Many solutes that must move into and out of cells exist in either a protonated or ionized form, or they have functional groups that can be either protonated or ionized. Simple molecules such as CO2CO2, H3PO4H3PO4 (phosphoric acid), and NH3NH3 (ammonia) are in this category, as are organic molecules with carboxylic acid groups, phosphate groups, and/or amino groups.

Instead of ammonia, consider the uptake of acetic acid. CH3COOH, an important intermediate in several biological pathways. What is the charged form in this case?