# Explain why the concentration of urea in the blood plasma leaving the liver is higher than the concentration of urea in the blood plasma entering the liver.

> OCR A-Level Biology A (H420) — 5.2 Excretion · Explain · 4 marks

> Urea is produced in the liver from the breakdown of excess amino acids. It is transported in the blood to the kidneys, where it is filtered and excreted in urine. A patient with liver disease may show abnormally low blood urea levels despite consuming a high-protein diet.

## Mark scheme (4 marks)

1. Amino acids cannot be stored, so excess amino acids must be broken down
2. The amino group is removed from excess amino acids by deamination
3. Deamination produces ammonia, which is toxic
4. Ammonia is converted to urea in the liver via the ornithine cycle, so blood leaving the liver contains more urea

## Key terms

- [urea](https://www.gradenine.co.uk/glossary/urea)

## Related

- [Revision notes for OCR A-Level Biology A (H420)](https://www.gradenine.co.uk/learn)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-the-concentration-of-urea-4d304bfc) · Published by Druglandscape Ltd.