For Each of the Following Sentences, Fill in the Blanks

For each of the following sentences, fill in the blanks with the best word or phrase selected from the list below.Not all words or phrases will be used; each word or phrase should be used only once.
$\begin{array}{l}\begin{array}{lll}\text { amino } & \text { ionized } & \text { polypeptides } \\\alpha \text {-carbon } & \text { length } & \text { protein } \\\text { carbon } & \text { noncovalent } & \text { R group } \\\text { carboxyl } & \text { peptide bonds } & \text { side chains }\end{array}\\\text { hydroxide }\end{array}$

Proteins are __________ built from amino acids, which each have an amino group and a __________ group attached to the central __________.There are 20 possible __________ that differ in structure and are generally referred to as "R." In solutions of neutral pH, amino acids are __________, carrying both a positive and a negative charge.When a protein is made, amino acids are linked together through __________, which are formed by condensation reactions between the carboxyl end of the last amino acid and the __________ end of the next amino acid to be added to the growing chain.