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 form Plants have the ability to take up several chemical forms of nitrogen. The most common are: ammonium (NH 4+), which has a positive charge; nitrate (NO 3-), which has a negative charge; and urea, ((NH 2)2CO), which has no charge.any commercial fertilizer mixes contain a combination of all three nitrogen forms. When these nitrogen forms are added to the growing medium natural processes can take place that convert one form to another (Figure 1). Bacteria can convert urea to ammonium or ammonium to nitrate.