2.1.1Inquiry Process
Properties of Carbon — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 37 of 160
Part 1 of 5Carbon Bonding Behaviour
Carbon is the central element of organic chemistry because of the way its atoms share electrons.
Four valence electrons
- A carbon atom has four electrons in its outer shell, so it needs to share four more to fill that shell and become stable.
Covalent bonds
- Carbon shares electron pairs with other atoms,
- giving strong bonds that do not break easily at the temperatures found inside living cells.
Tetravalence
- Each carbon can form four single bonds, or double bonds, to several different atoms at once, which allows branching in many directions.
Bonding partners
- Carbon bonds readily with hydrogen, oxygen, nitrogen, sulfur and phosphorus, the elements that make up most biological molecules.
Carbon-carbon bonds
- Carbon atoms bond strongly to each other, so long chains and rings can be built without the molecule falling apart.
| Feature | Consequence for living things |
|---|---|
| Four covalent bonds | Stable, branching molecular frameworks |
| C-C bond strength | Large molecules persist long enough to carry out a function |
| Bonds to H, O, N, P, S | Wide range of chemical properties from one backbone |
Part 2 of 5Building Diverse Skeletons
The same atom can be arranged in many ways, which is why a small set of elements produces millions of compounds.
Building Diverse Skeletons
Straight chains
- Carbon atoms joined in a line form the hydrocarbon tails of fatty acids and the backbones of many other organic molecules.
Branched chains
- Side chains attached to a carbon backbone change the shape of the molecule, as seen in the amylopectin of starch.
Rings
- Carbon can close into rings, such as the six-membered ring of glucose, giving compact and stable structures.
Isomers
- Molecules with the same molecular formula but different arrangements,
- such as glucose and fructose (both C₆H₁₂O₆), can have different properties.
Part 3 of 5Functional Groups
Small groups of atoms attached to the carbon skeleton decide how the whole molecule behaves.
Hydroxyl (-OH)
- Makes a molecule polar and able to form hydrogen bonds with water, so alcohols and sugars are often soluble.
Carboxyl (-COOH)
- Can release a hydrogen ion and so acts as an acid; it is found at one end of every amino acid and fatty acid.
Amino (-NH₂)
- Can accept a hydrogen ion and so acts as a base; it is found at the other end of every amino acid.
Phosphate (-PO₄)
- Carries a negative charge, links nucleotides in nucleic acids and takes part in energy transfers involving ATP.
| Group | Behaviour | Example molecule |
|---|---|---|
| Hydroxyl | Polar, forms hydrogen bonds | Glucose |
| Carboxyl | Acidic | Fatty acids, amino acids |
| Amino | Basic | Amino acids |
| Phosphate | Negatively charged | DNA, ATP, phospholipids |
Part 4 of 5Carbon in Living Systems
1
Carbon dioxide is fixed into organic molecules by photosynthesis, which turns inorganic carbon into sugars.
2
Small carbon compounds such as glucose, amino acids and fatty acids are the building blocks made from this fixed carbon.
3
Building blocks are joined by condensation reactions into larger molecules such as polysaccharides, proteins and lipids.
4
These carbon compounds form structures, store energy and carry information, covering the main needs of every cell.
5
Respiration and decomposition break organic molecules down again and return carbon to the atmosphere as carbon dioxide.
Part 5 of 5Exam Checklist & Tips
2.1.1 – Key Points for the Exam
- Covalent bond A bond formed when two atoms share a pair of electrons.
- Organic compound A compound based on a carbon skeleton, usually bonded to hydrogen and other non-metals.
- Functional group A small group of atoms attached to a carbon skeleton that gives the molecule its characteristic chemical properties.
- Isomers Compounds with the same molecular formula but different structural arrangements of atoms.
- Hydrocarbon A molecule made only of carbon and hydrogen atoms.
Common Mistakes
- Saying carbon forms ionic bonds; it shares electrons and forms covalent bonds.
- Thinking carbon dioxide and carbonates count as organic; they are simple inorganic carbon compounds.
- Forgetting that isomers share a formula but differ in structure and therefore in function.
- Giving 'strong bonds' as the only reason for carbon importance without mentioning four bonds and branching.
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