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A Handbook of Biology
ÜPTÅKË ÅÑD TRÅÑSPØRT ØF MÏÑËRÅL ÑÜTRÏËÑTS
ÜPTÅKË ØF MÏÑËRÅL ÏØÑS
Most minerals are actively absorbed by the roots because
1
Minerals occur in the soil as charged particles (ions)
which cannot move across cell membranes.
2
The concentration of minerals in the soil is usually
lower than the concentration of minerals in the
root.
The active uptake of ions is partly responsible for the water potential
gradient in roots, and therefore for the uptake of water by osmosis.
Some ions are absorbed passively.
The specific membrane proteins of root hair cells actively pump ions
from the soil into the epidermal cells.
Endodermal cell membrane also has transport proteins. They allow
some solutes cross the membrane, but not others. These proteins are
control points, where a plant adjusts quantity and types of solutes that
reach the xylem.
The suberin in the root endodermis allows the active transport of ions in
one direction only.
TRÅÑSLØÇÅTÏØÑ ØF MÏÑËRÅL ÏØÑS
The ions reached in xylem are further transported to all parts of the plant
through the transpiration stream.
The chief sinks for the mineral elements are:
Growing regions such as apical and lateral meristems.
Young leaves.
Developing flowers, fruits and seeds.
Storage organs.
Unloading of mineral ions occurs at the fine vein endings through
diffusion and active uptake by these cells.
Mineral ions are also frequently remobilised, particularly from older,
senescing parts (e.g. older dying leaves to younger leaves).