Urea

In humans, excess protein is broken down, leading to the formation of urea. Urea is a waste substance that is excreted in urine. Together, these examples show why metabolism is not a single reaction: it is a connected set of synthesis and breakdown reactions.

The liver produces urea, a waste substance, from this nitrogen-containing material. The key link is: excess amino acids → breakdown in the liver → urea.

Urea is a different kind of substance to transport: it is a waste product, carried in the blood to the kidneys and removed in urine. Transport therefore serves both delivery and disposal.

Substances such as glucose and urea can be carried while dissolved in it.