Final answer:
Chemical reactions can proceed in either direction, but many reactions in the human body follow a predictable path of least resistance.
Step-by-step explanation:
In theory, any chemical reaction can proceed in either direction under the right conditions. Reactants may synthesize into a product that is later decomposed. Reversibility is also a quality of exchange reactions. For instance, A + BC → AB + C could then reverse to AB + C → A + BC. This reversibility of a chemical reaction is indicated with a double arrow: A + BC ⇌ AB + C.
In the human body, many chemical reactions do proceed in a predictable direction, either one way or the other. You can think of this more predictable path as the path of least resistance because, typically, the alternate direction requires more energy.