Question

Why absorption is not completely passive ?

Answer 1

Heya @user
Here is your answer

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Minerals are present in the soil as changed particle which mostly cannot move across cell membranes passively.

The concentration of minerals is the soil is usually lower than the concentration of minerals in the roots

That's why Absolution is not completely passive.

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Hope helped !!

Answer 2

The absorption of dietary calcium occurs by two pathways: active transport, which is dependent on vitamin D2; and passive diffusion, which is independent of vitamin D. Establishing the relative contribution of these processes is an important nutritional issue because it could provide a rationale for supplementation. If passive diffusion were most important, then simply increasing the luminal concentration of calcium by increasing intake would, by definition, suffice to increase calcium absorption. If the vitamin D-dependent path were more critical, then the amount of calcium consumed would be important but insufficient to promote calcium absorption because it would be determined (regulated) by vitamin D-dependent mechanisms. In promoting calcium nutrition, the emphasis would presumably be different depending on which of these processes were most critical.