Biochemistry: The Chemical Basis of Life

Introduction to Biochemistry

Biochemistry is the study of the chemical composition and reactions of living matter. All living organisms are composed of chemicals, which can be classified as either organic or inorganic compounds. Both types are essential for life and play distinct roles in cellular structure and function.

• Inorganic compounds: Include water, salts, and many acids and bases. These compounds generally do not contain carbon.

• Organic compounds: Include carbohydrates, fats (lipids), proteins, and nucleic acids. These compounds always contain carbon, are usually large, and are covalently bonded.

Inorganic Compounds

Water: The Most Abundant Inorganic Compound

Water is the most abundant inorganic compound in living cells, accounting for 60%–80% of their volume. Its unique properties make it vital for life.

• High heat capacity: Water can absorb and release large amounts of heat with little temperature change, helping to prevent sudden changes in body temperature.

• High heat of vaporization: Evaporation of water requires significant energy, making sweating an effective cooling mechanism.

• Polar solvent properties: Water dissolves and dissociates ionic substances, forming hydration layers around large charged molecules (e.g., proteins). It is the body's major transport medium.

• Reactivity: Water is involved in many chemical reactions, including hydrolysis and dehydration synthesis.

Salts and Electrolytes

Salts are ionic compounds that dissociate in water, separating into positively charged cations and negatively charged anions (excluding H+ and OH- ions). When dissolved, salts form electrolytes, which can conduct electrical currents in solution.

• Physiological roles: Ions such as sodium (Na+), potassium (K+), calcium (Ca2+), and iron (Fe2+/3+) are essential for nerve impulse transmission, muscle contraction, and other body functions.

• Ionic balance: Maintaining the correct balance of ions is vital for homeostasis.

• Common body salts: Sodium chloride (NaCl), calcium carbonate (CaCO3), potassium chloride (KCl).

Acids and Bases

Definitions and Properties

• Acids: Substances that are proton donors; they release hydrogen ions (H+) in solution. Example: $\mathrm{HCl} \rightarrow \mathrm{H}^+ + \mathrm{Cl}^-$

• Important acids: Hydrochloric acid (HCl), acetic acid (HC2H3O2 or HAc), carbonic acid (H2CO3).

• Bases: Substances that are proton acceptors; they pick up H+ ions in solution. Example: $\mathrm{NaOH} \rightarrow \mathrm{Na}^+ + \mathrm{OH}^-$

• When a base dissolves, it releases a hydroxyl ion (OH-).

• Important bases: Bicarbonate ion (HCO3-), ammonia (NH3).

Acid-Base Concentration and the pH Scale

The pH scale measures the concentration of hydrogen ions [H+] in a solution. The more hydrogen ions, the more acidic the solution. The pH scale is logarithmic and ranges from 0 to 14.

• pH formula: $\mathrm{pH} = -\log_{10}[\mathrm{H}^+]$

• Each pH unit represents a tenfold difference in [H+]. For example, a solution with pH 5 is 10 times more acidic than one with pH 6.

Classification of Solutions by pH

• Pure water: pH 7, $[\mathrm{H}^+] = 10^{-7} \text{ M}$

Summary Table: Key Properties of Water and Electrolytes

Key Terms

• Electrolyte: A substance that dissociates into ions in solution and is capable of conducting electricity.

• Acid: A proton donor; increases [H+] in solution.

• Base: A proton acceptor; decreases [H+] in solution, often by releasing OH-.

• pH: A measure of hydrogen ion concentration; determines acidity or alkalinity.

Example: The stomach contains hydrochloric acid (HCl), which creates a highly acidic environment (pH ~2) necessary for digestion. Blood, in contrast, is slightly basic (pH ~7.4), and its pH is tightly regulated for proper physiological function.