2.3 pH, buffers, and cellular homeostasis

pH and Buffers

pH Scale and Biological Significance

pH measures how acidic or basic a solution is by quantifying the concentration of hydrogen ions ($H^+$) in that solution. It's defined mathematically as:

$pH = -\log[H^+]$

Because this is a logarithmic scale, each single-unit change in pH represents a tenfold change in $H^+$ concentration. A solution at pH 6 has 10 times more $H^+$ than one at pH 7, and 100 times more than one at pH 8. This is why even small pH shifts can have outsized biological effects.

The scale runs from 0 to 14:

• Acidic (pH below 7): higher $H^+$ concentration
• Neutral (pH 7): pure water at 25°C
• Basic/alkaline (pH above 7): lower $H^+$ concentration

Almost every cellular process is pH-sensitive. Enzymes only work well within narrow pH ranges, and proteins can denature (lose their 3D shape and function) if pH strays too far from their optimum. DNA replication, protein synthesis, and metabolic pathways all depend on a stable pH environment. Nucleic acids, lipids, and other biomolecules are similarly affected by shifts in acidity.

Buffer Systems in Organisms

A buffer is a solution that resists changes in pH when small amounts of acid or base are added. Buffers are made of a weak acid paired with its conjugate base (or a weak base paired with its conjugate acid).

Here's how they work:

1. When acid is added (excess $H^+$): The conjugate base component of the buffer binds the extra $H^+$ ions, preventing pH from dropping significantly.
2. When base is added (excess $OH^-$): The weak acid component releases $H^+$ ions to neutralize the $OH^-$, preventing pH from rising significantly.

The result is that pH stays relatively stable despite the addition of acid or base.

Buffering capacity refers to how much acid or base a buffer can absorb before pH starts to change noticeably. This depends on the concentration of the buffer components and the strength of the acid or base being added. A more concentrated buffer can handle larger disturbances.

Your body uses several buffering systems simultaneously:

• Bicarbonate buffer system in blood (the most important one for blood pH)
• Phosphate buffer system in the cytoplasm of cells
• Protein buffers like hemoglobin, which can bind or release $H^+$ depending on conditions

Bicarbonate Buffer in Blood

The bicarbonate buffer system is the primary mechanism keeping blood pH between 7.35 and 7.45. It consists of two components:

• Carbonic acid ($H_2CO_3$), the weak acid
• Bicarbonate ion ($HCO_3^-$), the conjugate base

This system is tightly linked to cellular respiration. Your cells constantly produce $CO_2$ as a metabolic byproduct, and that $CO_2$ reacts with water in the blood:

$CO_2 + H_2O \leftrightarrow H_2CO_3 \leftrightarrow H^+ + HCO_3^-$

Without buffering, the $H^+$ released in this reaction would make blood dangerously acidic. Instead, the equilibrium between carbonic acid and bicarbonate absorbs those extra $H^+$ ions and keeps pH stable.

Two organ systems work alongside this buffer to fine-tune blood pH:

• Lungs: Remove excess $CO_2$ through exhalation. Breathing faster lowers $CO_2$ levels and raises blood pH; breathing slower does the opposite.
• Kidneys: Adjust how much $HCO_3^-$ is reabsorbed or excreted in urine. This is a slower response but allows for longer-term pH correction.

pH Effects on Cellular Processes

Enzyme activity is one of the most pH-sensitive aspects of cell biology. Each enzyme has an optimal pH where it works best. For example, pepsin in the stomach operates optimally around pH 2, while trypsin in the small intestine prefers around pH 8. When pH shifts away from an enzyme's optimum, the ionization state of amino acid residues in the active site changes, reducing catalytic activity or denaturing the enzyme entirely.

Protein structure depends on pH as well. The hydrogen bonds, ionic interactions, and other forces holding a protein in its functional 3D shape are disrupted by pH changes. Denaturation (the unfolding of a protein's native conformation) often makes the protein non-functional.

Other cellular processes affected by pH include:

• Membrane permeability and transport: pH changes alter the charge on membrane lipids and transport proteins, affecting how ions and molecules move across the membrane through channels and carriers.
• Cell signaling: Some receptors and signaling molecules are pH-sensitive, so shifts in acidity can disrupt communication between cells (for example, hormone signaling).
• Cellular metabolism: Enzymes driving glycolysis, the citric acid cycle, and other metabolic pathways have pH-dependent activity, so pH shifts can speed up or slow down entire metabolic networks.

Cells actively regulate their internal pH using ion pumps, ion exchangers, and intracellular buffers. When these mechanisms fail, the consequences are serious. Acidosis (blood pH below 7.35) and alkalosis (blood pH above 7.45) are clinical conditions that can impair organ function and, if severe, become life-threatening.