Physical half-life: the time that it takes for 50% of an isotope to decay to a stable state.
Biological half-life: the time it takes for the substance to leave the body.
Conceptual relation (to remember): physical half-life focuses on radioactive decay; biological half-life focuses on biokinetics and elimination from the body.
Common way to express decay over time (conceptual formula):
N(t) = N0 \, \cdot \, 2^{-\frac{t}{t{1/2}}}
where N0 is the initial amount and t{1/2} is the physical half-life. After one half-life, N(t{1/2}) = \dfrac{N0}{2}.
Note: The transcript contrasts physical vs biological half-life to differentiate decay vs elimination.
Polarity, Solvent Properties, and Water
Polarity concept:
Molecules that share electrons unequally are polar.
Molecules that share electrons equally are nonpolar.
The water context in the transcript:
Water is polar.
Water is described as a universal solvent because it dissolves more substances than any other solvent.
Biological relevance:
Metabolic reactions in the body rely on the solvency of water to occur and to transport molecules.