Preview
IB DP Biology — B: Form and Function | SL | B1.1
📋 What You Will Learn
- Identify the molecular components of monosaccharides, disaccharides, and polysaccharides, and explain how condensation reactions link them
- Explain the structural differences between saturated and unsaturated fatty acids and describe how triglycerides and phospholipids are formed
- Analyse how the molecular structure of carbohydrates and lipids determines their biological functions — energy storage, structural support, and membrane formation
- Apply knowledge of carbohydrate and lipid structure to interpret unfamiliar biological contexts, including food science and cell biology data
- Evaluate the relationship between molecular structure and function as a unifying principle across B: Form and Function
🔍 Guiding Question How does the molecular structure of carbohydrates and lipids determine their diverse functions in living organisms?
🌍 Why This Matters A marathon runner's muscles run out of glycogen after roughly 90 minutes — at that point, the body switches to fat metabolism, unlocking a fuel reserve that could power hundreds of kilometres more. The reason fat stores so much more energy than carbohydrate is not magic: it is chemistry. The molecular structure of lipids, with their long hydrocarbon chains, makes them the most energy-dense biological molecules on Earth.
The Problem of Storing Energy and Building Structure
Every living cell faces two fundamental challenges: it must store chemical energy reliably, and it must build physical boundaries that separate inside from outside. Evolution has solved both problems using just two families of molecules — carbohydrates and lipids. Understanding why these molecules work so well requires looking at their architecture from the atoms up.
Carbon is the backbone of both families. Its ability to form four covalent bonds allows it to build chains, rings, and branching networks of extraordinary variety. Add oxygen and hydrogen in different ratios, and you get molecules with very different properties — some water-soluble and quick to release energy, others water-repelling and slow-burning. The contrast between carbohydrates and lipids is, at its heart, a contrast in the ratio of oxygen to carbon.
Carbohydrates: Structure from the Ground Up
Before we can understand starch or cellulose, we need to understand the unit from which they are built. The cell's strategy is elegant: make one small, soluble molecule, then link copies of it together to create something large, insoluble, and structurally powerful.
📖 Key Definition — Monosaccharide A monosaccharide is a single-unit carbohydrate with the general formula \( (\text{CH}_2\text{O})_n \), where \( n \) is typically 3–7. Monosaccharides are the monomers of all carbohydrates. Examples include glucose (\( n = 6 \)), fructose, and ribose (\( n = 5 \)).
Glucose — The Universal Currency
Glucose (\(\text{C}_6\text{H}_{12}\text{O}_6\)) is the most important monosaccharide in biology. In aqueous solution it exists predominantly as a six-membered ring — a pyranose ring — formed when the carbonyl group on carbon-1 reacts with the hydroxy group on carbon-6. This ring structure is not merely a curiosity: the orientation of the hydroxy group on carbon-1 determines whether glucose becomes alpha (\(\alpha\)) or beta (\(\beta\)) glucose, and that single difference in geometry has enormous consequences for the polysaccharides built from each form.
In \(\alpha\)-glucose, the hydroxy group on carbon-1 points downward (axial