucose units and 32 branch points, each made by one 1,6-glycosidic bond. Calculate the number of water molecules released when the whole molecule is formed from free glucose. [1]
6. A student wrote: "In photosynthesis, water is hydrolysed to release oxygen." Explain why the student is wrong to use the word hydrolysed. [2]
7. Explain why one enzyme can hydrolyse the glycosidic bonds in starch but not the peptide bonds in a protein. [2]
Answers
1. One mark per correct row; a row with any wrong or missing tick scores 0.
- contains glycosidic bonds: cellulose ✓, sucrose ✓ ;
- formed by condensation reactions: cellulose ✓, sucrose ✓, phospholipid ✓, polypeptide ✓ ;
- is a polymer: cellulose ✓, polypeptide ✓ ; [3] The traps are sucrose and phospholipid in the third row: both are made by condensation but are not polymers. A polypeptide has peptide bonds, not glycosidic bonds, between its amino acids; a phospholipid has ester bonds.
2. amino acid / nucleotide ; [1] Reject "protein" (a polymer).
3. 1. all organisms are made of the same groups of molecules / use the same monomers (amino acids, nucleotides) ; 2. which suggests they share a common ancestor ; [2] Accept "the genetic code is universal" for point 1.
4. 4.1 1. a water molecule is used ; 2. to break a (chemical) bond between two molecules / monomers ; [2] Reject "broken down" without a bond. 4.2 1. pectin in cell walls is hydrolysed, so cell walls are weakened / broken ; 2. so more cell contents / liquid can leave the cells ; [2]
5. Total bonds = 640 − 1 = 639, so 639 water molecules ; [1] The branch points do not change the total; 607 (639 − 32) is the number of 1,4-glycosidic bonds, not the water.
6. 1. water is not hydrolysed / hydrolysis uses water to break a (covalent) bond between two other molecules ; 2. water is split by light / photolysis (in the light-dependent reaction) ; [2] "The student is wrong" alone (no mark).
7. 1. enzymes are specific: the active site has a shape complementary to one substrate / one type of bond ; 2. a protein / peptide bond does not fit the active site, so no enzyme-substrate complex forms ; [2] Accept "amylase" named as the enzyme.