But that wasn't the question. The question was how such a small amount could be significant. This question can also be present by way of comparison with natural contributors to the greenhouse effect such as water vapor, natural CO2 sources, etc., and it can then be asked how the small amount contributed by human activity can matter.
The correct answer, or what approaches one I think, is that if human activity is having an effect, it is because there is essentially a homeostasis in play with respect to the natural contributors. In any system that is at homeostasis, a small change can make a difference. Further, if there is feedback in the system a small change can start to run away.
That's the proper answer, not a poor analogy to cyanide and the body's electron transport system.