Buffers in biology: What is the defining behavior of a buffer solution when small amounts of acid or base are added?

Introduction / Context:
Buffers are ubiquitous in cells, blood, fermentation broths, and analytical chemistry. They stabilize pH against additions of acids or bases, which is critical for enzyme activity, protein stability, and metabolic control.

Given Data / Assumptions:

• Buffer contains a weak acid and its conjugate base (or weak base and its conjugate acid).
• Small perturbations in H+ or OH– are expected.
• System is within the buffer’s effective pH range (pKa ± 1).

Concept / Approach:
By Le Châtelier's principle, added H+ is consumed by base; added OH– is neutralized by acid. The Henderson–Hasselbalch relation shows pH depends on the ratio base/acid; modest changes to this ratio produce small pH shifts, not large jumps.

Step-by-Step Solution:
Start with HA ⇌ H+ + A– (or BH+ ⇌ B + H+). Add acid → A– buffers it; add base → HA buffers it. Observe that pH remains comparatively stable within capacity limits. Hence, buffer behavior is to maintain a relatively constant pH.

Verification / Alternative check:
Blood bicarbonate and phosphate systems maintain physiological pH with minimal fluctuation despite metabolic acid/base loads.

Why Other Options Are Wrong:
A buffer need not be pH 7 (option A); buffers are common in life (option B false); option C contradicts the definition; option E is not defining behavior.

Common Pitfalls:
Exceeding buffer capacity or operating far from pKa; confusing strong acid/base solutions with buffers.

Final Answer:
It tends to maintain a relatively constant pH.