PhACs, among them lipophilicity and molecular diameter, strongly influence their
ability to passively pass through the membranes of plant cells. Higher lipophilicity
may allow faster diffusion between lipid bilayers but may impede translocation in
cell wall or the cytosol. However, the majority of PhACs for human or veterinary use
are polar or ionizable compounds. Plants absorb water and the mineral salts
dissolved in it from the soil, through the root. A hydraulic mechanism displaces
water from the roots to the leaves in response to an energy difference in water
potential from a region where the water potential is higher to one in which it is lower
by dragging inorganic elements and organic molecules in its movement. If polar and
ionizable compounds such as PhACs are present in the soil pore water, these are
captured by the roots and absorbed by the plant. Once absorbed, the water devoid of
nutrients retained by the cells is released into the atmosphere in the form of water
vapor (transpiration). Most of the water absorbed by the roots is lost by transpiration
through the leaves and returned to the atmosphere. It is estimated that only 1–5% of
all the water absorbed is retained by the plant, while the rest is emitted from the
leaves. This suction force of the leaves reaches values of tens of atmospheres. Water
and small solutes and consequently polar PhACs in ionized form can move from the
soil pore water to the vascularized tissue of the roots through three paths:
– Transmembrane pathway, from cell to cell (crossing twice the plasmatic membrane of each single cell, in and out)
– Symplastic pathway, after having crossed once the plasmalemma (through cells
via plasmodesmata)
– Apoplastic pathway (along the cell walls through the intercellular space) up to the
endoderm where it must necessarily cross the plasma membrane
In the symplastic and transmembrane way, water and salts pass through the
cytoplasmic membranes of the root hairs and penetrate into the symplast. In the
apoplastic pathway, water and salts pass through the apoplast without ever crossing a
plasmatic membrane.
In order to enter the stele, the water and minerals must pass into the symplast
because the passage through the apoplast is prevented by the walls of the endoderm
(Casparian strip). The endoderm acts as a hydrophobic barrier preventing any
substance from reaching the conductive tissue without crossing a membrane and
preventing the reflux of water and salts from the stele. After passing the endoderm,
the water and minerals will be translocated in the symplastic continuum and finally
reach the leaves.
The ions actively absorbed by the rhizodermic cells follow the symplastic pathway spread from cell to cell through plasmodesmata after having passed the
Casparian strip. At the xylematic parenchyma level, they are actively transferred
into the tracheas or tracheids. Ions transported together with the water via the
apoplastic way are stopped at the lipophilic endodermis and selected by the
cytoplasma membrane of the endodermis cells.
According to a recent study, the symplast pathway could allow the passage of the
small PAhCs absorbed with the flow of water through the Casparian strip toward the
xylem, while the large PAhCs would enter the root through the apoplastic path, and
20
N. Montemurro et al.
ability to passively pass through the membranes of plant cells. Higher lipophilicity
may allow faster diffusion between lipid bilayers but may impede translocation in
cell wall or the cytosol. However, the majority of PhACs for human or veterinary use
are polar or ionizable compounds. Plants absorb water and the mineral salts
dissolved in it from the soil, through the root. A hydraulic mechanism displaces
water from the roots to the leaves in response to an energy difference in water
potential from a region where the water potential is higher to one in which it is lower
by dragging inorganic elements and organic molecules in its movement. If polar and
ionizable compounds such as PhACs are present in the soil pore water, these are
captured by the roots and absorbed by the plant. Once absorbed, the water devoid of
nutrients retained by the cells is released into the atmosphere in the form of water
vapor (transpiration). Most of the water absorbed by the roots is lost by transpiration
through the leaves and returned to the atmosphere. It is estimated that only 1–5% of
all the water absorbed is retained by the plant, while the rest is emitted from the
leaves. This suction force of the leaves reaches values of tens of atmospheres. Water
and small solutes and consequently polar PhACs in ionized form can move from the
soil pore water to the vascularized tissue of the roots through three paths:
– Transmembrane pathway, from cell to cell (crossing twice the plasmatic membrane of each single cell, in and out)
– Symplastic pathway, after having crossed once the plasmalemma (through cells
via plasmodesmata)
– Apoplastic pathway (along the cell walls through the intercellular space) up to the
endoderm where it must necessarily cross the plasma membrane
In the symplastic and transmembrane way, water and salts pass through the
cytoplasmic membranes of the root hairs and penetrate into the symplast. In the
apoplastic pathway, water and salts pass through the apoplast without ever crossing a
plasmatic membrane.
In order to enter the stele, the water and minerals must pass into the symplast
because the passage through the apoplast is prevented by the walls of the endoderm
(Casparian strip). The endoderm acts as a hydrophobic barrier preventing any
substance from reaching the conductive tissue without crossing a membrane and
preventing the reflux of water and salts from the stele. After passing the endoderm,
the water and minerals will be translocated in the symplastic continuum and finally
reach the leaves.
The ions actively absorbed by the rhizodermic cells follow the symplastic pathway spread from cell to cell through plasmodesmata after having passed the
Casparian strip. At the xylematic parenchyma level, they are actively transferred
into the tracheas or tracheids. Ions transported together with the water via the
apoplastic way are stopped at the lipophilic endodermis and selected by the
cytoplasma membrane of the endodermis cells.
According to a recent study, the symplast pathway could allow the passage of the
small PAhCs absorbed with the flow of water through the Casparian strip toward the
xylem, while the large PAhCs would enter the root through the apoplastic path, and
20
N. Montemurro et al.