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

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 cross- walls 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. Mg 2+ for the formation of chlorophyll, Ca 2+ ions for the formation of the middle lamellae of cell walls, Mo 2+ (Molybdenum ions) used in the reduction of nitrates (NO 3- ) to nitrites (NO 2- )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