(a) Describe and explain how a protein molecule that is eaten becomes molecules of urea that are excreted.

Mark scheme

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

1. [1] digested / broken down /hydrolysis;
2. [1] into amino acids ;
3. [1] by protease or named enzyme ;
4. [1] absorbed in small intestine, duodenum / ileum ;
5. [1] in villi / by capillaries / blood ;
6. [1] hepatic portal vein ;
7. [1] to liver;
8. [1] deamination or described ;
9. [1] urea + in renal artery;
10. [1] urea + taken to kidney ;
11. [1] blood filtered / urea leaves blood;
12. [1] forms urine;
13. [1] bladder / urethra ;
14. [1] photosynthesis ;
15. [1] uses up CO2 ;
16. [1] faster than is made during respiration;