Sugars build, fats store. Two polymer families with very different jobs — and both built on carbon.
Every sub-topic below feeds at least one of these questions.
In what ways do variations in form allow diversity of function in carbohydrates and lipids?
How do carbohydrates and lipids compare as energy storage compounds?
The required syllabus content for B1.1, in order. Each card is one lesson-sized checkpoint.
Nature of Science: Students should understand that scientific conventions are based on international agreement (SI metric unit prefixes “kilo”, “centi”, “milli”, “micro” and “nano”).
Production of macromolecules by condensation reactions that link monomers to form a polymer
Digestion of polymers into monomers by hydrolysis reactions
Form and function of monosaccharides
Polysaccharides as energy storage compounds
Structure of cellulose related to its function as a structural polysaccharide in plants
Role of glycoproteins in cell–cell recognition
Hydrophobic properties of lipids
Formation of triglycerides and phospholipids by condensation reactions
Difference between saturated, monounsaturated and polyunsaturated fatty acids
Triglycerides in adipose tissues for energy storage and thermal insulation
Formation of phospholipid bilayers as a consequence of the hydrophobic and hydrophilic regions
Ability of non-polar steroids to pass through the phospholipid bilayer
A single property of carbon — its capacity to form four covalent bonds — opens up an essentially unlimited range of organic molecules.
Carbon has 4 valence electrons in its outermost shell. It can form up to four covalent bonds — single or double — with other carbon atoms or with non-metals like H, O, N, P, S. That structural flexibility lets carbon build branched chains, unbranched chains, single rings, fused rings — the underlying skeleton of every macromolecule in living things.
The four macromolecules of life — carbohydrates, lipids, proteins, nucleic acids — all share this carbon backbone. The differences are in the side groups and the way the carbons are arranged.
SI metric prefixes (kilo, centi, milli, micro, nano) are international conventions for shared measurement. You should be able to convert between mm, µm, nm — 1 mm = 1000 µm = 1,000,000 nm.
Macromolecules are assembled by condensation reactions and disassembled by hydrolysis. The same logic — minus or plus a water molecule — applies to all four macromolecule families.
Two monomers join, releasing one water molecule. Repeated thousands of times → a polymer. Builds polysaccharides, polypeptides, nucleic acids, triglycerides — every macromolecule.
A water molecule splits to provide an –H and an –OH, which add to two monomers on either side of a bond. The bond breaks. The reverse of condensation. The basis of digestion.
| Monomer | Polymer | Macromolecule examples |
|---|---|---|
| Glucose | Polysaccharide | Starch (amylose, amylopectin), glycogen, cellulose |
| Amino acid | Polypeptide | Proteins |
| Nucleotide | Polynucleotide | DNA, RNA |
| Fatty acid + glycerol | Triglyceride | Fats, oils |
Glucose's four critical properties make it the universal fuel: soluble, transportable, stable, and energy-rich.
Ribose (in RNA) and deoxyribose (in DNA). You should be able to draw both — they differ only in the –OH vs –H at carbon 2'.
Glucose, fructose, galactose. Glucose comes in α and β forms — same atoms, different orientation at carbon 1. Polysaccharides typically form via 1→4 glycosidic bonds.
Polar — many –OH groups → dissolves easily in water/cytoplasm.
Carried dissolved in blood, sap, lymph.
Doesn't break down spontaneously during transport.
Per glucose, via aerobic respiration.
Storing energy as glucose itself would create osmotic problems. Storing it as a coiled, branched polysaccharide keeps it compact and biologically inert until needed.
Two polymers together: amylose (long unbranched α-glucose chains, 1→4 bonds) and amylopectin (branched — 1→4 bonds plus 1→6 branches every ~20 residues). Coiled and packed into starch grains in plant cells.
Like amylopectin but more highly branched. Stored mainly in liver and skeletal muscle. The many branch tips make rapid addition/removal of glucose easy — useful for animals with rapidly fluctuating energy demands.
Large molecular size → relatively insoluble → doesn't change cell water potential. Branched/coiled → compact storage. Easy to add or remove monomers by condensation/hydrolysis.
Same monomer (glucose), different geometry — and you get a structural fibre instead of an energy store.
Cellulose is built from β-glucose, not α-glucose. The geometric consequence: every second glucose has to be flipped 180° to form the 1→4 bond. The result is a perfectly straight chain — no coiling, no branching.
Parallel cellulose chains hydrogen-bond to each other in bundles — cellulose microfibrils. The microfibrils have enormous tensile strength: they're what stops plant cells bursting under turgor pressure and what gives wood its load-bearing capacity.
A protein with a sugar chain attached becomes a glycoprotein — and that sugar chain often acts as a recognition signal. ABO blood groups are the classic example.
Glycoproteins sit in the plasma membrane with their carbohydrate chains projecting outward. Roles include: cell-cell adhesion (forming tissues), receptors for hormones and neurotransmitters, and immune system markers distinguishing self from non-self.
| Blood type | Antigens on RBC | Antibodies in plasma |
|---|---|---|
| A | A | Anti-B |
| B | B | Anti-A |
| AB | A and B | None — universal recipient |
| O | None — universal donor | Anti-A and anti-B |
The immune system does not produce antibodies against antigens that are already present on its own red blood cells — which is why a person with blood type A doesn't make anti-A antibodies, and why mismatched transfusions are catastrophic.
Lipids are a diverse family unified by one property: they don't dissolve in water. Their non-polar hydrocarbon chains make them all hydrophobic.
Lipids include:
All hydrophobic. Soluble in non-polar solvents (ether, chloroform), insoluble in water. Their functions — energy storage, membrane formation, hormone signalling across membranes — all depend on this hydrophobicity.
Triglycerides = glycerol + 3 fatty acids. Phospholipids = glycerol + 2 fatty acids + 1 phosphate. The phosphate changes everything.
Both molecules are assembled by condensation reactions between glycerol and fatty acids (and, for phospholipids, a phosphate group). Triglyceride formation releases 3 water molecules; phospholipid formation releases 3 as well.
The number of C=C double bonds in a fatty acid changes how tightly the chains pack — and that controls whether the lipid is a fat or an oil.
Single bonds only. Straight chains pack tightly. Higher melting points → solid at room temperature → fats. Common in animal energy stores (butter, lard).
One kink in the chain. Chains pack less tightly. Lower melting points. Common in olive oil.
Multiple kinks. Chains pack loosely. Lowest melting points → liquid at room temperature → oils. Common in plant and fish energy stores (sunflower oil, salmon).
Triglycerides in adipose tissue serve two functions at once: a high-density energy reserve and an insulating layer.
Examples: walruses in arctic waters have thick blubber (energy + insulation). Kangaroo rats in deserts have little subcutaneous fat — they store no insulation because they need to dump heat, not retain it.
Phospholipids have a hydrophilic head and a hydrophobic tail. Drop them in water and they self-assemble into bilayers without anyone's help.
The phosphate head of a phospholipid is polar and charged — hydrophilic. The two fatty acid tails are non-polar — hydrophobic. Together, that makes a phospholipid amphipathic: water-loving on one end, water-fearing on the other.
Place phospholipids in water and they spontaneously arrange themselves into a bilayer: heads outward (touching water on both sides), tails huddled inward away from water. No enzymes, no energy input. The basis of every cell membrane on Earth.
Steroids are non-polar lipids — they dissolve in the hydrophobic core of a phospholipid bilayer. So they cross membranes directly, without needing transporters.
Steroids have a characteristic shape: four fused carbon rings plus a short hydrocarbon chain. Like all lipids, they are hydrophobic. Cholesterol is a steroid — a component of cell membranes. Oestradiol and testosterone are steroid hormones.
Because they're hydrophobic, steroid hormones can diffuse straight through the phospholipid bilayer of cell membranes. Polar hormones (insulin, glucagon) cannot — they have to bind receptors on the cell surface. The difference in transport mode is a direct consequence of polarity.
If you can't define one of these in a sentence, that's where to revise next.
“How can compounds synthesized by living organisms accumulate and become carbon sinks?”
“What are the roles of oxidation and reduction in biological systems?”