AQA A-Level Biology

## Monomers and polymers Living things are built from a small number of carbon-based molecules, and the same few building blocks turn up in every organism. A **monomer** is a small unit that can be joined to many others, and a **polymer** is the long molecule made when a large number of monomers are joined in a chain. Monosaccharides, amino acids and nucleotides are the three monomers you meet across the course. Two reactions make and break these chains. - A **condensation reaction** joins two molecules by forming a new chemical bond, and a molecule of water is released as it happens. - A **hydrolysis reaction** breaks a bond by using up a molecule of water, which is added across the bond. $$\ce{C6H12O6 + C6H12O6 <=> C12H22O11 + H2O}$$ Here two glucose molecules form maltose. The reaction runs to the right by condensation and to the left by hydrolysis. Each type of bond in the course has its own name (glycosidic, ester, peptide, phosphodiester), but the chemistry of making and breaking it is the same. ## Monosaccharides Monosaccharides are single sugar units. **Glucose**, **galactose** and **fructose** are the three to know, and each has the formula $\ce{C6H12O6}$, so they are isomers of one another: the same atoms arranged differently. Glucose exists in two forms, **α-glucose** and **β-glucose**. Both are six-carbon rings, and they differ only in the position of the hydroxyl group on carbon 1. In α-glucose the $\ce{-OH}$ on carbon 1 sits below the plane of the ring, and in…