Unit 1: Chemistry of Life

Unit 1 is the chemistry that the rest of the course is built on. Expect it to come back in almost every other unit: enzymes in Unit 3, membranes in Unit 2, DNA in Unit 6.

Water and hydrogen bonding

Water is a polar molecule: oxygen is more electronegative than hydrogen, so the oxygen end carries a partial negative charge and the hydrogen ends carry partial positive charges. Neighbouring water molecules are held together by hydrogen bonds.

Hydrogen bonding explains the properties you need to know:

• Cohesion and adhesion: water molecules stick to each other and to other polar surfaces, which drives transpiration in xylem.
• High specific heat: a lot of energy is needed to break hydrogen bonds, so water resists temperature change.
• Ice floats: hydrogen bonds hold ice in an open lattice that is less dense than liquid water.
• Universal solvent: water dissolves ions and other polar molecules.

On the exam, name the property and link it to hydrogen bonding. "Water has high cohesion" earns less than "hydrogen bonds between water molecules cause cohesion, which pulls a continuous column of water up the xylem."

The four macromolecules

Macromolecule	Monomer	Bond	Main elements
Carbohydrates	Monosaccharides	Glycosidic	C, H, O
Lipids	(not true polymers)	Ester	C, H, O (phospholipids also P)
Proteins	Amino acids	Peptide	C, H, O, N (some S)
Nucleic acids	Nucleotides	Phosphodiester	C, H, O, N, P

Polymers are built by dehydration synthesis (a water molecule is removed) and broken down by hydrolysis (a water molecule is added).

Structure determines function

Protein shape comes from four levels of structure. The R-groups of the amino acids decide how the chain folds: hydrophobic R-groups cluster inside, charged and polar R-groups interact with water and with each other. Change one amino acid and the shape, and therefore the function, can change.