1.7 – THE TRANSPORT OF SUBSTANCES IN PLANTS

LEARNING OUTCOMES To state the necessity for transport of substances in plants To identify the vascular tissue in stem, root & leaf To state the role of vascular tissue in the transport of substances To describe the structure of vascular tissue To relate the structure of xylem to transport

plants need Water & mineral Food Capillary action Transported by involves Root pressure involves xylem phloem translocation Transpirational pull Relate to structure Results in Relate to Transpiration Vascular tissues affecting Factors Air movement Temperature Light intensity Relative humidity Found in stem root leaf

NECESSITY Essential to transport water & mineral salts absorbed by the roots have to be transported to all parts of the plants. Water – as a solvent for biochemical rxns, as reactants in cell metabolism (ex.:light rxn)

Mineral ions – for chlorophyll synthesis, healthy plant growth & development. Organic food materials synthesised by the leaves during photosynthesis have to be transported to the growing regions, storage organs & other parts of the plant. Small, simple multicellular plants no need vascular system b’coz have a large TSA/V ratio.

Large flowering plant @ angiosperms , conifer n ferns have a small TSA/V ratio n the substances have to move a greater distance  need vascular system Unlike animals, plants are unable to pump necessary substances through great distances, sometimes requiring the need to defy gravity. To overcome, plants use a combination of root pressure, capillary action n transpiration to provide enough force to transport water to shoot

Vascular Tissue in Stem, Root & Leaves XYLEM – transport water & mineral salts PHLOEM – transport organic substances Vascular system is not involved in the transport of oxygen & carbon dioxide.

STRUCTURE OF XYLEM IN RELATION TO TRANSPORT It has two important functions : It transport water & mineral ions from roots to the upper parts of the plant It also provide mechanical support to the plant In flowering plants, the xylem consists of xylem vessels, tracheids, parenchyma & fibres. The vessels are elongated cells arranged end to end. To allow water to flow in a continuous column.

The end walls of the vessels have broken down to provide an uninterrupted flow of water up the plant. The side walls of xylem vessels are perforated by pits, which allow water & mineral salts to pass sideways. The lignified walls make the xylem vessels rigid to prevent them from collapsing under the large tension forces set up by the transpiration pull.

The narrowness of the lumen of xylem vessels increases the capillarity forces. The lignified walls of the xylem vessels increase the adhesion of water molecules & helps the water to rise by capillarity. Mature vessels are dead cells with no protoplasm to obstruct the flow.

Conifers & ferns do not have xylem vessels Conifers & ferns do not have xylem vessels. They only have tracheids which are less efficient in water transport. Tracheids do not have open ends to form a continuous hollow tube to pass water from cell to cell through pits.

STRUCTURE OF PHLOEM IN RELATION TO TRANSPORT Transports organic food substances (sucrose & amino acids) from leaves to various plant parts Consists mainly of sieve tubes & companion cells A sieve tube is a cylindrical tube made up of elongated living sieve tube cells.

The cross-walls separating the sieve tube cells are perforated by small pores. The cross-walls with the pores look like a sieve & are called sieve plates. There are cytoplasmic connections between the sieve tube cells through the sieve pores. (allow the flow of dissolved food materials from one sieve tube cell to the next).

Mature sieve tubes  many cell organelles including nucleus degenerate. A thin layer of cytoplasm & some mitochondria are found lining the inside of the thin cellulose cell wall  less resistance to the rapid flow of nutrient solution through the sieve tube cells. Companion cells are only found in flowering plant, not in conifers or ferns. Its adjacent & closely associated with the sieve tube cells.

Each companion cells has a nucleus, dense cytoplasm & many mitochondria. Help to transport manufactured food from leaf cells into the sieve tubes. Many mitochondria to generate ATP needed for active transport of sucrose from companion cells into the sieve tubes.

When a stem of a woody plant is ringed, the bark containing tissues external to the xylem (including phloem tissue) is removed Nutrient solution containing organic substances is prevented from being transported to the roots. Food materials would accumulate in the outer stem, above the ringed region of the plant.

In early stage, no wilting because xylem vessels can still transport water & minerals from the roots to the upper parts of the plant. The plant eventually wilt & die because food synthesised in the leaves cannot be transported to the root cells  can no longer absorb water & mineral salts from the soil solution.

FIRE!!! WACHA!!!! END OF 1.7