Discussion
It is known that aquatic animals excrete ammonia and that terrestrial organisms largely do not. When animals catabolize nitrogen-containing molecules for energy or to convert them into carbohydrates or lipids, they produce nitrogen-containing by-products called nitrogenous wastes, which must be eliminated from the body. These are formed form nitrogen-containing molecules for example amino acids. The first step in this process is deamination (removal of NH2 group) and its combination with H+ to form ammonia in the liver. Ammonia is quite toxic to cells because the NH4+ ion interferes with oxidative phosphorylation. It can therefore only be transported and excreted in large volumes of very dilute solutions. Aquatic animals are therefore able to excrete ammonia because they have access to large amounts of water. Ammonia is highly soluble and therefore easily passes through membranes and is readily lost by diffusion into the surrounding water. In many invertebrates, ammonia release occurs across the whole body surface. In fish it is lost mainy as NH4+ across the epithelium of the gills. However terrestrial animals very often simply do not have access to sufficient water to routinely excrete ammonia. If they tried to excrete ammonia, it would build up in their cells and kill them, eventually leading to the death of the organism. Terrestrial organisms therefore mainly excrete urea, which is less toxic and can therefore be stored and transported safely. Other terrestrial organisms such as birds and many reptiles excrete uric acid. Another aspect to consider is water loss. Animals which need to limit water loss, for example desert organisms, cannot afford to excrete ammonia as it takes far too much water to do this safely.
It can be seen that the rate of ammonia excretion is therefore affected by the diet of the organism. In this experiment, one group of crayfish were fed bloodworms. This is a high-protein diet. These crayfish would therefore catabolize...