Concentration Gradient

To understand how net movement of particles occurs, consider an imaginary horizontal section taken perpendicularly across the concentration gradient, and imagine two thin, equal elements of volume, one downstream and one upstream, from our horizontal section. Though we cannot say which direction any particle will move in an interval of time, we can say that on the average, a definite fraction of the molecules in the upstream element will cross the section moving down and that the same fraction of molecules will cross the section moving up in the same interval. Because the concentration of particles is greater in the upstream element, however, a net transfer of nutrient particles occurs from the uphill side where nutrient particles are in greater concentration towards the downhill side where nutrient particles are in lesser concentration. If nutrients continually enter the hemolymph from the gut and are taken up by muscles in the thorax, our driving gradients will transfer mass and energy directionally through the hemocoel even though all particles move by random molecular motions.

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