‹ All simulationsBiology · AS Level 9700 · 2 Biological moleculesSeven simulations covering Topic 2: carbohydrates and lipids (2.2), proteins (2.3), water (2.4) and the tests for biological molecules (2.1). Rotate the 3D models with drag, zoom with the wheel or pinch, and step through every reaction one bond at a time.
Glucose is a hexose that closes into a six-membered ring. The only difference between α-glucose and β-glucose is which way the OH group on carbon 1 points, and that one detail decides whether a chain becomes starch or cellulose. Ribose is the pentose in RNA.
Every large biological molecule is built the same way: two units join with the loss of a water molecule, and are split apart by adding one back. Step through the three bond types the syllabus needs: glycosidic, ester and peptide.
All three are polymers of glucose, yet one is a coiled energy store, one is a heavily branched energy store, and one is a straight structural fibre. The shape of the chain comes straight from the α or β form of the monomer and from where the bonds form.
A fatty acid is a hydrocarbon tail with a carboxyl head. One C=C double bond puts a kink in the tail. Swap one fatty acid for a phosphate group and the molecule has a water-loving head and water-hating tails, which is why phospholipids line up into the bilayer of every membrane.
Twenty amino acids that differ only in their R group, joined by peptide bonds into a chain. The chain then folds in stages. Step up through the four levels of structure, then compare a globular protein with a fibrous one.
A small, bent, polar molecule that hydrogen-bonds to its neighbours. Almost every property that makes water fit for life follows from those hydrogen bonds: it is a solvent, it resists temperature change, it is cohesive, and ice floats.
| property | because | matters for |
|---|---|---|
| solvent | polar molecules surround ions and polar solutes | transport in blood and xylem; reactions in solution |
| high specific heat capacity | energy goes into breaking hydrogen bonds | stable temperatures inside cells and in lakes |
| high latent heat of vaporisation | many hydrogen bonds to break to escape as gas | sweating and transpiration cool effectively |
| cohesion | molecules pull on each other | continuous columns in xylem; surface tension |
| ice is less dense | open hydrogen-bonded lattice | ice floats and insulates the water below |
Choose a sample and a test, then watch the tube. Benedict's is semi-quantitative: the colour tells you roughly how much reducing sugar there is. For a non-reducing sugar such as sucrose you must hydrolyse it first. Try the mystery samples and work out what they contain.