Biology XI: 11 Transport in plants(4 Mechanism of Water Absorption

Mechanism of Water Absorption by Plants Long Distance Transport of Water Diffusion: Slow process and cannot account for the transport of molecules in a 10-m-tall tree Mass flow system: Movement of water, minerals and food in bulk from one part to the other in a plant Results due to pressure difference between two points (either positive hydrostatic pressure gradient or negative) Translocation: Bulk flow through conducting or vascular tissues of plants Conducting tissues − 2 types Xylem Phloem Transports water, salts, nitrogen and hormones Transports organic and inorganic solutes From roots to the other parts − unidirectional From the source to the sink − multidirectional Absorption of Water by Plants Water is absorbed through roots by diffusion. Root hairs (slender, thin-walled extensions of root epidermal cells) increase the surface area for absorption. Once absorbed by root hairs, water moves into deeper layers by 2 pathways − Apoplast Pathway or Symplast Pathway. Apoplast Pathway Symplast Pathway Apoplast is a system of adjacent cell walls that are continuous, except at the casparian strips of the endodermis. Symplast is a system of interconnected protoplasts. Movement occurs through these intercellular spaces or walls of the cells, without entering the cytoplasm. Water enters the cell through the cell membrane and travels intracellularly through plasmodesmata. As water evaporates into the atmosphere, tension develops in the continuous stream of water in Apoplast, and mass flow occurs due to adhesive and cohesive properties. The movement may be aided by cytoplasmic streaming. Movement is fast. Movement is slow as water has to enter the cell. Most of the water flow in roots occurs via apoplast, except at the casparian strip. At the casparian strip region, water moves through the symplast. Some additional structures may also be associated with plants for water absorption.E.g.Mycorrhiza − symbiotic fungus living with the roots of higher plants.Hyphae provide a large surface area for absorption of molecules. In return, the fungus gets sugar and nitrogen-containing compounds from the roots.Pinus shows an obligate association with mycorrhiza.