The big idea: Almost every molecule in a living thing is built around the element carbon.
Carbon is special because one carbon atom can form four covalent bonds.
This lets carbon atoms link into long chains, branches and rings, so a small set of elements can be arranged into a huge variety of different molecules — the basis of all the macromolecules of life.
One carbon atom forms four covalent bonds, letting carbon skeletons grow into chains, branches and rings.
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- Carbon compound (organic molecule)
- A molecule built around a skeleton of carbon atoms — for example a carbohydrate, lipid, protein or nucleic acid.
- Covalent bond
- A bond in which two atoms share a pair of electrons.
- Macromolecule
- A very large molecule built from many smaller repeating subunits (for example a polysaccharide, a protein or a nucleic acid).
- Monomer
- A single small subunit that can be joined to others to build a larger molecule (for example glucose or an amino acid).
- Polymer
- A large molecule made of many monomers joined together (for example starch, a polymer of glucose).
Why four bonds matters: Because each carbon can bond four times, carbon skeletons can grow and branch in almost endless ways.
No other common element of life builds such varied frameworks — this single property is why carbon is the backbone of every biological molecule.
Macromolecules are far too large to be made in one step. Instead the cell joins many small monomers together, one at a time, using a reaction called condensation.
To use a macromolecule for energy or recycle its parts, the cell breaks it back down into monomers using the reverse reaction, hydrolysis.
- Condensation
- A reaction that joins two subunits together and releases one molecule of water (H₂O).
- Hydrolysis
- A reaction that uses one molecule of water (H₂O) to break a bond and split a molecule into two subunits.
- Anabolic reaction
- A reaction that builds larger molecules from smaller ones (condensation is anabolic).
- Catabolic reaction
- A reaction that breaks larger molecules into smaller ones (hydrolysis is catabolic).
Condensation — joining with water removed: In condensation, two monomers are joined and one water molecule (H₂O) is released.
Repeating this many times links monomers into a polymer (a macromolecule).
Examples: glucose + glucose → maltose + water; many glucose → starch; glycerol + fatty acids → a triglyceride; amino acid + amino acid → a dipeptide.
Condensation: two monomers join, a new bond forms, and one H₂O is removed.
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Hydrolysis — breaking with water added: Hydrolysis is the reverse. One water molecule (H₂O) is added across the bond, splitting the molecule into two subunits.
The word itself is a clue: hydro = water, lysis = splitting — 'splitting with water'.
Examples: starch + water → glucose; a disaccharide + water → two monosaccharides; a protein + water → amino acids.
Hydrolysis: a water molecule is added to split the bond, releasing the two subunits.
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Condensation
- Joins subunits together
- Releases one water molecule
- Builds larger molecules (polymers)
- Is an anabolic reaction
Hydrolysis
- Breaks a molecule apart
- Uses one water molecule
- Breaks down larger molecules into monomers
- Is a catabolic reaction
A memory hook: Condensation constructs (and water comes out). Hydrolysis uses hydro (water goes in) to split the molecule.
What all macromolecules share: Polysaccharides (like starch), triglycerides and proteins look very different, but they are built the same way: smaller subunits joined by condensation, releasing water each time.
That shared feature is a favourite exam point — the common thread is condensation of subunits with the loss of water.
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How this is tested: On Paper 2 a 4-mark Outline question can ask for the chemical properties of carbon that let it form diverse compounds — give separate, scoring points (four bonds, strong covalent bonds, bonds to many elements, chains/branches/rings).
A 1-mark State question often asks for the reaction type that breaks a macromolecule or disaccharide — the answer is hydrolysis.
On Paper 1A you may have to identify a feature common to polysaccharides and triglycerides (built by condensation), or classify a reaction as anabolic or catabolic.
IB-style question — outline carbon's bonding properties
Outline the chemical properties of carbon that allow it to form a wide variety of compounds. [4]
How to score all four marks
- Four covalent bonds. Each carbon atom can form four covalent bonds, so many other atoms can join to it.
- Bonds to itself and to other elements. Carbon bonds strongly to other carbon atoms and to elements such as hydrogen, oxygen and nitrogen, adding different chemical groups.
- Many shapes. Carbon skeletons can be straight chains, branched chains or rings, so the same atoms can be arranged in many different ways.
- Strong, stable bonds. Carbon–carbon bonds are strong and stable, so large, long-lasting molecules can be built. (Award 1 mark for each distinct point, up to 4.)
Final answer
Carbon forms four covalent bonds; bonds to itself and to other elements (H, O, N); makes chains, branches and rings; and forms strong, stable bonds — together these build a huge variety of molecules.
✓ Why this scores full marks: Each sentence is a separate, distinct property — four bonds, bonds to many elements, many shapes, strong bonds.
An 'outline' worth 4 marks needs four scoring points, not one idea written four ways.
| Property of carbon | What it means | Why it gives diverse molecules |
|---|---|---|
| Forms four covalent bonds | Each carbon atom can bond to up to four other atoms | A huge number of different structures can be built around each carbon |
| Strong, stable bonds | Carbon–carbon and carbon–hydrogen bonds are strong | Large molecules hold together and last a long time |
| Bonds to many elements | Carbon bonds to H, O, N, P, S and to itself | Different elements add different chemical groups and properties |
| Chains, branches and rings | Carbon skeletons can be straight, branched or ring-shaped | The same atoms can be arranged into many different shapes |