The nitrogen cycle makes nitrogen available for the production of proteins in plants. These plants may then be eaten by humans and the proteins digested. (a) Outline the roles of bacteria in the nitrogen cycle.

Mark scheme

Marking points from the official mark scheme (May/June 2020). The mark or mark type is shown in brackets.

1. [1] nitrogen fixation ;
2. [1] nitrogen + gas / in air / atmosphere ;
3. [1] to ammonium ;
4. [1] decomposers / decomposition ;
5. [1] breakdown + of organic compounds or named ;
6. [1] nitrifying / nitrification ;
7. [1] ammonium + to nitrites / nitrates;
8. [1] denitrifying / denitrification ;
9. [1] nitrates + to nitrogen + gas / in air / in atmosphere ;
10. [1] protease;
11. [1] stomach;
12. [1] pepsin;
13. [1] small intestine ;
14. [1] trypsin;
15. [1] duodenum;
16. [1] to amino acids ;
17. [1] reference to a pH linked to correct location ;