The characteristics of an organic molecule depend not only on its carbon skeleton, but also on groups of atoms attached to the skeleton that are involved in chemical reactions. These groups of atoms are called functional groups. This is an organic molecule called a tripeptide. Let's review the characteristics of the tripeptide’s functional groups, beginning with the phosphate group. The electronegative oxygens of the phosphate group attract electrons to themselves, and as a result the phosphate group acts as an acid, losing hydrogen ions to the surrounding solution. This dissociation leaves the phosphate group with two negative charges. The phosphate group is important in ATP and the transfer of energy between organic molecules. There are two carbonyl groups, C double bond O, in this tripeptide. The carbonyl group is polar because oxygen has a strong attraction for electrons. If a carbonyl group is at the end of a carbon skeleton, the molecule is called an aldehyde; if a carbonyl group is within a carbon skeleton, the molecule is called a ketone, as shown here. Next is the sulfhydryl group, -SH. Molecules termed thiols have a sulfhydryl group. Two SH groups can react, forming a cross-link that helps stabilize the structure of many proteins. The two electronegative oxygens of the carboxyl group, -COOH, pull electrons away from the hydrogen atom. This weakens the bond between oxygen and hydrogen, and the hydrogen atom tends to dissociate from the molecule as a hydrogen ion (H+). Because the carboxyl group donates hydrogen ions, this group is acidic, and molecules that contain these groups are known as carboxylic acids. The hydroxyl group, -OH, is polar because the electronegative oxygen has a strong attraction for electrons. Water is attracted to the hydroxyl group, and therefore molecules that have the hydroxyl group, such as sugars, tend to dissolve in water. Alcohols have a hydroxyl group. Finally, the slightly electronegative nitrogen atom in the amino group, -NH2, tends to pick up hydrogen ions from the surrounding solution, and thus the amino group acts as a base. Because of the added hydrogen ion, the amino group usually has a plus one charge in the cell, NH3+. The amino group is characteristic of organic molecules called amines.