Excretion

These chemical reactions produce waste products such as carbon dioxide, water, salts, urea and uric acid.

Oxygen is a byproduct generated during photosynthesis, and exits through stomata, root cell walls, and other routes.

It has been shown that the leaf acts as an 'excretophore' and, in addition to being a primary organ of photosynthesis, is also used as a method of excreting toxic wastes via diffusion.

[5] In animals, the main excretory products are carbon dioxide, ammonia (in ammoniotelics), urea (in ureotelics), uric acid (in uricotelics), guanine (in Arachnida), and creatine.

[6] Aquatic animals usually excrete ammonia directly into the external environment, as this compound has high solubility and there is ample water available for dilution.

In terrestrial animals, ammonia-like compounds are converted into other nitrogenous materials, i.e. urea, that are less harmful as there is less water in the environment and ammonia itself is toxic.

[citation needed] In insects, a system involving Malpighian tubules is used to excrete metabolic waste.