Example of Diffusion in Biology

In this problem, we are asked to identify which of the provided options is an example of diffusion within the human respiratory system.

First, let's define diffusion. Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration. In biology, this is a passive process, meaning it does not require energy input from the cell.

Now, let's evaluate each option. Option one is warming and moistening of inhaled air. This is a thermal and humidifying process occurring in the nasal cavity and upper airways, not a movement of gas molecules across a membrane driven by concentration.

Option two is the build-up of carbon dioxide in the cells. While this creates the gradient necessary for future diffusion, the 'build-up' itself is a result of metabolic reactions, not the movement of the gas across a membrane.