IB Biology Carbohydrates & Lipids: Your Questions Answered

Start by naming the molecule type correctly (monosaccharide, disaccharide, polysaccharide, triglyceride, phospholipid), then link structure to function using the command term in the question. Examiners want the reaction named explicitly — condensation or hydrolysis — the correct bond type (glycosidic or ester), and a structure-function link, not just a description.

Carbohydrates are built from monosaccharide units joined by glycosidic bonds and act as the body's main short-term energy source, plus structural material like cellulose. Lipids are triglycerides or phospholipids built from glycerol and fatty acids, storing more than double the energy per gram and forming cell membranes.

A condensation reaction joins two monomers by removing a water molecule, forming a covalent bond — a glycosidic bond between sugars, an ester bond between glycerol and a fatty acid. Hydrolysis is the reverse: water is added to break that bond, releasing the monomers, exactly as happens in digestion.

Lipids store roughly 38 kJ per gram compared with about 17 kJ per gram for carbohydrates. Fatty acid chains hold far more carbon-hydrogen bonds and are already less oxidised than carbohydrate molecules, so more energy releases when those bonds break down during aerobic respiration.

Saturated fatty acids have no carbon-carbon double bonds, so their chains stay straight and pack tightly, making the fat solid at room temperature — think butter. Unsaturated fatty acids contain one or more double bonds that kink the chain, keeping the fat liquid at room temperature — think olive oil.

Carbohydrates and lipids is core content taught identically to SL and HL students, sitting within Theme B: Form and function in the current DP Biology guide. There's no additional HL-only extension on this specific sub-topic, though HL students meet the same structure-function logic again in enzymes and respiration.

In the current DP Biology guide, first examined 2025, carbohydrates and lipids sits as topic B1 within Theme B, Form and function. It comes straight after the cell biology theme and feeds directly into later topics on membranes, enzymes and cell respiration.

The biggest mark loss is describing a structure without naming the reaction — writing 'glucose molecules join together' instead of 'condensation reaction forms a glycosidic bond.' A close second is mixing up starch and cellulose's monomer: starch is built from alpha-glucose, cellulose from beta-glucose.

You're expected to calculate body mass index using mass in kilograms divided by height in metres squared, then interpret it against WHO weight categories. Exam questions typically ask you to discuss correlation — not causation — between high-fat, high-sugar diets and obesity-related conditions such as type 2 diabetes.

Focus on the monomer first: starch and glycogen are built from alpha-glucose, which kinks the chain into a coil ideal for compact storage. Cellulose is built from beta-glucose, which keeps the chain straight so hydrogen bonds link parallel chains into strong microfibrils for plant cell walls.

Expect a Paper 1 multiple-choice question identifying a monosaccharide or fatty acid from a diagram, a Paper 2 short-answer command like 'outline the relationship between the structure of triglycerides and their function,' and occasionally a data-based question comparing energy content or melting points across different fats.

The DP Biology guide lists molecular visualisation software as a recommended tool for viewing the 3D shape of glucose, starch, cellulose and triglycerides. Seeing the models in 3D makes it obvious why alpha-glucose coils into a helix and beta-glucose stays flat — something a 2D textbook diagram rarely shows clearly.

Not conceptually. Most students find it more manageable than enzymes or respiration because it's largely descriptive rather than mathematical. Where marks get lost is precision — mixing up condensation with hydrolysis, or forgetting that phospholipids carry only two fatty acid tails, not three like a triglyceride.

Yes — you need to recognise and sometimes sketch the general structure of a monosaccharide, a glycosidic bond, a triglyceride and a phospholipid, since both Paper 1 and Paper 2 test structure recognition directly. You don't need obscure sugars, just glucose, starch, glycogen, cellulose and the two main lipid types.

Test recall the simple way: ask your child to sketch a triglyceride from memory and label the ester bonds, or explain why cellulose is stronger than starch. Past-paper questions and topic-specific worksheets are more useful at this revision stage than re-reading notes, and take far less of your evening to organise.