Urea is formed in liver cells from
(a) Ammonia and nitrogen
(b) Ammonia and carbon dioxide
(c) Ammonia, carbon dioxide and aspartic acid
(d) Ammonia and carbon monoxide

Options

A. Ammonia and nitrogen
B. Ammonia and carbon dioxide
C. Ammonia, carbon dioxide and aspartic acid
D. Ammonia and carbon monoxide

Answer

To solve the question regarding the formation of urea in liver cells, we can break down the information step-by-step:

### Step-by-Step Solution:

1. **Understanding Urea Formation**: Urea is primarily formed in the liver cells (hepatocytes) through a specific metabolic cycle known as the ornithine cycle or urea cycle.

2. **Deamination Process**: The first step in the formation of urea involves the breakdown of excess proteins in the body through a process called deamination. During deamination, the amino groups (NH2) are removed from amino acids.

3. **Formation of Ammonium Ions**: The amino groups that are removed during deamination are converted into ammonium ions (NH4+). This is a critical step because ammonia is toxic to the body and needs to be converted into a less toxic form.

4. **Combination with Carbon Dioxide**: The ammonium ions then combine with carbon dioxide (CO2) in the liver. This reaction is catalyzed by the enzyme carbamoyl phosphate synthetase, which is essential for the urea synthesis process.

5. **Conclusion**: The combination of ammonium ions and carbon dioxide leads to the formation of urea. Therefore, the correct answer to the question is option (b) Ammonia and carbon dioxide.

### Final Answer:
(b) Ammonia and carbon dioxide

---