UNIT 1: pH, Acids, Bases, and Buffers in Biology

Study Guide - Smart Notes

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pH and Its Biological Importance

Definition and Measurement of pH

The concept of pH is fundamental in biology, as it describes the concentration of hydrogen ions (H+) in a solution. The pH scale is a negative logarithmic scale used to quantify the acidity or basicity of a solution.

pH is defined as the negative logarithm (base 10) of the hydrogen ion concentration:

Acid and base are terms used to describe solutions based on their H+ concentration.
The pH scale typically ranges from 0 (very acidic) to 14 (very basic), with 7 being neutral.
Each unit change in pH represents a tenfold change in H+ concentration.

Example: A solution with [H+] = 1 × 10-7 M has a pH of 7 (neutral).

Water and Ionization

Water (H2O) can dissociate into hydrogen ions and hydroxide ions (OH-), a process that is essential for understanding pH.

At neutral pH (7), the concentration of H+ ions equals the concentration of OH- ions.
Most water molecules remain intact, but a small fraction dissociates into ions.

Example: In pure water at 25°C, [H+] = [OH-] = 1 × 10-7 M.

Acids, Bases, and Their Biological Effects

Acids and Bases: Definitions and Properties

Acids are substances that increase the concentration of H+ ions in a solution. They are also called proton donors.
When acids dissolve in water, they release H+ ions, which can react with OH- ions.
Bases are substances that decrease the concentration of H+ ions, often by increasing OH- ions. They are called proton acceptors.
Bases can react with H+ ions, reducing their concentration in solution.

Example: Hydrochloric acid (HCl) is a strong acid; sodium hydroxide (NaOH) is a strong base.

Changes in Ion Levels and Biological Impact

As H+ concentration increases, pH decreases (solution becomes more acidic).
As OH- concentration increases, pH increases (solution becomes more basic).
Even small changes in pH can have dramatic effects on living organisms, as pH influences the shape and function of biological molecules (such as proteins and enzymes).

Example: Enzyme activity is highly sensitive to pH; deviations can denature proteins and disrupt cellular processes.

pH in Biological Systems

pH of Body Fluids

The pH of most body fluids is tightly regulated to maintain proper physiological function.

Common body fluids with regulated pH include blood, tears, saliva, and vaginal fluid.
Acids and bases are present in the body; for example, stomach acid (HCl) and bicarbonate (HCO3-).

Example: Blood pH is normally maintained around 7.4; deviations can be life-threatening.

Acid-Base Balance and Buffer Systems

Biological systems use buffers to minimize changes in pH and maintain homeostasis.

A buffer consists of a weak acid and its corresponding base.
Buffers absorb excess H+ or OH- ions to stabilize pH.
In the human body, the bicarbonate buffer system is a key example:

Example: The bicarbonate buffer system helps maintain blood pH despite metabolic or respiratory changes.

Environmental and Physiological Relevance

Ocean acidification is a symptom of climate change, caused by increased CO2 dissolving in seawater and forming carbonic acid, which lowers ocean pH.
Coral bleaching is linked to decreased pH, which harms marine organisms.

Example: Lowered ocean pH can kill corals and disrupt marine ecosystems.

Summary Table: Acids, Bases, and Buffers

Term	Definition	Effect on [H+]	Example
Acid	Proton donor; increases [H+]	Increases	HCl (hydrochloric acid)
Base	Proton acceptor; decreases [H+]	Decreases	NaOH (sodium hydroxide)
Buffer	Mixture of weak acid and base that resists pH changes	Stabilizes	Bicarbonate buffer system

Additional info: The notes reference the importance of buffers in various body fluids (blood, tears, saliva, vaginal fluid) and the environmental impact of pH changes, such as ocean acidification and coral bleaching. These are key applications of acid-base chemistry in biology.