4.1 Water
Water is an inorganic compound essential for life, making up 70% to 90% of cellular mass in organisms.
Properties of Water and Their Biological Importance
- Polarity of Water: Water is a polar molecule where oxygen attracts shared electrons more strongly, forming a delta negative ($\delta^-$) charge on oxygen and delta positive ($\delta^+$) charges on hydrogen atoms. This allows water molecules to form hydrogen bonds with one another and with other polar substances.
- Universal Solvent: Water dissolves polar molecules (e.g., glucose, amino acids) and ionic substances (e.g., $Na^+$, $Cl^-$), serving as a transport medium in blood, xylem, and phloem, as well as a medium for biochemical reactions inside cells.
- High Specific Heat Capacity: Water requires $4.2\text{ kJ kg}^{-1}\text{ }^\circ\text{C}^{-1}$ of heat energy to raise the temperature of $1\text{ kg}$ of water by $1^\circ\text{C}$. This buffers organisms against rapid internal temperature changes.
- Cohesive and Adhesive Forces:
- Cohesion: Attraction between water molecules via hydrogen bonds, producing high surface tension.
- Adhesion: Attraction between water molecules and other polar surfaces (e.g., cellulose in xylem walls).
- Together, cohesion and adhesion generate capillary action to pull water upwards through xylem vessels.
- Density Anomalies: Water reaches maximum density at $4^\circ\text{C}$. Ice is less dense than liquid water and floats, insulating aquatic life under frozen surface layers.
4.2 Carbohydrates
Carbohydrates are organic compounds composed of carbon (C), hydrogen (H), and oxygen (O) in a ratio of $1:2:1$ with the empirical formula $(CH_2O)_n$.
Three Main Classes of Carbohydrates
- Monosaccharides (Simple Sugars): Monomers of carbohydrates, soluble in water, sweet, and crystalline.
- Glucose: Sugar found in plants and animals; primary substrate for cellular respiration.
- Fructose: Sugar found in sweet fruits and honey.
- Galactose: Sugar found in milk.
- All monosaccharides are reducing sugars (reduce blue Benedict's reagent to a brick-red precipitate of cuprous oxide upon heating).
- Disaccharides (Double Sugars): Formed by joining two monosaccharides via a condensation reaction with the removal of one water molecule, forming a glycosidic bond.
- Maltose: $\text{Glucose} + \text{Glucose}$ (Reducing sugar; found in germinating grains).
- Sucrose: $\text{Glucose} + \text{Fructose}$ (Non-reducing sugar; transport sugar in phloem).
- Lactose: $\text{Glucose} + \text{Galactose}$ (Reducing sugar; found in milk).
- Disaccharides are broken down into monosaccharides through a hydrolysis reaction (addition of water molecule).
- Polysaccharides (Complex Polymers): Insoluble in water, non-sweet polymers formed by condensation of hundreds of glucose monomers.
- Starch: Storage carbohydrate in plants (helical polymers of amylose and amylopectin); turns iodine solution from brown to dark blue.
- Glycogen: Highly branched storage carbohydrate in human/animal liver and muscle cells.
- Cellulose: Straight-chain glucose polymer forming tough, structural plant cell walls; un-digestible human dietary fiber.
4.3 Proteins
Proteins are complex organic macromolecules containing carbon (C), hydrogen (H), oxygen (O), nitrogen (N), and often sulfur (S) or phosphorus (P).
Amino Acids and Polypeptide Formation
- Monomers of proteins are amino acids. Each amino acid possesses an amino group ($-NH_2$), a carboxyl group ($-COOH$), a hydrogen atom ($-H$), and a variable side chain ($-R$) attached to a central carbon atom.
- Two amino acids combine via a condensation reaction to form a dipeptide joined by a peptide bond, releasing one water molecule.
- Multiple amino acids link to form a linear polypeptide chain.
Four Structural Levels of Proteins
- Primary Structure: Linear sequence of amino acids in a polypeptide chain linked by peptide bonds.
- Secondary Structure: Folding or coiling of the polypeptide chain into an $\alpha$-helix or $\beta$-pleated sheet stabilized by hydrogen bonds between peptide backbones.
- Tertiary Structure: Complex 3D globular shape formed by folding of secondary structures, stabilized by hydrophobic interactions, hydrogen bonds, ionic bonds, and disulfide bridges (e.g., enzymes, antibodies, myoglobin).
- Quaternary Structure: Association of two or more polypeptide chains working together as a functional protein complex (e.g., Hemoglobin with 4 polypeptide subunits, Collagen).
4.4 Lipids
Lipids are hydrophobic organic molecules composed mainly of carbon (C), hydrogen (H), and oxygen (O), with a much higher proportion of hydrogen to oxygen than carbohydrates. They are insoluble in water but soluble in organic solvents (alcohol, ether).
Types of Lipids
- Fats and Oils (Triglycerides): Formed by condensation of 1 glycerol molecule and 3 fatty acid molecules, yielding 3 water molecules and forming 3 ester bonds.
- Saturated Fats: Contain single covalent bonds between carbon atoms ($C-C$), straight hydrocarbon chains, high melting points, solid at room temperature (e.g., butter, lard).
- Unsaturated Fats: Contain one or more double bonds ($C=C$) between carbon atoms, bent hydrocarbon chains, lower melting points, liquid at room temperature (e.g., olive oil, vegetable oil).
- Waxes: Composed of a long-chain alcohol combined with a fatty acid; waterproof coating found on leaf cuticles, insect exoskeletons, and mammalian skin.
- Phospholipids: Major structural component of plasma membranes consisting of 1 glycerol, 2 fatty acid tails (hydrophobic), and 1 phosphate group (hydrophilic).
- Steroids: Complex lipids containing four fused carbon rings without fatty acid chains (e.g., cholesterol, estrogen, progesterone, testosterone).
4.5 Nucleic Acids
Nucleic acids are organic macromolecules containing carbon (C), hydrogen (H), oxygen (O), nitrogen (N), and phosphorus (P). They store and express genetic information in cells.
Nucleotide Structure
The monomer of a nucleic acid is a nucleotide, which consists of three components:
- A five-carbon pentose sugar (Deoxyribose in DNA or Ribose in RNA).
- A nitrogenous base: Adenine (A), Guanine (G), Cytosine (C), Thymine (T - DNA only), or Uracil (U - RNA only).
- A phosphate group.
DNA vs RNA
- Deoxyribonucleic Acid (DNA): Double-stranded helix composed of antiparallel polynucleotide chains held by hydrogen bonds between complementary nitrogenous base pairs:
- Adenine (A) pairs with Thymine (T) via 2 hydrogen bonds ($A=T$).
- Guanine (G) pairs with Cytosine (C) via 3 hydrogen bonds ($G\equiv C$).
- Ribonucleic Acid (RNA): Single-stranded polynucleotide chain containing ribose sugar and Uracil (U) instead of Thymine. Three main types: messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA).