2.2 Biomolecules: nucleic acids, proteins, carbohydrates, and lipids

Biomolecules are the building blocks of life. Nucleic acids, proteins, carbohydrates, and lipids play crucial roles in cellular structure and function. Each type has unique properties that contribute to the complex workings of living organisms.

Understanding these biomolecules is key to grasping cellular and molecular biology. They're involved in genetic information storage, energy production, cellular structure, and countless biochemical reactions. Their interactions form the basis of life processes.

Nucleic Acids

DNA (Deoxyribonucleic Acid)

• Stores and transmits genetic information in all living organisms
• Consists of two antiparallel polynucleotide strands coiled around each other to form a double helix
• Composed of four types of nucleotide bases: adenine (A), thymine (T), guanine (G), and cytosine (C)
• Complementary base pairing: A pairs with T, and G pairs with C through hydrogen bonds
• Replication occurs through the action of DNA polymerase, ensuring genetic information is passed on to daughter cells during cell division (mitosis and meiosis)

RNA (Ribonucleic Acid)

• Single-stranded nucleic acid involved in various cellular processes, including protein synthesis, gene regulation, and catalytic activities
• Composed of four types of nucleotide bases: adenine (A), uracil (U), guanine (G), and cytosine (C)
• Three main types of RNA:
  • Messenger RNA (mRNA): Carries genetic information from DNA to ribosomes for protein synthesis
  • Transfer RNA (tRNA): Transfers specific amino acids to the ribosome during protein synthesis
  • Ribosomal RNA (rRNA): Forms the structural and catalytic components of ribosomes

Proteins

Amino Acids and Protein Structure

• Proteins are polymers composed of amino acids linked together by peptide bonds
• There are 20 different amino acids, each with a unique side chain (R group) that determines its properties
• Primary structure: The linear sequence of amino acids in a polypeptide chain
• Secondary structure: Local folding patterns of the polypeptide chain, such as alpha helices and beta sheets, stabilized by hydrogen bonds
• Tertiary structure: Three-dimensional folding of a polypeptide chain, stabilized by interactions between side chains (disulfide bonds, hydrophobic interactions, ionic bonds, and hydrogen bonds)
• Quaternary structure: The arrangement of multiple polypeptide subunits to form a functional protein complex (hemoglobin and DNA polymerase)

Enzymes

• Proteins that catalyze chemical reactions in living organisms by lowering the activation energy
• Highly specific to their substrates and reactions, ensuring efficient and precise cellular processes
• Active site: The region of an enzyme where the substrate binds and the reaction occurs
• Cofactors: Non-protein molecules or ions (coenzymes and metal ions) that assist in enzyme function
• Factors affecting enzyme activity: Temperature, pH, substrate concentration, and the presence of inhibitors or activators

Carbohydrates

Monosaccharides

• Simple sugars that serve as the building blocks of larger carbohydrates
• Examples include glucose, fructose, and galactose
• Can exist in linear or ring forms (pyranose and furanose)
• Monosaccharides can be linked together through glycosidic bonds to form disaccharides (sucrose and lactose) and polysaccharides

Polysaccharides

• Complex carbohydrates composed of multiple monosaccharide units linked together by glycosidic bonds
• Plant polysaccharides:
  • Cellulose: Structural component of plant cell walls, provides support and protection
  • Starch: Energy storage molecule in plants, composed of amylose (linear) and amylopectin (branched)
• Animal polysaccharides:
  • Glycogen: Energy storage molecule in animals, highly branched for rapid mobilization of glucose
• Other polysaccharides: Chitin (structural component in fungal cell walls and arthropod exoskeletons) and peptidoglycan (component of bacterial cell walls)

Lipids

Fatty Acids and Phospholipids

• Fatty acids: Long-chain hydrocarbons with a carboxyl group at one end, can be saturated (single bonds) or unsaturated (one or more double bonds)
• Triacylglycerols (triglycerides): Energy storage molecules composed of three fatty acids esterified to a glycerol backbone
• Phospholipids: Main components of cell membranes, consist of two fatty acids and a phosphate group attached to a glycerol backbone
  • Hydrophilic head (phosphate group) and hydrophobic tails (fatty acids) give phospholipids their amphipathic properties
  • Arrange into bilayers to form cell membranes, with hydrophilic heads facing the aqueous environment and hydrophobic tails facing each other

Steroids

• Lipids characterized by a four-ring structure (three six-carbon rings and one five-carbon ring)
• Examples include cholesterol (component of animal cell membranes and precursor to steroid hormones), testosterone, and estrogen
• Steroid hormones: Regulate various physiological processes, such as metabolism, immune function, and reproductive development
  • Cortisol: Stress hormone that regulates glucose metabolism and immune responses
  • Aldosterone: Regulates blood pressure and electrolyte balance in the kidneys