Natural Sciences Concepts
Natural sciences concepts
This domain tests how well you apply biology, biochemistry, chemistry, and physics to living systems. Expect questions that combine a short experiment or physiological scenario with quantitative reasoning: enzyme kinetics, acid–base balance, thermodynamics, fluid flow, electrical circuits, and genetics.
Core rules and concepts
- Amino acids and proteins: side-chain charge depends on pH relative to pKa; peptide bonds are planar and resonance-stabilized; tertiary structure is driven by hydrophobic burial, hydrogen bonds, salt bridges, and disulfides.
- Enzymes: Michaelis–Menten v = Vmax[S]/(Km+[S]). Competitive inhibitors raise apparent Km with unchanged Vmax; noncompetitive inhibitors lower Vmax with unchanged Km.
- Thermodynamics: ΔG = ΔH − TΔS; ΔG°' = −RT ln Keq. A negative ΔG indicates spontaneity under stated conditions, not necessarily speed.
- Acid–base: Henderson–Hasselbalch pH = pKa + log([A−]/[HA]). Buffers work best within ±1 pH unit of pKa.
- Fluids and circuits: flow Q = ΔP/R; resistance rises with length and falls with radius to the fourth power. In circuits, V = IR and power = IV.
Worked example
A weak acid has pKa 4.8. At pH 5.8, find the base-to-acid ratio. Use pH − pKa = log([A−]/[HA]) = 1.0, so [A−]/[HA] = 10^1 = 10:1. The solution is mostly deprotonated. If pH were 3.8, the ratio would be 10^−1 = 0.1:1, mostly protonated.
Traps that cost marks
Do not confuse ΔG with ΔG°'; conditions matter. Competitive inhibition changes Km, not Vmax. In fluid problems, radius dominates resistance. Check units and whether a system is open or closed. In genetics, distinguish genotype from phenotype and remember that linked genes violate independent assortment.