A level Biology exam revision resources written by A level Examiners


In humans nitrogen containing waste is removed from the body in the form of urea. Most waste, containing nitrogen, is generated from excess amino acids. The conversion of excess amino acids to urea takes place in the liver and occurs in stages.

The first stage is the deamination of the excess amino acids. This involves the removal of the amino group and is shown in the equations in figure 1, below.

Figure 1 : Deamination of the excess amino acids

Exam Advice

The organic acid molecule that is generated by deamination can be used in respiration.  Exam Advice

The second stage is converting ammonia to urea. It is necessary to quickly convert ammonia because ammonia is a toxic molecule. The ammonia is taken up by an enzyme-controlled cycle of reactions called the ornithine cycle. This cycle converts the ammonia into urea, which is less toxic. Figure 2, below, shows the operation of the ornithine cycle.

Figure 2 : Overview of the Ornithine Cycle

Exam Advice

The urea generated is transported to the kidneys by the bloodstream.

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