Top Key Points: Lipids and Nucleic Acids

Lipids:

• Hydrophobic (water-insoluble) biomolecules. Diverse class — not all lipids are the same
• Triglycerides: glycerol + 3 fatty acids (ester bonds). Non-polar. Function: long-term energy storage $\frac{9 kcal}{g}$
• Saturated fatty acids: no C=C double bonds → straight chains → tight packing → SOLID at room temperature (animal fats: butter, lard)
• Unsaturated fatty acids: one or more C=C double bonds → kinked chains → loose packing → LIQUID at room temperature (plant oils: olive, sunflower)
• Phospholipids: glycerol + 2 fatty acids + phosphate head group (amphipathic). Form biological membrane bilayers
• Amphipathic = hydrophilic head (phosphate) + hydrophobic tails (fatty acids) → bilayer self-assembly in water
• Cholesterol (steroid): membrane fluidity buffer. At low temp: prevents solidification. At high temp: prevents excessive fluidity
• Steroids: 4-ring carbon skeleton. Derived from cholesterol. Examples: testosterone, oestrogen, cortisol
• Steroids are lipid-soluble → cross plasma membrane → act on nuclear receptors
• Phospholipids ≠ Triglycerides: Phospholipid has 2 FA + phosphate head; triglyceride has 3 FA + NO phosphate

Nucleic Acids:

• DNA: Deoxyribose sugar, Thymine base, typically Double-stranded, STABLE genetic storage
• RNA: Ribose sugar (has 2'-OH), Uracil base (not Thymine), typically Single-stranded, LESS stable
• Chargaff's rules: A = T (2 H-bonds) and G = C (3 H-bonds) in dsDNA
• High GC content → more H-bonds → higher melting temperature (Tm) → more thermally stable DNA
• Nucleotide = nitrogenous base + pentose sugar + phosphate group(s)
• Purines: Adenine (A) and Guanine (G). Pyrimidines: Cytosine (C), Thymine (T, DNA only), Uracil (U, RNA only)
• DNA double helix: antiparallel strands (one 5'→3', complementary strand 3'→5'). Right-handed helix
• Three RNA types: mRNA (carries code), tRNA (brings amino acids; cloverleaf structure), rRNA (ribosome)
• Ribose makes RNA reactive → allows ribozymes to exist and RNA to be chemically catalytic
• Chargaff calculation: if A = x%, then T = x%; G = C = $\frac{100-2x}{2}$%