 
Plants' Mineral Selection Abilities
How Roots Take in Ions from the Soil
 The
cells in the roots of a plant select particular ions from
the soil to use in cell reactions. Plant cells can easily
take these ions inside themselves, despite the internal concentration
of some ions in the plant being a thousand times greater than
that in the soil solution. So, this is a most important process.
Under normal conditions, a transfer of materials will occur from an area with a higher concentration to one with a lower concentration. But as we have seen, just the opposite takes place in the roots' absorbing ions from the soil. For this reason the process requires quite substantial amounts of energy.
Two factors influence the passage of the ions through the cell membrane: the membrane's permeability and the concentration of the ions on either side of the membrane.
Let us examine these two factors by asking some questions
about them. What does a plant's choosing the required elements
from those in the soil actually mean? Let us first take the
concept of "requirements." A root cell has to know
all the elements in the plant, one by one, to meet its requirements.
It has to establish which of all the elements it knows are
lacking in all parts of the plant and identify them as needs.
Let us ask another question. How is an element known? If the
soil is not in a pure state, in other words if there are other
elements mixed up in it, what has to be done to distinguish
one element from all the rest?
Will it be possible for someone to tell which is which if elements such as iron, calcium, magnesium, and phosphorus are put in front of him all mixed up? How can he tell them apart? If he has received training in the subject, he may be able to identify some of them. It will be impossible for him to identify the rest, however. So how do plants make the distinction? Or rather, how is it possible for a plant to know elements by itself, and to find those ones most useful for it? Is it possible that such a process should have been carried out in the right way every time for millions of years by chance? In order to think about all of these questions-to which the answer is "Impossible!"-in a more detailed and deeper way, let us examine what kind of selective property roots possess and what happens at the time of selection.
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