Key Idea: Every molecule of life is built around carbon, because one carbon atom forms four covalent bonds and links into chains, branches and rings. This topic covers the two carbon-based food groups: carbohydrates (sugars β disaccharides β polysaccharides) and lipids (triglycerides and phospholipids). They are all macromolecules built from smaller subunits by the same reaction β condensation (which removes water) β and broken back down by hydrolysis (which adds water). It is a regular on Paper 1A (structure/reaction MCQs) and on Paper 2 (identifying parts of labelled molecules and explaining how a structure suits its function).
𧬠Carbon & building macromolecules
Carbon forms four covalent bonds, bonds to itself and to other elements (H, O, N), and makes chains, branches and rings with strong, stable bonds β so a small set of elements builds a huge variety of molecules. Macromolecules are assembled from monomers by condensation (a new bond forms and one water molecule is released) and broken back into monomers by hydrolysis (one water molecule is added to split a bond).
One carbon atom forms four covalent bonds, so carbon skeletons grow into chains, branches and rings.
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| Feature | Condensation | Hydrolysis |
|---|---|---|
| What it does | joins subunits together | breaks a molecule apart |
| Water | released (removed) | used (added) |
| Builds or breaks | builds polymers | breaks into monomers |
| Type of reaction | anabolic (building up) | catabolic (breaking down) |
Condensation joins two subunits and releases one water molecule; hydrolysis is the reverse β water is added to break the bond.
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Condensation constructs, and water comes out. Hydrolysis uses hydro (water goes in) to split the molecule. Every macromolecule β polysaccharide, triglyceride, phospholipid, protein β is built this same way.
π¬ Monosaccharides & disaccharides
Monosaccharides are single sugar units β the monomers of carbohydrates. Glucose is the most important (formula CβHββOβ, drawn as a six-carbon ring). Glucose has two isomers β alpha- and beta-D-glucose β with the same formula but one difference: the -OH on carbon 1 points down in alpha and up in beta. That tiny change decides whether the glucose builds an energy store or a structural fibre. Join two monosaccharides by condensation and a glycosidic bond forms, releasing water β the product is a disaccharide.
Alpha- and beta-D-glucose are isomers: the only difference is whether the -OH on carbon 1 points DOWN (alpha) or UP (beta).
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| Disaccharide | Made from | Where you meet it |
|---|---|---|
| Maltose | glucose + glucose | germinating grain; breakdown of starch |
| Sucrose | glucose + fructose | table sugar; transported sap in plants |
| Lactose | glucose + galactose | milk sugar |
Do not say the formula differs (it doesn't). The single difference is the direction of the carbon-1 -OH: Ξ± down, Ξ² up.
π½ Polysaccharides: structure & function
A polysaccharide is a giant polymer of many glucose units. The three you must know are all made of glucose, yet do different jobs β because of which form of glucose is used and how the chains are shaped. Storage polysaccharides (starch in plants, glycogen in animals) use alpha-glucose, are branched and insoluble β a compact, quick-release store. The structural polysaccharide (cellulose, in plant cell walls) uses beta-glucose in straight, unbranched chains that hydrogen-bond into strong fibres.
Same monomer (glucose), different jobs: starch (alpha-glucose, coiled + branched) and glycogen (alpha-glucose, highly branched) store energy; cellulose (beta-glucose, straight fibres) gives structure.
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| Polysaccharide | Monomer | Shape | Job |
|---|---|---|---|
| Starch | alpha-glucose | coiled (helix) + some branching | energy storage in plants |
| Glycogen | alpha-glucose | highly branched | energy storage in animals |
| Cellulose | beta-glucose | long, straight, unbranched fibres | structural support in plant cell walls |
Alpha β Around (coils, branches) β Available energy (starch, glycogen). Beta β Bars (straight) β Building material (cellulose).
π§ Triglycerides & fatty acids
A triglyceride (the main fat/oil) = 1 glycerol + 3 fatty acids, joined by 3 ester bonds. Building it is condensation (3 water molecules removed); breaking it is hydrolysis (3 water molecules added β back to glycerol + 3 fatty acids). Fatty-acid tails are saturated (only single CβC bonds, straight, usually solid) or unsaturated (one or more C=C double bonds, kinked, usually a liquid oil).
A triglyceride: one glycerol backbone joined to three fatty acids by three ester bonds.
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Saturated fatty acid (straight chain, single CβC bonds only) vs unsaturated fatty acid (a C=C double bond puts a kink in the chain).
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| Feature | Saturated fatty acid | Unsaturated fatty acid |
|---|---|---|
| Bonds between carbons | only single (CβC) | one or more double (C=C) |
| Shape of chain | straight β pack closely | kinked β pack loosely |
| State at room temperature | usually solid (e.g. butter) | usually liquid oil (e.g. olive oil) |
Look only for a C=C double bond: present β unsaturated (kinked); absent β saturated (straight). The bond joining a fatty acid to glycerol is an ester bond β not glycosidic (sugars) or peptide (proteins).
π Lipids as energy stores
Triglycerides are the body's long-term energy store, kept as fat in adipose tissue (under the skin, around organs). They suit storage because of three structure-linked properties. Always pair the property with its reason β that is where the marks are.
Why triglycerides make great stores
- ~2Γ the energy per gram of carbohydrate β because the long fatty-acid tails are packed with energy-rich CβH bonds and little oxygen
- Insoluble β because the tails are hydrophobic, so the store has no osmotic effect on the cell's water balance and large amounts can be stored safely
- Compact and light β energy-dense and water-free, so the same energy is stored for less mass
- The fat layer also gives thermal insulation and protection / cushioning
| Feature | Lipid store (triglyceride) | Carbohydrate store (glycogen) |
|---|---|---|
| Energy per gram | about twice as much | about half as much |
| Solubility | insoluble (hydrophobic) | soluble |
| Osmotic effect on cell | none | can draw water in |
| Time scale of use | long-term store | short-term / quick store |
It comes down to polarity: glucose has many polar -OH groups that attract water, so it dissolves; a triglyceride's fatty-acid tails are non-polar / hydrophobic, so they don't mix with water and stay insoluble.
π§ Phospholipids & amphipathic molecules
A phospholipid = glycerol + two fatty acids + a phosphate head (a triglyceride with one tail swapped for a phosphate), built by condensation. Its phosphate head is hydrophilic (polar, water-loving) and its two fatty-acid tails are hydrophobic (non-polar, water-fearing). Having both in one molecule makes it amphipathic β and that is why, in water, phospholipids self-arrange into a bilayer (heads out, tails in): the basis of every cell membrane.
A single phospholipid: a hydrophilic phosphate head joined through glycerol to two hydrophobic fatty-acid tails.
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| Feature | Phospholipid | Triglyceride |
|---|---|---|
| Fatty-acid tails | TWO fatty acids | THREE fatty acids |
| Third glycerol position | a phosphate-containing head | a third fatty acid |
| Behaviour in water | amphipathic (head β tails) | all hydrophobic |
| Main job | builds cell membranes | stores energy |
In water, phospholipids line up into a bilayer: hydrophilic heads face the water on both sides, hydrophobic tails tuck away in the middle.
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Head = phosphate = hydrophilic (faces water). Tails = fatty acids = hydrophobic (tuck inwards, away from water). One head + two tails is the phospholipid 'tadpole'; three tails and no head is a triglyceride.
βοΈ Worked examples
IB-style question β 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 bonds. Each carbon atom can form four covalent bonds, so many atoms can join to it.
Bonds widely. Carbon bonds to itself and to other elements such as H, O and N.
Many shapes. Carbon skeletons can be straight chains, branched chains or rings.
Strong bonds. Carbonβcarbon bonds are strong and stable, so large, long-lasting molecules can be built. (1 mark per distinct point, max 4.)
Four covalent bonds; bonds to itself and to other elements (H, O, N); chains, branches and rings; strong, stable bonds β together giving huge molecular variety.
IB-style question β cellulose structure and function
Explain how the structure of cellulose relates to its function in plant cells. [3]
Model answer:
Structure. Cellulose is made of beta-glucose joined into long, straight, unbranched chains.
Bonding. Many straight chains lie parallel and form hydrogen bonds between them, bundling into strong fibres.
Function. These fibres make the plant cell wall strong, giving the cell support and helping it resist bursting.
Straight beta-glucose chains hydrogen-bond into strong fibres, which strengthen the plant cell wall and provide support.
IB-style question β a labelled triglyceride
A triglyceride diagram shows a short backbone joined to three long chains, one containing a C=C double bond. (a) Name the backbone molecule. (b) Name the reaction that joins the chains to it. (c) State the type of fatty acid shown by the chain with the double bond. [3]
How to score all three marks:
(a) The backbone is glycerol (the 3-carbon molecule with three -OH groups).
(b) The chains join by condensation, which forms an ester bond and removes a water molecule each time.
(c) A chain with a C=C double bond is an unsaturated fatty acid (single bonds only = saturated).
(a) Glycerol. (b) Condensation (forming ester bonds). (c) Unsaturated, because of the C=C double bond.
IB-style question β why phospholipids form a bilayer
Explain why phospholipid molecules arrange themselves into a bilayer when surrounded by water. [3]
Model answer:
Phospholipids are amphipathic β each has a hydrophilic phosphate head and hydrophobic fatty-acid tails.
The hydrophilic heads are attracted to water, so they face outwards towards the water on both sides.
The hydrophobic tails are repelled by water, so they turn inwards and tuck together in the middle β forming a stable bilayer.
Amphipathic phospholipids point their hydrophilic heads to the water on both sides and tuck the hydrophobic tails into the middle, forming a bilayer.
β Quick self-check
Tap each card to check yourself.
What is the difference between condensation and hydrolysis? Condensation joins subunits and RELEASES water (anabolic); hydrolysis ADDS water to break a molecule into subunits (catabolic).
What structurally distinguishes alpha- from beta-D-glucose? Only the direction of the -OH on carbon 1: DOWN in alpha, UP in beta. Same formula (CβHββOβ) β they are isomers.
Why is starch a good energy store but cellulose a good structural molecule? Starch is alpha-glucose, branched and insoluble β a compact quick-release store. Cellulose is beta-glucose in straight chains that hydrogen-bond into strong supporting fibres.
What is a triglyceride made of, and how do you spot an unsaturated fatty acid? One glycerol + three fatty acids joined by three ester bonds. An unsaturated fatty acid has one or more C=C double bonds (a kink in the chain).
Why are triglycerides good long-term energy stores? High energy per gram (many CβH bonds) and insoluble (hydrophobic, no osmotic effect), so a compact, light store; the fat layer also insulates.
Why is a phospholipid amphipathic, and what does that let it do? Its phosphate head is hydrophilic and its two fatty-acid tails are hydrophobic β both in one molecule. In water this lets phospholipids form the bilayer of cell membranes.
Exam Tips
- Condensation RELEASES water (builds); hydrolysis USES water (breaks) β swapping these is the most common error.
- Alpha vs beta glucose = the direction of the carbon-1 -OH only (Ξ± down, Ξ² up); never say the formula differs.
- Same monomer, different job: storage polysaccharides are alpha-glucose & branched; cellulose is beta-glucose & straight.
- Make structureβfunction links explicit β e.g. 'straight beta-glucose chains hydrogen-bond into fibres β support'.
- Triglyceride numbers: 1 glycerol + 3 fatty acids = 3 ester bonds = 3 water removed (condensation) or added (hydrolysis).
- To name a fatty-acid type, look only for a C=C double bond: present = unsaturated, absent = saturated.
- For lipid storage, pair each property with its reason: insoluble BECAUSE hydrophobic; high energy BECAUSE many CβH bonds.
- 'Why oils are insoluble but glucose dissolves' is about polarity β non-polar tails vs polar -OH groups.
- Phospholipid = 2 tails + a phosphate head; head = hydrophilic, tails = hydrophobic; amphipathic = BOTH in one molecule.
Key Idea: Carbon's four bonds build all biological molecules. Carbohydrates go monosaccharide (glucose) β disaccharide β polysaccharide (starch/glycogen store, cellulose supports). Lipids are triglycerides (1 glycerol + 3 fatty acids; long-term energy store) and phospholipids (2 fatty acids + a phosphate head; amphipathic β cell membranes). Everything is built by condensation (water out) and broken by hydrolysis (water in), and in every case the structure explains the function.