Download Transport in Flowering Plants

Transport in Flowering Plants
Vascular Bundle
Consists mainly of
 Xylem
 Phloem
Referred to as vascular bundles in stems and steles
in roots
TS of the Stele of a Root
Structure of Xylem Vessels
Structure of Xylem Vessels
Xylem
 a complex tissue composed of xylem vessels,
xylem tracheids, xylem fibres and xylem
parenchyma.
Xylem vessels
 comprise a vertical chain of lengthened, dead
cells known as vessel elements.
 The cells are arranged end to end and the crosswalls dissolve completely or have simple or
complex perforation plates between successive
cells.
 The secondary walls of vessels are impregnated
with lignin and are thickened unevenly.
 The walls of the vessels may be thickened in different
ways, e.g. annular, spiral and pitted
thickening may be observed.
Xylem vessels
 contain pits which are gaps between these vessel
units which allow for the sideways movement of
water into the xylem vessels from the roots to
neighbouring vessels.
Xylem tracheid
 single cells that are long and elongated with tapering




ends which overlap with neighbouring tracheids
the contents of which are non-living.
The cell walls are thickened, impregnated with
lignin and the lumen is smaller.
there is a differentiation between annular, spiral
and pitted tracheids again caused by the type
of thickening of the secondary walls.
have no perforation plates.
Xylem tracheid
 originally thought to be the conducting vessels of
primitive plants, but have now become obsolete
in the common modern types as conducting
vessels.
 provides mechanical strength and support for the
plants.
Xylem parenchyma
 bear a strong resemblance to normal
parenchyma.
 the walls of xylem parenchyma are
sometimes thicker than those of normal
parenchyma.
 used for the storage of substance such as starch,
tannins, crystals.
 living cells with a thin cellulose cell wall.
Xylem fibres
 long cells with thickened cell walls
 much shorter and thicker than tracheids.
 They provide structural support for the plant.
Functions of Xylem
 xylem is an important strengthening tissue,
 xylem vessels and tracheids transport water and
mineral salts,
 starch is sometimes stored in the xylem fibres and
xylem parenchyma.
Xylem Thickening
 Annular
 Spiral
 Reticulate
 Pitted
Uptake of Ions by Active Transport in
Roots
 Active transport-the movement of ions, derived from
the dissociation of mineral salts in solution, from an area
of low concentration to an area of higher concentration,
which is against the concentration gradient, using energy
from ATP.
 Mineral nutrients absorbed from the root has to be
carried to the xylem. This transport follows two
pathways namely apoplastic pathway and symplastic
pathway.
Root
Structure
Uptake of Ions
 In apoplastic pathway, mineral nutrients along with
water moves from cell to cell through spaces between
cell wall by diffusion. The ions, which enter the cell wall
of the epidermis move across cell wall of cortex,
cytoplasm of endodermis, cell walls of pericycle and
finally reach the xylem.
 In symplastic pathway, mineral nutrients entering the
cytoplasm of the epidermis move across the cytoplasm of
the cortex, endodermis of pericycle through
plasmodesmata and finally reach the xylem.
Uptake of Ions
 Plant cells need ions for their efficient functioning eg. Mg2+ for
the formation of chlorophyll, Ca2+ ions for the formation of the
middle lamellae of cell walls, Mo2+ (Molybdenum ions) used in
the reduction of nitrates (NO3-) to nitrites (NO2-)in the synthesis
of amino acids by plants.
 However, there are two hurdles to cross: (1) The ions are polar and have to pass across the cell
membrane which is a lipid bilayer and possess hydrophobic
regions which do not mix with polar components.
 (2) Ions in the external soil where the plant resides are of a
lower concentration than what is found inside.
Uptake of Ions
 The problem is overcome by employing the process of active
transport which moves the ions against their concentration
gradient with the use of energy as ATP and also provides a
channel through which they pass.
 Once the ions enter the root hairs they use the apoplastic and
the symplastic pathway to move through the cells of the
epidermis in solution where water is taken up
 This acts as the medium of transport of the ions through this
pathway.
Uptake of Ions
 the solution with the desired ions now reach the
endodermis of the root which have strips of suberin
coatings within their cell walls called Casparian strips
 this halts the flow of solution via the apoplastic pathway.
 The solution is now forced to take the symplastic
pathway (because of the endodermis) where, if ions are
involved, they are transported into the cytoplasm by
specific and selective ion channels via the process of
active transport
Uptake of Ions
 Once the solution is across the endodermis (controls and
monitors the types of ion that eventually enter the xylem
vessels) it can now enter the vascular bundle via the same
symplastic pathway and eventually the xylem vessels.
Uptake of Ions
Entry of Water Into Plant Roots
 water must first get to the xylem vessels from the soil
 This is done by the process of osmosis across the root
hairs, where the water potential in the soil is higher than
the water potential of the cells of the root hairs so the
water molecules move down the concentration gradient
into the cells of the root hairs.
 Once in the cells of the root hairs (higher water
potential) the water molecules again move down a
concentration gradient into the neighbouring cells of the
epidermis and cortex as these cells have a lower water
potential
 this movement is done via the apoplastic and symplastic
pathways.
Entry of Water Into Plant Roots
 Upon reaching the endodermis water becomes
impermeable to the suberin coatings (Casparian Strips)
in the cell wall of the endodermis, thus movement is
halted via the apoplastic pathway and restricted to
movement only through the symplastic pathway again
down a concentration gradient from high water potential
to low water potential.
 Finally the water enters the xylem vessels through the
pits in their walls, where the journey up the xylem and
into the leaves now begins.
REMEMBER! WATER POTENTIAL
1. Pure water has a max. ᴪ of zero
2. Water always moves from a region of higher ᴪ to a
region of lower ᴪ
3. All solutions have a lower ᴪ than pure water ie.
solutions have –ve values
4. Osmosis can be defined as the movement of water
molecules from a region of high ᴪ to a region of lower
ᴪ through a partially permeable membrane