The Foundations of Biochemistry

Key Elements and Composition of the Human Body

The human body is primarily composed of a few essential elements and a variety of chemical compounds that are critical for life. Understanding the elemental and molecular composition is foundational to biochemistry.

• Four Key Elements: Oxygen, Carbon, Hydrogen, Nitrogen

• Body Composition (by mass):

  • Water: 60%

  • Protein: 16%

  • Triglyceride (fat): 13%

  • Salt: 6.2% (Insoluble: 5.5%, Soluble: 0.7%)

  • Carbohydrate: 1.5%

  • Nucleic acid: 0.2%

• Main Inorganic Components: Water & salts

• Main Organic Components: Proteins, triglycerides, carbohydrates, nucleic acids

Water in Biochemistry

Properties and Importance of Water

Water is the most abundant inorganic compound in the human body and is essential for numerous physiological processes.

• Vital Properties of Water:

  • Universal solvent

  • Neutral pH

  • Transport medium

  • Regulation of body temperature

  • Cushioning and lubrication

  • High surface tension

• Water as a Universal Solvent: Water's polarity and ability to form hydrogen bonds allow it to dissolve a wide range of substances, making it an excellent medium for biochemical reactions.

Solutions, Solvents, and Solutes

• Solution: A uniform mixture of two or more substances (e.g., electrolytes or polar non-electrolytes in water).

• Solvent: The liquid in which substances are dissolved (water in biological systems).

• Solutes: The dissolved substances (electrolytes and polar non-electrolytes).

• Dissolve: Solutes are separated but not broken apart in the solvent (e.g., glucose in water).

• Dissociate: Solutes are broken into ions in the solvent (e.g., NaCl in water).

Electrical Dipoles and Hydrogen Bonds

Water molecules are polar, creating electrical dipoles that enable hydrogen bonding. This property is crucial for water's solvent abilities and for the structure of biomolecules.

• Electrical Dipole: Separation of charge within a molecule due to differences in electronegativity.

• Hydrogen Bond: Attraction between the hydrogen atom of one water molecule and the oxygen atom of another.

Body Fluids and Fluid Compartments

Classification of Body Fluids

Body fluids are categorized based on their location, which is essential for understanding fluid balance and transport in physiology.

• Intracellular Fluid (ICF): Fluid within cells (~64% of total body fluid; also called cytosol).

• Extracellular Fluid (ECF): Fluid outside cells (~36% of total body fluid), including:

  • Interstitial fluid (tissue space)

  • Plasma (within blood vessels)

  • Lymph (within lymphatic vessels)

  • Transcellular fluids (e.g., urine, digestive juices, cerebrospinal fluid, synovial fluid)

Fluid Balance and Homeostasis

• Fluid Intake: Beverages, foods, metabolic water production

• Fluid Output: Urine, skin, lungs, sweat, feces

• Consequences of Imbalance:

  • Dehydration: Insufficient water

  • Oedema/Water intoxication: Excessive water

Electrolytes and Non-Electrolytes

Definitions and Examples

• Electrolytes: Substances that dissociate into ions in water and conduct electricity (e.g., NaCl, HCl, NaOH).

• Non-Electrolytes: Substances that dissolve but do not dissociate into ions (e.g., glucose, urea, ethanol).

Major Electrolytes in Body Fluids

• Intracellular Fluid: K+, Mg2+, HPO42-

• Extracellular Fluid: Na+, Cl-, HCO3- (for pH regulation)

Acid-Base Balance in the Human Body

Acids, Bases, and pH

• Acids: Substances that release H+ in water (pH < 7).

• Bases: Substances that accept H+ or release OH- (pH > 7).

• pH: A measure of hydrogen ion concentration; optimal plasma pH is 7.35–7.45.

Types of Acids in the Body

• Fixed Acids: From diet, eliminated by kidneys (e.g., sulfuric acid, phosphoric acid)

• Metabolic Acids: From metabolism, eliminated by kidneys (e.g., lactic acid, ketone bodies)

• Volatile Acids: Carbonic acid, eliminated as CO2 by lungs

Significance of pH in Physiology

• Changes in [H+] can disrupt cell membranes, alter protein structures, and affect enzyme activity.

• Blood pH below 6.8 or above 7.7 is incompatible with life.

Buffering Systems in the Body

The body uses chemical and physiological buffering systems to maintain pH within the narrow optimal range.

• Chemical Buffering Systems (fast):

  • Carbonic acid-bicarbonate buffer system (plasma)

  • Phosphate buffer system (cytosol)

  • Protein buffer system (ECF and cytosol)

• Physiological Buffering Systems (slower):

  • Urinary system (excretes acids/bases)

  • Respiratory system (eliminates CO2)

Carbonic Acid-Bicarbonate Buffer System

This system is crucial for maintaining the pH of extracellular fluid, especially blood plasma.

• Key Equation:

• When plasma pH < 7.35 (acidosis): HCO3- binds excess H+, forming H2CO3, which dissociates into CO2 and H2O, raising pH.

• When plasma pH > 7.45 (alkalosis): More CO2 and H2O combine to form H2CO3, which dissociates into HCO3- and H+, lowering pH.

Acid-Base Disorders

Acid-base imbalances occur when there is excessive input or output of acids or bases, leading to acidosis or alkalosis.

• Acidosis: Plasma pH < 7.35 (too much H+)

• Alkalosis: Plasma pH > 7.45 (insufficient H+)

Major Types of Acid-Base Disorders

Examples and Clinical Applications

• Diabetic Ketoacidosis (DKA): A condition where excessive fat metabolism leads to accumulation of ketone bodies, causing metabolic acidosis (blood pH < 7.1).

• Antacids: Used to neutralize excess stomach acid, raising pH and relieving heartburn.

Water Interactions: Hydrogen Bonds and Ion-Dipole Forces

Hydrogen Bonds in Water

Hydrogen bonds are responsible for many of water's unique properties, including its high boiling point and solvent capabilities.

• Hydrogen bonds form between the partially positive hydrogen of one water molecule and the partially negative oxygen of another.

• These interactions are crucial for the dissolution of polar molecules and the structure of macromolecules like proteins and nucleic acids.

Ion-Dipole Interactions

Ion-dipole interactions occur when charged ions interact with polar water molecules, facilitating the dissolution of salts and other ionic compounds in biological fluids.

• Example: Na+ ions are surrounded by the negative (oxygen) end of water molecules, while Cl- ions are surrounded by the positive (hydrogen) ends.

Summary Table: Key Concepts