Biological and Physical Sciences: Reasoning Across Disciplines
What you must know
This section rewards conceptual understanding across biology, chemistry, and physics, applied to unfamiliar data. Key areas: cell biology and genetics, human physiology and homeostasis, organic and acid-base chemistry, mechanics and energy, electricity and waves, and data interpretation. You are not expected to memorise every formula; you are expected to reason from principles.
Reasoning habits
Translate word problems into quantities with units before calculating. Check dimensional consistency. Estimate first, then compute - an answer an order of magnitude off is wrong regardless of arithmetic. For graphs, identify axes, units, and whether the relationship is linear, inverse, or exponential.
Traps
- Unit slips. Millilitres versus litres, kilojoules versus joules, minutes versus seconds.
- Confusing correlation with causation in experimental data; look for controls and sample size.
- Misreading inverse relationships - doubling one variable halves the other, not doubles it.
- Ignoring homeostasis. Physiological systems often respond in the opposite direction to the disturbance (negative feedback).
- Over-applying a formula outside its valid conditions, such as ideal-gas assumptions.
Worked example
A 0.10 mol/L solution of a weak monoprotic acid has pH 3.0. Estimate Ka. From pH, [H⁺] = 10⁻³ mol/L. Assuming the undissociated concentration is essentially unchanged, Ka ≈ [H⁺]²/[HA] = (10⁻³)²/0.10 = 10⁻⁵. The reasoning - approximating a small degree of dissociation - matters more than the arithmetic.
Rule of thumb
When stuck, ask: what is conserved? Energy, mass, charge, and momentum conservation solve many problems that appear to require a formula you cannot recall.