2.3 pH, buffers, and cellular homeostasis

pH is the key to life's delicate balance. From enzymes to proteins, cellular processes depend on maintaining the right acidity or alkalinity. Buffers, like the bicarbonate system in blood, help keep pH stable, allowing our bodies to function smoothly.

Understanding pH and buffers is crucial for grasping how cells work. These concepts explain why small changes in acidity can have big impacts on our health, and how our bodies fight to keep everything in check.

pH and Buffers

pH scale and biological significance

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Top images from around the web for pH scale and biological significance

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  14.3 pH and pOH | General College Chemistry II View original

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• 

  17.14 Acid-Base Balance – Fundamentals of Anatomy and Physiology View original

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  Unit 9: Fluids & Electrolytes – Douglas College Human Anatomy & Physiology II (2nd ed.) View original

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• pH measures the concentration of hydrogen ions (H+) in a solution
  • Mathematically defined as the negative logarithm of the hydrogen ion concentration: $pH = -log[H+]$
  • Logarithmic scale means each unit change in pH represents a tenfold change in H+ concentration
    • A solution with a pH of 6 has 10 times more H+ than a solution with a pH of 7 (neutral)
• pH scale ranges from 0 (acidic) to 14 (basic/alkaline)
  • Acidic solutions have a pH below 7 and a higher concentration of H+
  • Basic solutions have a pH above 7 and a lower concentration of H+
• Most biological processes are sensitive to changes in pH
  • Enzymes have optimal pH ranges for maximum activity
  • Proteins can denature if the pH deviates too far from their optimal range
  • Cells must maintain a relatively stable pH for proper functioning of cellular processes (DNA replication, protein synthesis)
• Changes in pH affect the structure and function of nucleic acids, lipids, and other biomolecules

Buffer systems in organisms

• Buffers are solutions that resist changes in pH when small amounts of acid or base are added
  • Composed of a weak acid and its conjugate base, or a weak base and its conjugate acid
• Buffers work by absorbing excess H+ or OH- ions, preventing significant changes in pH
  • When acid is added, the conjugate base of the buffer binds to the excess H+, limiting the increase in H+ concentration
  • When base is added, the weak acid of the buffer releases H+ to neutralize the excess OH-, limiting the decrease in H+ concentration
• Buffering capacity is the amount of acid or base a buffer can absorb before the pH changes significantly
  • Determined by the concentration of the buffer components and the strength of the acid or base
• Cells and body fluids contain various buffering systems to maintain relatively constant pH
  • Bicarbonate buffer system in blood
  • Phosphate buffers in cells (cytoplasm)
  • Proteins that act as buffers (hemoglobin)

Bicarbonate buffer in blood

• The bicarbonate buffer system is the primary buffer in the blood, maintaining a pH between 7.35 and 7.45
  • Consists of carbonic acid (H2CO3) and bicarbonate ion (HCO3-)
  • Carbonic acid is the weak acid, and bicarbonate ion is its conjugate base
• Bicarbonate buffer system regulates blood pH during cellular respiration
  • Cellular respiration produces carbon dioxide (CO2) as a byproduct
  • CO2 reacts with water to form carbonic acid, which can lower blood pH
    • $CO2 + H2O <-> H2CO3 <-> H+ + HCO3-$
  • The bicarbonate buffer system neutralizes the excess H+ ions, preventing acidification of the blood
• The lungs and kidneys work with the bicarbonate buffer system to maintain blood pH
  • Lungs remove excess CO2 from the blood through exhalation
  • Kidneys reabsorb or excrete HCO3- to adjust blood pH

pH effects on cellular processes

• Enzymes have optimal pH ranges for maximum activity
  • Most enzymes function best at a neutral or slightly alkaline pH (pepsin in stomach, trypsin in small intestine)
  • Deviations from the optimal pH can reduce enzyme activity or even denature the enzyme
    • Changes in pH alter the ionization of amino acid residues, affecting the enzyme's structure and function
• pH changes affect protein structure and stability
  • Proteins have a specific three-dimensional structure crucial for their function
  • Changes in pH disrupt hydrogen bonds, ionic interactions, and other forces that maintain protein structure
    • Denaturation occurs when a protein loses its native conformation, often rendering it non-functional
• Other cellular processes affected by pH changes
  • Membrane permeability and transport
    • pH changes alter the ionization of membrane lipids and proteins, affecting their function (ion channels)
  • Cell signaling and communication
    • Some signaling molecules and receptors are sensitive to pH changes, which can disrupt cellular communication (hormones)
  • Cellular metabolism
    • pH changes affect the activity of metabolic enzymes and alter the rates of cellular reactions (glycolysis, Krebs cycle)
• Maintaining a stable pH is crucial for cellular homeostasis
  • Cells employ various mechanisms to regulate their internal pH (ion pumps, exchangers, buffers)
  • Disruptions in pH homeostasis can lead to cellular dysfunction and contribute to various diseases (acidosis, alkalosis)