---
title: "Transamination, Ammonia and the Urea Cycle"
description: "Amino groups are moved onto keto acids rather than released, and only at the end is nitrogen freed as ammonia, which is toxic. The liver converts it into urea in a cycle that costs ATP, and the carbon"
canonical: https://lightmysky.com/learn/science/transamination-ammonia-and-the-urea-cycle-mt_MFu5uQKyFL
source: https://lightmysky.com/learn/science/transamination-ammonia-and-the-urea-cycle-mt_MFu5uQKyFL.md
retrieved: 2026-09-12
---

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# Transamination, Ammonia and the Urea Cycle

Amino groups are moved onto keto acids rather than released, and only at the end is nitrogen freed as ammonia, which is toxic. The liver converts it into urea in a cycle that costs ATP, and the carbon skeletons left behind enter metabolism elsewhere.

Subject: Science · Area: Biochemistry & Molecular Biology · Ages 21 to 22
Page: https://lightmysky.com/learn/science/transamination-ammonia-and-the-urea-cycle-mt_MFu5uQKyFL

## Ready when they can

- Traces an amino group from an amino acid to urea, naming what happens at each transfer.
- Explains why the cell moves amino groups around instead of releasing ammonia immediately.
- Says what becomes of the carbon skeleton after the amino group is removed.

## Lesson: Passing nitrogen safely to urea

Cells never let nitrogen travel alone. First, amino acids hand their amino groups to alpha ketoglutarate in a move called transamination, forming glutamate. Each transfer keeps the nitrogen bound to a carrier, so toxic free ammonia stays out of circulation from the very start.

Only in the liver is nitrogen finally freed. Glutamate releases ammonia there, and the urea cycle captures it into urea, a safe soluble carrier worth its ATP price. Mammals ship urea in blood and leave it in urine. Birds and reptiles instead pay a higher energy price for uric acid, which saves water.

**Example.** Picture the carbon skeleton left behind after the amino group departs. It becomes pyruvate, acetyl CoA, or a cycle intermediate, and the body either burns it for fuel or rebuilds it into something new. Nothing is thrown away: the nitrogen goes to urea, and the carbon rejoins metabolism elsewhere.

**Tip.** Trace any amino group in three hops: transamination onto glutamate, ammonia release in the liver, capture by the urea cycle.

**Recap.** Nitrogen rides carriers from amino acid to urea because free ammonia is poison.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [The Pentose Phosphate Pathway and Reducing Power for Biosynthesis](https://lightmysky.com/learn/science/the-pentose-phosphate-pathway-and-reducing-power-for-biosynthesis-mt_2p-JnaEaTO)
- [Osmoregulation and the Action of ADH](https://lightmysky.com/learn/science/osmoregulation-and-the-action-of-adh-mt_Rz_MmL-Pa-)

## Opens up

- [Flux Control and Following Atoms with Labelled Substrates](https://lightmysky.com/learn/science/flux-control-and-following-atoms-with-labelled-substrates-mt_PzhsDwD_aW)