Question

how does the way a buffer stabilizes ph during addition of acid differ from the way the same buffer stabilizes ph during addition of base? in one case it adds h+; in the other case it adds oh-. the same buffer cant work for both acid and base. in one case the buffer is strong; in the other case its weak. its the same reaction running backward or forward. part e which answer helps to explain why all living cells need ph buffers? atp will not deliver energy if it is ionized. nucleic acids must have positive charges to form double helices. amino acid side chains have many carboxyl and amino groups. hydrogen bonds only form at medium ph values.

Explanation:

For the first question, a buffer contains a weak - acid/weak - base conjugate pair. When an acid is added, the conjugate base of the buffer reacts with the added $H^+$; when a base is added, the weak acid of the buffer reacts with the added $OH^-$. It's the same equilibrium reaction shifting in different directions. For the second question, amino acid side - chains have carboxyl ($-COOH$) and amino ($-NH_2$) groups which are sensitive to pH changes. pH buffers help maintain an appropriate pH so that these functional groups can function properly.

Answer:

First question: It's the same reaction running backward or forward.
Second question: Amino acid side chains have many carboxyl and amino groups.