Water, the Elements of Life and Condensation

Polarity: oxygen is much more electronegative than hydrogen, so water has a partial negative charge $\delta^-$ on O and partial positive charges $\delta^+$ on each H. The molecule is bent (about 104.5°), so the charges do not cancel.

• A hydrogen bond is the attraction between a hydrogen atom that is covalently bonded to O or N (or F) and a lone pair on an O or N atom of another molecule (or a distant part of the same large molecule).
• It is weak (roughly twenty times weaker than the O-H covalent bond) and temporary: in liquid water hydrogen bonds break and re-form all the time.
• One water molecule can hydrogen bond to up to four neighbours.
• Hydrogen bonds also hold the two strands of DNA together and hold the secondary structure of proteins.

• Solvent: water dissolves ions and polar molecules (they are hydrophilic); non-polar molecules such as fats are hydrophobic and do not dissolve. Most reactions in cells happen in solution.
• High specific heat capacity (about $4.2\ \text{J g}^{-1}\ ^\circ\text{C}^{-1}$): a lot of energy is needed to raise its temperature, so the body and lakes stay at a steady temperature.
• High latent heat of vaporisation: evaporating a little water removes a lot of heat, so sweating and transpiration cool.
• Cohesion (water to water) gives high surface tension and lets columns of water be pulled up the xylem; adhesion (water to other surfaces) gives capillary rise.
• Density: water is densest at about 4 °C, and ice is less dense than liquid water, so ice floats and insulates the water beneath it.

• Carbohydrates contain C, H and O only (general formula of a simple sugar $\mathrm{(CH_2O)}_n$). Many hydroxyl (OH) groups.
• Lipids contain C, H and O, with much less O than carbohydrates. Phospholipids add P (and often N).
• Proteins contain C, H, O and N, and usually S (in the amino acids cysteine and methionine).
• Nucleic acids contain C, H, O, N and P.
• Key groups: carboxyl $\mathrm{-COOH}$ (acidic) in every amino acid and every fatty acid; amino $\mathrm{-NH_2}$ in every amino acid; hydroxyl $\mathrm{-OH}$: many on sugars, three on glycerol.
• Water (H and O) is not a biomolecule polymer, but it takes part in building and breaking all of them.

• Condensation: two molecules join by a covalent bond and release one water molecule.
• Hydrolysis: a covalent bond is broken by adding one water molecule. It is the reverse of condensation.
• A chain of $n$ units has $n-1$ bonds, so making it releases $n-1$ water molecules, and breaking it completely uses $n-1$.
• A triglyceride is the exception to the chain rule: one glycerol with three fatty acids makes three ester bonds, so three water molecules.
• Digestion is hydrolysis; building starch, proteins and DNA in a cell is condensation.