Biochemistry: The Chemistry of Life

Introduction to Biochemistry

Biochemistry is the study of the chemical processes and substances that occur within living organisms. Understanding biochemistry is essential for grasping how cells and tissues function in the human body.

• Organic compounds include carbohydrates, lipids, proteins, and nucleic acids.

• These molecules are vital for energy, structure, and information storage in cells.

Carbohydrates

Classification and Structure

Carbohydrates are organic molecules composed of carbon (C), hydrogen (H), and oxygen (O), typically in a 2:1 ratio of hydrogen to oxygen. They serve as a primary energy source for cells.

• Monosaccharides: Single sugar units; smallest carbohydrate monomers.

• Disaccharides: Two monosaccharides joined together.

• Polysaccharides: Long chains of monosaccharides; polymers.

Monosaccharides

Monosaccharides are simple sugars containing three to seven carbon atoms. Their general formula is .

• Pentose sugars (5 carbons): Ribose and deoxyribose (important in nucleic acids).

• Hexose sugars (6 carbons): Glucose (blood sugar), fructose, and galactose.

Disaccharides

Disaccharides are formed by dehydration synthesis, joining two monosaccharides and releasing water.

• Sucrose = glucose + fructose

• Maltose = glucose + glucose

• Lactose = glucose + galactose

Disaccharides are too large to pass through cell membranes and must be broken down into monosaccharides for absorption.

Polysaccharides

Polysaccharides are large, insoluble molecules formed by the dehydration synthesis of many monosaccharide units.

• Starch: Storage form of carbohydrates in plants.

• Glycogen: Storage form in animals, especially in liver and muscle cells.

Polysaccharides are not very soluble in water.

Lipids

General Properties

Lipids are organic compounds containing C, H, and O (less O than carbohydrates), and sometimes phosphorus (P). They are insoluble in water and serve as energy storage, insulation, and cell membrane components.

• Types of lipids: Triglycerides, phospholipids, steroids, eicosanoids.

Triglycerides

Triglycerides (fats and oils) are composed of three fatty acids bonded to a glycerol molecule by dehydration synthesis.

• Main functions: Energy storage, insulation, protection.

Saturated vs. Unsaturated Fatty Acids

• Saturated fatty acids: All carbons linked by single covalent bonds; molecules are linear and pack closely, solid at room temperature (e.g., animal fats, butter).

• Unsaturated fatty acids: One or more double bonds between carbons; molecules have kinks, cannot pack closely, liquid at room temperature (e.g., plant oils).

• Trans fats: Modified unsaturated fats, unhealthy.

• Omega-3 fatty acids: Considered heart-healthy.

Phospholipids

Phospholipids are modified triglycerides with a glycerol, two fatty acids, and a phosphorus-containing group.

• Head: Polar, hydrophilic (water-attracting).

• Tails: Nonpolar, hydrophobic (water-repelling).

• Essential for cell membrane structure.

Steroids

Steroids consist of four interlocking hydrocarbon rings. The most important steroid in the body is cholesterol.

• Functions: Precursor for vitamin D, steroid hormones, bile salts; important in plasma membrane structure.

Eicosanoids

Eicosanoids are derived from fatty acids (arachidonic acid) and include prostaglandins, which play roles in blood clotting, blood pressure regulation, inflammation, and labor contractions.

• Anti-inflammatory drugs (NSAIDs) like aspirin inhibit prostaglandin synthesis.

Proteins

Structure and Function

Proteins comprise 20-30% of cell mass and perform diverse functions, including structural support, catalysis (enzymes), and contraction (muscles). They contain C, H, O, N, and sometimes S and P.

• Polymers of amino acids joined by peptide bonds.

• Shape and function determined by four structural levels.

Amino Acids and Peptide Bonds

Proteins are made from 20 types of amino acids, each containing an amine group, acid group, and a unique "R group." Amino acids are joined by covalent peptide bonds.

• Amino acids can act as acids or bases.

• Sequence of amino acids determines protein structure.

Levels of Protein Structure

• Primary: Linear sequence of amino acids.

• Secondary: Alpha helix (spring-like) and beta pleated sheet (accordion-like) formed by hydrogen bonding.

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

• Quaternary: Association of two or more polypeptide chains.

Fibrous vs. Globular Proteins

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

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

Protein Denaturation

Denaturation is the unfolding and loss of a protein's functional 3D shape, often caused by changes in pH or temperature. It is usually reversible unless the changes are extreme.

• Example: Cooking an egg irreversibly denatures its proteins.

Enzymes and Enzyme Activity

Enzymes are globular proteins that act as biological catalysts, speeding up chemical reactions by lowering activation energy without being consumed.

• Most enzymes are holoenzymes (protein + cofactor/coenzyme).

• Names often end in "-ase" and reflect their function.

Mechanism of Enzyme Action

1. Substrate binds to enzyme's active site, forming an enzyme-substrate complex.

2. Complex undergoes rearrangement, resulting in product formation.

3. Enzyme releases product and is ready for another reaction.

Activation energy is the energy required to initiate a chemical reaction. Enzymes lower this threshold, allowing reactions to proceed rapidly at body temperature.

Nucleic Acids

Structure and Types

Nucleic acids are the largest molecules in the body, composed of C, H, O, N, and P. They are polymers of nucleotides, each consisting of a nitrogen base, pentose sugar, and phosphate group.

• Two major classes: Deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).

DNA

DNA holds the genetic blueprint for protein synthesis. It is a double-stranded helical molecule located in the cell nucleus.

• Nucleotides contain deoxyribose sugar, phosphate group, and one of four nitrogen bases:

  • Purines: Adenine (A), Guanine (G)

  • Pyrimidines: Cytosine (C), Thymine (T)

• Complementary base pairing: A-T, G-C

RNA

RNA links DNA to protein synthesis. It is a single-stranded molecule, mostly found outside the nucleus, and contains ribose sugar. Thymine is replaced by uracil (U).

• Three main types: messenger RNA (mRNA), transfer RNA (tRNA), ribosomal RNA (rRNA).

ATP (Adenosine Triphosphate)

Role and Structure

ATP is the primary energy carrier in cells. Chemical energy released from glucose breakdown is stored in ATP, which directly powers cellular reactions.

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

• Energy is released when the terminal phosphate bond is hydrolyzed.

• Loss of a phosphate group converts ATP to ADP; loss of another converts ADP to AMP.

• ATP provides immediate, usable energy for cellular work.

Examples of Cellular Work Driven by ATP

• Transport of substances across membranes

• Muscle contraction

• Synthesis of macromolecules

Additional info: Some explanations and examples have been expanded for clarity and completeness, including the table summarizing macromolecules.