Acidic and Basic Conditions Affect Living Organisms

Rucete ✏ Campbell Biology In a Nutshell

Understanding how acids and bases behave in aqueous solutions is fundamental to biology. This section explores how even the smallest fluctuations in pH can dramatically influence life—both at the cellular level and across ecosystems.

Unit 1 THE CHEMISTRY OF LIFE

Concept 3.3 Acidic and Basic Conditions Affect Living Organisms

1. Acids, Bases, and Hydrogen Ions

In water, acids donate hydrogen ions (H⁺) while bases reduce H⁺ concentration, either by accepting H⁺ or releasing hydroxide ions (OH⁻). For example:

• Strong Acid: HCl → H⁺ + Cl⁻

• Strong Base: NaOH → Na⁺ + OH⁻

• Weak Base: NH₃ + H⁺ ⇌ NH₄⁺

• Weak Acid: H₂CO₃ ⇌ HCO₃⁻ + H⁺

Strong acids and bases dissociate completely in water, while weak ones exist in a dynamic equilibrium.

2. The pH Scale

The pH scale compresses an enormous range of H⁺ concentrations into a simple 0–14 scale:

$$\text{pH}=-\log [{\text{H}}^{+}]$$

• pH < 7 → Acidic

• pH = 7 → Neutral

• pH > 7 → Basic

Each step on the scale represents a tenfold difference in hydrogen ion concentration. So, pH 3 is not "twice" but 1,000 times more acidic than pH 6.

3. Buffers: Nature’s pH Stabilizers

Life is pH-sensitive. Human blood, for instance, must stay close to pH 7.4. Buffers maintain stability by reversibly absorbing or donating H⁺ ions.

Example: The Carbonic Acid–Bicarbonate Buffer

$${\text{H}}_2{\text{CO}}_3\rightleftharpoons {\text{HCO}}_3^{-}+{\text{H}}^{+}$$

• If blood becomes too basic (low H⁺), carbonic acid donates H⁺.

• If too acidic (high H⁺), bicarbonate removes H⁺.

This equilibrium keeps our internal environment safe—even against sudden acid/base changes.

4. Ocean Acidification: A Global Buffer Crisis

Burning fossil fuels releases CO₂, and oceans absorb ~25% of it. CO₂ reacts with seawater:

$${\text{CO}}_2+{\text{H}}_2\text{O}\to {\text{H}}_2{\text{CO}}_3\to {\text{H}}^{+}+{\text{HCO}}_3^{-}$$

This added H⁺ lowers ocean pH (acidification) and reduces carbonate ions (CO₃²⁻) essential for calcification, the process marine organisms use to build shells and reefs:

$${\text{Ca}}^{2+}+{\text{CO}}_3^{2-}\to {\text{CaCO}}_3$$

If CO₃²⁻ drops, organisms like corals can’t form skeletons properly. Studies predict up to a 40% decrease in carbonate ion availability by 2100—endangering entire marine ecosystems.

In a Nutshell

• Acids increase [H⁺], bases decrease it.

• The pH scale measures this, with each step a 10x change in acidity.

• Buffers, like the carbonic acid–bicarbonate system, maintain biological pH stability.

• Ocean acidification, driven by CO₂, disrupts marine life by lowering pH and harming calcifying organisms.

Even subtle pH changes ripple through cells, systems, and entire ecosystems. Understanding this chemistry isn’t just academic—it’s ecological survival.