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.
Nitrogen is one of the most important chemical elements for plants. If there is not enough nitrogen available in the soil plants look pale and their growth is stunted. Nitrogen fixing plants are called legumes. Legumes - and all peas and beans are legumes - are plants that work together with nitrogen fixing bacteria called Rhizobia, to fix nitrogen. The Rhizobia chemically convert the nitrogen from the air to make it available for the plant. Legume plants live in a symbiotic relationship with the nitrogen fixing bacteria - the Rhizobia live in nodules in the plant's roots. This way the plant can look after its own nitrogen needs. Fertilizer is not required.