Isolated mitochondria were used to investigate respiration. • Mitochondria were extracted from respiring mammalian cells and incubated in a buffer solution. • Pyruvate and inorganic phosphate (Pi) were added at time zero. • ADP was added one minute later. • The oxygen concentration of the buffer solution containing mitochondria was monitored throughout the investigation. The results of the investigation are shown in [Figure 7.1].

Mark scheme

Marking points from the official mark scheme (Feb/Mar 2019). The mark or mark type is shown in brackets.

1. [3] any three from: 1 because there is a faster rate of (aerobic) respiration (than in phase A); 2 (because) in phase B, ADP is present so oxygen concentration decreases faster ; 3 ADP is needed for ATP synthesis ; 4 oxygen used, as final electron acceptor / for oxidative phosphorylation / for aerobic respiration ;
2. [1] ADP / Pi / oxygen / pyruvate, becomes limiting / runs out ;
3. [5] any five from: 1 low concentration of oxygen in water; 2 ref. to aerenchyma / description ; 3 gases diffuses (through aerenchyma) down to root cells ; 4 allows aerobic respiration; 5 some leaves trap air underwater due to ridges on leaves ; 6 ethanol produced from respiration under anaerobic conditions ; A alcoholic fermentation 7 (root cells) can tolerate ethanol ; 8 (root cells) produce, alcohol / ethanol, dehydrogenase (to break down ethanol); 9 some varieties of rice have high rate of respiration under anaerobic conditions to generate more ATP ;