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1 mM = 10^{-3} M = 1000 μM = 10^{6} nM
Prefix fluency (common SI prefixes):
- p = 10^{-12}
- n = 10^{-9}
- μ = 10^{-6}
- m = 10^{-3}
- c = 10^{-2} (centi)
- d = 10^{-1} (deci)
- 1 = base unit
- k = 10^{3} (kilo)
- M = 10^{6} (mega) [note: in chemistry, M also denotes molarity; avoid ambiguity in problems]
- G = 10^{9} (giga)
Rule of thumb for body compartments (percent of body mass):
- Total body water (TBW) ≈ 60% of body weight
- Intracellular fluid (ICF) ≈ 40% BW
- Extracellular fluid (ECF) ≈ 20% BW (plasma ≈ 5% BW; interstitial fluid ≈ 15% BW)
Ideal gas at STP: P = 1 atm, T = 273.15 K; used for gas-law estimates
About diffusion and transport: patterns that set rate and timing scales; relate to Fick’s law and Stokes–Einstein; diffusion time scales with distance as t ∝ L^2
Fast diffusion in water tends to occur for small, less hindered species; diffusion rate increases with temperature and decreases with viscosity and particle size
Basic gas–solid–liquid intuition helps frame how molecules move across barriers and through compartments

Diffusion basics:
- Net movement of solute from high to low concentration driven by random thermal motion
- Thermal motion is the random molecular motion due to temperature; collisions are elastic in idealized models
Fick’s law (through a slab):
- Flux: J = -D rac{dC}{dx}
- For a slab of thickness $T$ with concentrations C1 and C2 on each side, the rate of transfer is ext{Rate} = D rac{A (C1 - C2)}{T}
- Here: D is the diffusion coefficient (units: $L^2/T$ ), A is area, T is thickness
Diffusion coefficient (D) factors:
- D↑ with increasing temperature (T)
- D↓ with higher solvent viscosity (η) and larger particle radius (r)
Stokes–Einstein equation (diffusion in liquids):
- $$D = rac{k_B T}{6 \,
  {b1} \, 6 \infty} \, \