Outline the bonding between DNA nucleotides.

Explain how chemical bonding between water molecules makes water a valuable coolant in living organisms.

State a word equation for anaerobic cell respiration in humans.

hydrogen bonds between nucleotides on opposite strands/ complementary bases/adenine and thymine and cytosine and guanine;
(reject letters instead of base names)
covalent bonds between nucleotides within strands/between sugar/deoxyribose and phosphate;

hydrogen bonding between water molecules;
breaking (hydrogen bonds) needs/removes energy/heat;
hydrogen bonds must break when water evaporates/vaporizes;

pyruvate/pyruvic acid →lactate/lactic acid;
glucose → (pyruvate/pyruvic acid) → lactate/lactic acid;
Accept correct chemical equation with formulae.

Almost all candidates knew something about the bonding between nucleotides but in some cases the answers were not precise enough to be awarded marks. A common fault was to describe bonding within nucleotides rather than between nucleotides.

Many candidates knew that hydrogen bonds form between water molecules, but often the remainder of the answer was weak. A common misunderstanding was revealed by candidates who stating that hydrogen bonds are strong and therefore take large amounts of energy to break. It should be stressed that individual hydrogen bonds are in fact weak, but because water molecules are small, large numbers of hydrogen bonds are formed within water so collectively they have significant effects. Another area of misunderstanding was the difference between the energy needed to heat up water (heat capacity) and the energy needed to evaporate water. Sweating and transpiration have cooling effects because of the energy needed for evaporation, not raising the temperature of water.

Less than half of candidates were able to give an equation for anaerobic cell respiration in humans. Many included oxygen, carbon dioxide or water in their equation and so were not awarded the mark. Others gave an equation for yeast rather than humans. In some cases the answer was not given in the form of an equation but as long as the substrate and product was correct, the mark was awarded. Both pyruvate and glucose were accepted as the substrate, though glucose was preferable.