Excess amino acids are broken down in liver cells to form molecules of urea.

Mark scheme

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

1. [1] transported in blood / plasma ;
2. [1] dissolved;
3. [1] hepatic vein to vena cava / heart;
4. [1] renal artery (to kidney);
5. [1] to kidney;
6. [1] urea removed / filtered + from blood;
7. [1] part of urine;
8. [1] ureter (to bladder) ;
9. [1] bladder;
10. [1] (bladder to) urethra ;
11. [1] cycling / replenishing nitrogen ;
12. [1] used / converted by nitrifying bacteria / nitrogen fixing bacteria ;
13. [1] form + nitrates ;
14. [1] nitrates + taken up by plants ;
15. [1] production of proteins / amino acids ;
16. [1] increased soil fertility (for crops) / plants grow faster/bigger/AW ;
17. [1] animals / herbivores obtain protein / nitrogen from plants ;