Respiration is a reaction that takes place in living cells to release energy. The overall reaction involves the oxidation of glucose. C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O The reaction is exothermic and is catalysed by enzymes. (a) Draw an energy profile diagram for respiration using the axes shown. [Blank axes are provided in the question paper.] Label • the axes, • the enthalpy change, • the reactants and products.

Mark scheme

Marking points from the official mark scheme (May/June 2017). The mark or mark type is shown in brackets.

1. A6(a) Energy / enthalpy on vertical axis AND progress of reaction / course of reaction on horizontal axis (1) Reactant level above product level and to the left of product AND reactants and products labelled (1) Enthalpy change shown by downward arrow AND labelled enthalpy change or ∆H (1)
2. A6(b) 1 mark each for any two of: • Lower activation energy (1) • More particles have energy equal to / greater than the activation energy (1) • Different pathway / different mechanism / via an enzyme complex (1) • more successful collisions (between groups on enzyme and substrates) / number of effective collisions increase (with specific groups on enzyme surface) (1)
3. A6(c) Idea that combustion AND respiration increase levels of carbon dioxide / carbon in the atmosphere (1) Idea that photosynthesis reduces levels of carbon dioxide / carbon in the atmosphere (1) Idea that these processes balance each other (1)