Condensation and Hydrolysis Reactions
How monomers join into polymers by condensation (a bond forms and water is released) and how hydrolysis breaks them apart again using water. Includes the 3-mark exam question, a worked model answer and the examiner trap on which way the water moves.
Chapters (7)
Exam question [3 marks]
A protein is a polymer made of amino acids. Describe how amino acids are joined to form a protein, and how a protein is broken down into amino acids.
Try it on paper first, then check the mark scheme below. The question is original and was written for this lesson.
Mark scheme
| Mark | Mark point | Also accept | Do not accept |
|---|---|---|---|
| 1 | Amino acids are joined by condensation (reactions) | "condensation reaction", "condensation polymerisation" | "amino acids are digested together"; "condensation uses water" |
| 2 | A (chemical) bond forms AND a molecule of water is released / eliminated (both needed for the mark) | "peptide bond", "water given off / lost / removed" (as the water half) | "water is used", "water is added", bond named incorrectly as "glycosidic" |
| 3 | Broken down by hydrolysis, which uses a molecule of water to break the (chemical) bond | "adding water breaks the bond", "water splits the bond" | "hydrolysis removes water"; "hydrolysis releases water"; "water is used to break down the amino acids" with no bond |
The three points are the process that builds (condensation), what happens in it (bond forms, water out), and the reverse process (hydrolysis, water in). Naming the reaction is needed for marks 1 and 3; the direction of the water is the point examiners test. "Chemical bond" or "peptide bond" both score; a bond without the water, or water without the bond, does not score mark 2.
Model answer (3/3)
Amino acids are joined by condensation reactions, in which a chemical bond forms and a molecule of water is released. The protein is broken down by hydrolysis, which uses a molecule of water to break the bonds.
Examiner trap
- Saying condensation 'uses' water (it releases it)
- Writing hydrolysis as 'removing water'
- Calling a polymer a monomer
Key terms
- monomer
- polymer
- monosaccharide
- amino acid
- nucleotide
- condensation reaction
- hydrolysis
- chemical bond
Lesson notes
A written version of the video, chapter by chapter, for revision. Times link to the video.
0:20Monomers: the building blocks
Life is built from a small set of building blocks. A single block is called a monomer. It is one smaller unit, from which larger molecules are made. You need three examples. Monosaccharides, amino acids and nucleotides. The same kinds of molecules build every living thing, from bacteria to you.
That is what the specification means when it says the biochemical basis of life is similar for all living things. Different organisms, same toolkit.
0:51Polymers: many monomers joined
Join lots of monomers together and you get a polymer. Polymer just means a molecule made from a large number of monomers. Many sugars make a polysaccharide, many amino acids make a protein, and many nucleotides make DNA. Think of a train. Each carriage is a monomer, and the whole train is the polymer.
Small units, joined many times, give a big molecule.
1:16Condensation: building a polymer
So how do two monomers join? By a condensation reaction. Watch the ends. An OH comes off one monomer, and a hydrogen comes off the other. Those combine to make a molecule of water, which is released. And with the water gone, a chemical bond forms between the two monomers.
So condensation forms a bond, and eliminates water. It does not use water.
1:42Hydrolysis: breaking it back down
Now run it backwards. A hydrolysis reaction breaks a chemical bond between two molecules. It does that by using a molecule of water, which splits and adds to the two ends. So hydrolysis uses water, and condensation releases it.
A handy trick. Hydro means water, and lysis means splitting. Hydrolysis is splitting with water. You do this every time you eat. Digestion breaks big polymers in your food down by hydrolysis into monomers small enough to absorb. And your cells then use condensation to build the polymers they need.
2:41Examiner trap and recap
The trap. Never say condensation uses water, and never say hydrolysis removes water. Condensation releases water. Hydrolysis uses water.
Recap. Monomers join into polymers by condensation, and hydrolysis splits them apart. Next up, the monomers behind carbohydrates. Sugars, and the bonds that join them.
Keep going
More from 3.1 Biological molecules
- Maltose, Sucrose, Lactose: Glycosidic Bonds · AQA 7402 · 3.1.2
- Alpha vs Beta Glucose: Starch vs Cellulose · AQA 7402 · 3.1.2
- Benedict's and Iodine Tests Explained · AQA 7402 · 3.1.2
- Triglycerides: Saturated vs Unsaturated Fats · AQA 7402 · 3.1.3
All 21 3.1 Biological molecules lessons Whole playlist on YouTube
Independent revision resource, not affiliated with or endorsed by AQA. The exam question is original and was written for this lesson; it is not an AQA question. Spotted an error? Email info@medservatory.com with the lesson code AB-001.