Chemical Principles in Anatomy & Physiology: Acids, Bases, Biomolecules, and Cellular Energy

Chemical Principles

Acids and Bases

Acids and bases are fundamental to understanding the chemical environment of the human body. Their concentrations and interactions are crucial for physiological processes and homeostasis.

• Acids release hydrogen ions (H+) when dissolved in water. The concentration of hydrogen ions determines the acidity of a solution.

• Bases release hydroxyl ions (OH-) or absorb hydrogen ions. Bicarbonate ion (HCO3-) is an important base in the body, especially in blood, where it acts as an alkaline reserve.

• Acids are produced as end products of metabolism of proteins, carbohydrates, and fats.

• Types of acids produced:

  • Respiratory acids (volatile) – e.g., carbonic acid (H2CO3), eliminated as CO2 gas.

  • Non-respiratory acids (non-volatile or fixed) – e.g., sulfuric, phosphoric, and organic acids, eliminated by the kidneys.

• Bases are used in metabolic disturbances to return plasma pH to normal.

• Homeostasis: For physiological functioning, acid-base balance must be kept within a narrow range.

Acids and Bases: Regulation and pH

The body regulates acid-base balance primarily through the lungs and kidneys.

• The lungs secrete acid (as CO2).

• The kidneys secrete metabolic acids.

• Hydrogen ions are protons with a positive charge (H+).

• Acids release/donate hydrogen ions; bases receive/absorb hydrogen ions and neutralize positively charged ions.

pH and Its Importance

pH is a measure of hydrogen ion concentration and is critical for biological systems.

• pH scale: Ranges from 0 (most acidic) to 14 (most basic). Neutral pH is 7.0.

• Each unit change in pH represents a tenfold change in hydrogen ion concentration.

• Normal blood pH is tightly regulated between 7.35 and 7.45.

• Acidosis: pH below 7.35; Alkalosis: pH above 7.45.

• Extreme pH values (<6.8 or >8.0) are incompatible with life.

Equation:

$\text{pH} = -\log_{10}[\text{H}^+]$

Examples of pH Values

Substance	pH
Stomach acid	1-2
Lemon juice	2
Vinegar, beer	3
Blood	7.4
Sea water	8
Baking soda	9
Oven cleaner	13-14

Body Acids: Volatile and Nonvolatile Forms

Body acids exist in two forms, each eliminated by different organs.

• Volatile acids (e.g., carbonic acid) can be eliminated as CO2 gas via the lungs.

• Nonvolatile (fixed) acids (e.g., sulfuric, phosphoric acids) are eliminated by the kidneys.

• Nonvolatile acids are secreted in urine, about 150 mEq of hydrogen per day.

Major Buffer Systems

Buffers help maintain pH stability in body fluids. The body uses three main buffer systems:

1. Carbonic Acid-Bicarbonate Buffer

   • Operates in both lungs and kidneys; major buffer in extracellular fluid (ECF).

   • Lungs decrease carbonic acid by exhaling CO2.

   • Kidneys reabsorb or generate new bicarbonate from carbon dioxide and water.

   • Equation: $\text{CO}_2 + \text{H}_2\text{O} \leftrightarrow \text{H}_2\text{CO}_3 \leftrightarrow \text{H}^+ + \text{HCO}_3^-$

2. Protein Buffer System

   • Proteins can act as acids or bases (amphoteric).

   • Albumin and plasma globulins buffer in the vascular (extracellular) compartment.

   • Hemoglobin (Hb) is an excellent intracellular buffer, binding H+ and CO2.

   • Hemoglobin not saturated with oxygen (venous blood) is a better buffer than oxygenated hemoglobin (arterial blood).

3. H+/K+ Exchange System

   • On bone surfaces, 40% of acute acid load buffering occurs.

   • Hydrogen is exchanged for sodium and potassium; this can lead to dissolution of bone minerals and release of buffers.

   • Alterations in potassium and hydrogen movement between intracellular (ICF) and extracellular (ECF) fluid affect acid-base balance.

   • Buffers absorb excess H+ (acid) or OH- (base) to prevent significant pH changes.

Respiratory Control Mechanism

The respiratory system regulates acid-base balance by controlling CO2 elimination, a waste product of cellular metabolism.

• Large lung surface area allows efficient CO2 diffusion and exhalation.

• In acidosis (pH < 7.0), compensatory hyperventilation may occur to increase CO2 elimination.

• Limitations: Hypoventilation can occur as a compensatory response to metabolic alkalosis, but ongoing hypoventilation leads to hypoxemia, limiting compensation.

Renal Control Mechanism

The renal system is essential for maintaining acid-base balance by regulating bicarbonate and nonvolatile acids.

• Urine is normally acidic due to excretion of metabolic acids.

• Kidneys maintain acid-base balance by:

  • Recovering bicarbonate (filtered into renal tubules).

  • Excreting hydrogen ions.

  • Excreting ammonium ions (NH4+), a product of protein metabolism.

• Renal compensation can take hours to days to restore normal pH.

Compensation in Acid-Base Balance

Compensation refers to renal and respiratory adjustments to changes in pH.

• Respiratory system compensates by altering ventilation to change CO2 levels (minutes).

• Renal system compensates by producing more acidic or alkaline urine (hours to days).

• Correction occurs when both buffer pair components (carbonic acid and bicarbonate) return to normal values.

Biomolecules

Carbohydrates

Carbohydrates are organic molecules that include sugars and starches, serving as a primary energy source for cells.

• Represent 1-2% of cell mass.

• Contain carbon, hydrogen, and oxygen.

• Classified by size and solubility:

  • Monosaccharides (one sugar) – building blocks.

  • Disaccharides (two sugars).

  • Polysaccharides (many sugars).

• Larger carbohydrate molecules are less soluble in water.

• Main function: Provide a ready, easily used source of cellular fuel.

• Glucose is broken down in cellular respiration to produce carbon dioxide and water.

Lipids

Lipids are organic compounds insoluble in water but soluble in organic solvents. They are important for energy storage, cell membrane structure, and hormone production.

• Contain carbon, hydrogen, and oxygen (less oxygen than carbohydrates).

• Types of lipids:

  • Neutral fats (triglycerides): Stored in subcutaneous tissue and organs; protect and insulate organs; major source of stored energy.

  • Phospholipids: Chief component of cell membranes; prevalent in nervous tissue.

  • Steroids: Cholesterol is the most important steroid, essential for human life; found in cell membranes and is the raw material for vitamin D, steroid hormones, and bile salts.

Proteins

Proteins are the most varied biomolecules, serving structural and functional roles in the body.

• Compose 10-30% of cell mass.

• Contain carbon, oxygen, hydrogen, and nitrogen.

• Made of amino acids (20 common types).

• Classified by structure:

  • Structural proteins: Fibrous proteins (e.g., collagen, keratin, elastin) provide strength and support to tissues.

  • Functional proteins: Water-soluble, mobile, chemically active molecules (e.g., antibodies, hormones, enzymes).

Nucleotides and Nucleic Acids

Nucleotides are the building blocks of nucleic acids, which store and transmit genetic information and energy.

• Components:

  • Nitrogen-containing base

  • Pentose sugar

  • Phosphate group

• Functions:

  • Transfer energy (ATP)

  • Storage and expression of genetic information (DNA, RNA)

• Five nitrogenous bases:

  • Adenine (A)

  • Guanine (G)

  • Cytosine (C)

  • Thymine (T)

  • Uracil (U)

Enzymes

Enzymes are functional proteins that act as biological catalysts, regulating and accelerating biochemical reactions without being consumed.

• Enzymes increase the speed of reactions but cannot force reactions to occur.

• Each enzyme is highly specific, controlling only a single chemical reaction or a small group of related reactions.

• Some enzymes are produced in inactive forms and must be activated.

• Enzymes may be inactivated after performing their function (e.g., blood clotting enzymes).

Nucleic Acids: DNA and RNA

Nucleic acids are the largest biomolecules, composed of nucleotides. DNA and RNA differ in structure and function.

• DNA (Deoxyribonucleic acid):

  • Found in the nucleus; contains genetic material (genes).

  • Double-stranded polymer; bases are A, G, C, T; sugar is deoxyribose.

  • Functions:

    • Replicates before cell division.

    • Provides instructions for protein synthesis.

  • Bases pair specifically: A-T, G-C.

  • Structure: Double helix.

• RNA (Ribonucleic acid):

  • Single-stranded; bases are A, G, C, U; sugar is ribose.

  • Three types:

    • mRNA (messenger RNA): Transcribes DNA instructions.

    • tRNA (transfer RNA): Carries amino acids during protein synthesis.

    • rRNA (ribosomal RNA): Structural component of ribosomes.

Adenosine Triphosphate (ATP)

ATP is the universal energy currency of cells, storing and providing energy for cellular processes.

• ATP is an adenine-containing RNA nucleotide with three phosphate groups.

• High-energy phosphate bonds store energy; breaking these bonds releases energy for cellular work.

• Cleavage of the terminal phosphate yields ADP and inorganic phosphate.

• ATP is replenished as glucose and other fuel molecules are metabolized.

Equation:

$\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i + \text{energy}$

Summary Table: Key Biomolecules

Biomolecule	Main Elements	Function
Carbohydrates	C, H, O	Energy source
Lipids	C, H, O	Energy storage, cell membranes, hormones
Proteins	C, H, O, N	Structure, enzymes, transport, immunity
Nucleic Acids	C, H, O, N, P	Genetic information, energy transfer

Additional info: These notes cover foundational chemical principles relevant to Anatomy & Physiology, including acid-base balance, biomolecules, and cellular energy, which are essential for understanding physiological processes at the molecular and cellular levels.