Carboxyl groups have an electronegative oxygen atom double bonded to a carbon atom. This carbon-oxygen bond is very polar and the fact that its a double bond increases the polarity of the bond.
PEA agar, also referred to as Phenylethyl Alcohol Agar is a medium  used by microbiologists to grow microorganisms. The ingredient that  supplies nitrogen is casein. Making the world better, one answer at a time.
1 //used to make fertilisers / added to soil / leakage of ions into river. 2  By Nitrogen-fixing bacteria by anaerobic nitrogenase. 3  Live free in soil/nodules on the roots of legumes (peas, beans). 4  NH4+ is assimilated by legumes into amino acids.