Biochemistry: The Chemical Basis of Life

Introduction to Biochemistry

Biochemistry is the study of the chemical composition and reactions of living matter. It distinguishes between organic compounds (containing carbon, such as carbohydrates, lipids, proteins, and nucleic acids) and inorganic compounds (such as water, salts, acids, and bases, which generally do not contain carbon). Both types are essential for life and underpin all physiological processes.

Inorganic Compounds

Water: Properties and Biological Importance

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

• High heat capacity: Absorbs and releases heat with minimal temperature change, stabilizing body temperature.

• High heat of vaporization: Requires significant energy to evaporate, providing an effective cooling mechanism (e.g., sweating).

• Reactivity: Participates in hydrolysis and dehydration synthesis reactions essential for metabolism.

• Cushioning: Protects organs from physical trauma (e.g., cerebrospinal fluid around the brain).

• Polar solvent properties: Dissolves and dissociates ionic substances, forming hydration layers around charged molecules, and serves as the body's major transport medium.

Salts

Salts are ionic compounds that dissociate in water to form electrolytes—cations and anions (excluding H+ and OH–). These ions are crucial for physiological functions such as nerve impulse transmission and muscle contraction. Common body salts include NaCl, CaCO3, KCl, and calcium phosphates. Maintaining ionic balance is vital for homeostasis.

Acids, Bases, and pH

Acids and bases are electrolytes that ionize and dissociate in water. Acids are proton donors (release H+), while bases are proton acceptors (often releasing OH–). The pH scale (0–14) measures hydrogen ion concentration; lower pH is more acidic, higher pH is more basic. The scale is logarithmic: each unit represents a tenfold difference in [H+]. Enzymes function within narrow pH ranges, making pH regulation critical for life.

• Acidic solutions: pH 0–6.99, high [H+]

• Neutral solutions: pH 7, equal [H+] and [OH–]

• Alkaline (basic) solutions: pH 7.01–14, low [H+]

Buffers

Buffers are mixtures of weak acids and bases that resist abrupt changes in pH by binding or releasing H+ as needed. The carbonic acid–bicarbonate system is a key buffer in blood, maintaining physiological pH.

Organic Compounds

Carbohydrates

Carbohydrates are organic molecules containing C, H, and O, with hydrogen and oxygen typically in a 2:1 ratio. They serve as the primary energy source for cells and are classified as:

• Monosaccharides: Simple sugars (e.g., glucose, fructose, galactose, ribose, deoxyribose).

• Disaccharides: Two monosaccharides joined by dehydration synthesis (e.g., sucrose, maltose, lactose).

• Polysaccharides: Long chains of monosaccharides (e.g., starch in plants, glycogen in animals).

Lipids

Lipids are hydrophobic organic molecules containing C, H, and O (less O than carbohydrates). They include:

• Triglycerides: Three fatty acids bonded to glycerol; main functions are energy storage, insulation, and protection.

• Phospholipids: Glycerol, two fatty acids, and a phosphate group; major component of cell membranes.

• Steroids: Four interlocking hydrocarbon rings; cholesterol is the most important, serving as a precursor for hormones and vitamin D.

• Eicosanoids: Derived from arachidonic acid; include prostaglandins, which regulate inflammation and other processes.

Proteins

Structure and Function

Proteins are polymers of amino acids joined by peptide bonds. They perform diverse functions: structural support, enzymes, transport, movement, communication, and defense. Protein structure is organized into four levels:

• Primary: Linear sequence of amino acids.

• Secondary: Alpha helices and beta sheets formed by hydrogen bonding.

• Tertiary: Folding of secondary structures into a 3D shape.

• Quaternary: Association of multiple polypeptide chains.

Fibrous vs. Globular Proteins

• Fibrous proteins: Structural, water-insoluble, stable (e.g., collagen, keratin).

• Globular proteins: Functional, water-soluble, sensitive to environmental changes (e.g., enzymes, antibodies, hemoglobin).

Enzymes

Enzymes are globular proteins that act as biological catalysts, increasing the rate of chemical reactions by lowering activation energy. They are highly specific for their substrates and often require cofactors or coenzymes to function.

Nucleic Acids

DNA and RNA

Nucleic acids are polymers of nucleotides (composed of a nitrogenous base, pentose sugar, and phosphate group). DNA stores genetic information and directs protein synthesis, while RNA translates genetic instructions into proteins. DNA is double-stranded (A-T, G-C pairing); RNA is single-stranded and uses uracil instead of thymine.

ATP: The Energy Currency

ATP (adenosine triphosphate) stores and transfers energy within cells. Hydrolysis of ATP releases energy for cellular work:

Cells: The Smallest Living Units

Cell Theory and Structure

Cells are the basic structural and functional units of life. All organisms are composed of cells, which arise from preexisting cells. Human cells share common features: plasma membrane, cytoplasm, and nucleus.

Plasma Membrane

The plasma membrane separates intracellular from extracellular environments, controls substance movement, and is composed of a phospholipid bilayer with embedded proteins. The fluid mosaic model describes its dynamic structure.

Lipid Bilayer and Membrane Proteins

The lipid bilayer consists of phospholipids (hydrophilic heads, hydrophobic tails), cholesterol (stability), and glycocalyx (cell recognition). Membrane proteins serve as channels, receptors, enzymes, and anchors.

Glycocalyx

The glycocalyx is a carbohydrate-rich area on the cell surface, functioning in cell recognition and immune response.

Discussion Questions

• Why does water alone not remove oily residue from a salad bowl? Lipids are hydrophobic and do not dissolve in water; detergents are needed to emulsify and remove them.

• Which is more acidic: lemon juice (pH 2) or orange juice (pH 4), and by how much? Lemon juice is 100 times more acidic than orange juice (each pH unit is a tenfold difference).

• Is the plasma membrane a solid or a liquid? The plasma membrane is a dynamic, fluid structure (fluid mosaic model), not a rigid solid.