Is NH4+ Polar or Nonpolar?

Answer: NH4+ is a "nonpolar" molecule according to its tetrahedral structure since all of the hydrogen atoms are arranged symmetrically around the central nitrogen atom. However the molecule has more polar characteristics since it an ion with one electron missing from its central nitrogen atom. 

Ammonium is formed by the addition a hydrogen atom to ammonia (NH3). This forms a positive cation with a +1 charge on the nitrogen atom. As a result, NH4+ is weakly acidic, wishing to exchange a hydrogen with a lone pair of electrons to achieve a neutral charge. Ammonium is also capable of forming a wide variety of salts. However, some of these are noted to be quite explosive.

To learn more about the polarity and lewis dot structure of NH3, feel free to check out this article and this brief, respectively.

NH4+ Ball and Stick Model. Created with MolView.

What is the role of ammonium in biological systems?

One of the most recognizable functional groups within biology amine group (NH, NH2 or NH3). This is an important feature of amino acids, allowing primary structures to form. Moreover, ammonium ions are a byproduct of metabolism within the human body. While many species convert this product to urea for disposal, there are certain fish that release it directly into the surrounding water. NH4+ is also an important nitrogen source for plants; however, it is toxic at high concentrations and therefore cannot be relied upon solely.