All organisms respire to release energy from energy-rich molecules such as glucose. Some factory workers in the early 20th century were exposed to chemical X and experienced serious side-effects. Some of the effects of exposure to chemical X are: • decreased production of ATP • increased lipid metabolism, with weight loss • increased production of pyruvate and lactate • excess heat energy release, causing an increase in body temperature, which can be fatal. Chemical X increases the permeability of the inner mitochondrial membrane to protons (hydrogen ions), causing some protons to leak out into the matrix.

Mark scheme

Marking points from the official mark scheme (May/June 2021). The mark or mark type is shown in brackets.

1. [1] idea of synthesis from smaller units
2. [1] DNA replication / transcription;
3. [1] to, make / build up, (named) protein;
4. [1] to, make / build up, (named), lipid / triglyceride / phospholipid ;
5. [1] to, make / build up, (named) carbohydrate / polysaccharide ;
6. [1] phosphorylation ;
7. [1] phosphorylates glucose or glucose → glucose / fructose / hexose, phosphate ;
8. [1] second phosphorylation or glucose / fructose / hexose, phosphate → fructose / hexose, bi(s)phosphate ;
9. [1] so glucose cannot leave the cell;
10. [1] activates/ increases energy of/ decreases stability of, glucose ;
11. [1] smaller/less steep, proton gradient ;
12. [1] few(er) protons go through, ATP synth(et)ase / stalked particle ;
13. [1] use/burn / respire, body fat / lipid stores ;
14. [1] as less ATP made from, glucose / food;
15. [1] more / faster/increased, (rate of) glycolysis ;
16. [1] oxygen, decreased / used faster / shortage;
17. [1] (so) more / increased, anaerobic respiration ;
18. [1] energy from, reduced NAD / ETC, released as heat ;
19. [1] energy not used to join ADP and Pi so released as heat ;