2.1.2Inquiry Process
Macromolecules — IB Biology HL Revision Notes & Topical Questions
5 min read 6 parts Lesson 38 of 160
Part 1 of 6Monomers and Polymers
Cells build large molecules from small repeating units, which keeps construction simple and flexible.
Monomer
- A small unit that can bond to others of its kind, such as a monosaccharide, an amino acid or a nucleotide.
Polymer
- A long chain made of many monomers joined by covalent bonds, giving a large molecule with a specific function.
Macromolecule
- Any very large molecule built from thousands of atoms, including polysaccharides, proteins and nucleic acids.
Lipids
- Large non-polar molecules such as triglycerides that are grouped with macromolecules but are not true polymers of repeating monomers.
| Macromolecule | Monomer | Bond formed |
|---|---|---|
| Polysaccharide | Monosaccharide | Glycosidic |
| Polypeptide | Amino acid | Peptide |
| Nucleic acid | Nucleotide | Phosphodiester |
| Triglyceride | Glycerol + fatty acids | Ester |
Part 2 of 6Condensation Reactions
Condensation joins monomers while releasing water.
1
Two monomers line up so that a hydroxyl group on one is next to a hydrogen on the other.
2
An enzyme catalyses the removal of these atoms as one molecule of water.
3
A covalent bond forms between the two monomers, joining them into a dimer.
4
The process repeats at the free end, so the chain lengthens one monomer at a time.
5
Each new bond needs energy, which is why building polymers is an anabolic process.
Part 3 of 6Hydrolysis Reactions
Hydrolysis splits polymers into monomers by adding water.
Hydrolysis Reactions
Water is added
- A molecule of water is used to break the bond between two monomers, the reverse of condensation.
Enzyme control
- Digestive enzymes such as amylase and protease catalyse hydrolysis in the gut and inside cells.
Energy release
- Breaking the bonds of a polymer lets the monomers be absorbed or used for respiration.
Recycling
- Cells can hydrolyse their own polymers and reuse the monomers to build different molecules.
Part 4 of 6Why Polymers Matter
Storage
- Large polymers like starch hold many glucose units while exerting very little osmotic effect inside the cell.
Structure
- Long strong chains such as cellulose and collagen give plant walls and animal tissues their strength.
Information
- The sequence of nucleotides in DNA stores genetic instructions in a compact form.
Function
- The order of amino acids in a protein sets its shape and so determines its role as enzyme, carrier or receptor.
Part 5 of 6Worked Example
Question
- A polypeptide contains 120 amino acids in one unbranched chain.
- How many peptide bonds and how many water molecules were released in making it?
- Each peptide bond forms between two neighbouring amino acids.
- A linear chain of 120 amino acids has 120 - 1 = 119 links.
- Each condensation reaction releases one water molecule, so 119 water molecules were released.
Answer 119 peptide bonds and 119 molecules of water.
Part 6 of 6Exam Checklist & Tips
2.1.2 – Key Points for the Exam
- Condensation A reaction that joins two molecules together with the loss of a water molecule.
- Hydrolysis A reaction that breaks a bond by adding a water molecule.
- Polymer A large molecule made of many repeating monomer units.
- Anabolic reaction A reaction that builds larger molecules from smaller ones and requires energy.
Common Mistakes
- Writing that water is used in condensation; water is released in condensation and used in hydrolysis.
- Calling all lipids polymers; they are not made of repeating monomers.
- Counting one bond per monomer; a chain of n monomers has n - 1 bonds.
- Confusing the bond namesglycosidic, peptide and ester belong to different macromolecules.
Last updated:
\n