AS Biology - Molecules, Bonding, Water, Carbohydrates, Lipids, Proteins, and Circulation (Notes)

Chemical Bonds

• Ionic bonds: complete electron transfer → cation (+ ) + anion (-); metal donates, non-metal accepts; compound overall charge 0
• Covalent bonds: electrons shared when electronegativity difference <1.7
  • Non-polar 0\text{–}0.4; equal sharing (e.g. \text{O}2, \text{CH}4)
  • Polar 0.5\text{–}1.7; unequal sharing → \delta^+ / \delta^- (e.g. \text{H}_2\text{O})
• Hydrogen bonds: weak attractions between \delta^+ H and \delta^- O/N; numerous in water and biomolecules

Key Inorganic Ions

• \text{NO}_3^- – DNA, amino acid synthesis in plants
• \text{PO}_4^{3-} – ATP, ADP, DNA/RNA
• \text{Cl}^- – nerve impulses, sweating, secretion
• \text{HCO}_3^- – blood pH buffer
• \text{Na}^+ – nerve impulses, secretion
• \text{Ca}^{2+} – middle lamella, bones, muscle contraction
• \text{H}^+ – respiration, photosynthesis, pH

Water: Structure & Properties

• Polar molecule → hydrogen bonding → cohesion, adhesion, surface tension
• High specific heat 4.18\,\text{J g}^{-1}\text{K}^{-1} → thermally stable habitats
• Ice less dense than water → insulation of aquatic life
• Excellent solvent for polar/ionic solutes; hydrophilic vs. hydrophobic substances
• Cohesion supports transpiration stream in xylem; surface tension enables water-surface habitats

Carbohydrates

• General formula (CH2O)n
• Monosaccharides: triose (n=3), pentose (5, e.g. ribose), hexose (6, e.g. glucose)
• \alpha- and \beta-glucose differ at \text{C}_1; \alpha forms helical starch, \beta forms straight cellulose chains → microfibrils
• Disaccharides: formed by condensation → glycosidic bond + \text{H}_2\text{O}; e.g. sucrose (glucose+fructose), lactose (glucose+galactose), maltose (glucose+glucose)
• Polysaccharides: energy storage (starch – amylose 1\rightarrow4 helix, amylopectin 1\rightarrow6 branches; glycogen – highly branched) and structure (cellulose – \beta 1\rightarrow4, high tensile strength)

Lipids

• Triglyceride = glycerol + 3 fatty acids via esterification (condensation, ester bond + \text{H}_2\text{O})
• Fatty acids: saturated (no C=C), monounsaturated (1 C=C), polyunsaturated (>1 C=C); cis (bent, low m.p.) vs trans (straight, high m.p.)
• Hydrophobic; functions: energy storage, membranes (phospholipids), hormones, insulation

Proteins

• Amino acid structure: central C with \text{NH}_2, \text{COOH}, H, and variable R
• Peptide bond: condensation between \text{NH}2 and \text{COOH} → \text{H}2\text{O} released
• Structural levels:
  • Primary – AA sequence
  • Secondary – \alpha-helix or \beta-sheet via H-bonds
  • Tertiary – 3-D folding via H-, ionic, disulfide bonds; forms globular proteins
  • Quaternary – >1 polypeptide chain (e.g. haemoglobin)
• Fibrous (collagen: triple helix, tensile) vs. globular (enzymes, antibodies, hormones); conjugated proteins include prosthetic group (haem in haemoglobin); glycoproteins & lipoproteins important in lubrication and cholesterol transport (HDL vs LDL)

Mass Transport in Organisms

• Need increases as surface area : volume ratio decreases; diffusion alone insufficient in large organisms
• Effective transport systems share: exchange surface, vessels, one-way flow, fast movement (pump/concentration gradient), transport medium

Circulatory Systems

• Open (insects) vs. closed (vertebrates)
• Single circulation (fish): heart → gills → body → heart; lower pressure post-gills
• Double circulation (birds, mammals): pulmonary (heart ↔ lungs) + systemic (heart ↔ body); keeps oxygenated/deoxygenated blood separate, allows high systemic pressure

Mammalian Heart & Cardiac Cycle

• Four chambers; septum separates sides; valves ensure unidirectional flow (atrioventricular: tricuspid, bicuspid; semilunar)
• Cardiac muscle supplied by coronary arteries; myoglobin stores O$_2$
• Cycle: atrial systole → ventricular systole → diastole (total ≈0.8\,\text{s}); heart sounds "lub-dub" = valve closures

Blood Components & Functions

• Plasma (>50%): transports nutrients, wastes (CO$_2$, urea), hormones, heat
• Cells: red blood cells (O$_2$ transport), white blood cells (immune), platelets (clotting)
• Capillary walls one-cell thick → short diffusion paths for exchange