The Black Death was an epidemic of bubonic plague, a disease caused by the bacterium Yersinia pestis that circulates among wild rodents where they live in great numbers and density. Such an area is called a âplague focusâ or a âplague reservoirâ.
Diffusion is the net movement of molecules or atoms from a region of high concentration to a region of low concentration.his is also referred to as the movement of a substance down a concentration gradient. A gradient is the change in the value of a quantity (e.g., concentration, pressure, temperature) with the change in another variable (usually distance).
But as organisms increase in size, they need a specialized transport system to exchange substances between the internal environment and the external environment. Interestingly enough, osmosis and diffusion are still the important mechanisms at the cellular level. + 60 others found this useful. Helena Trang.ut as organisms increase in size, they need a specialized transport system to exchange substances between the internal environment and the external environment. Interestingly enough, osmosis and diffusion are still the important mechanisms at the cellular level. + 60 others found this useful. Helena Trang.