Transport in Vascular Plants

Transport in Plants Overview movement of materials from one part of a plant to another involves 2 specialized tissues: Xylem Phloem

Xylem Tissue Overall Structure: made of thick cell walls most cells are dead cells are stacked up on top of another to form a hollow tube throughout the plant Overall Function: provides structural support to the plant carries water and minerals from roots to leaves See Table 1 on pg. 555

Phloem Tissue Overall Structure cells are living cell walls are porous, allowing exchange of materials with neighbouring cells Overall Function carries sugars and hormones throughout the plant See Table 1 on pg. 555

Water Transport in Xylem (1 of 2) Roots take in water through the root hairs & epidermal cells by osmosis Water flows through the epidermal cells, cortex and into the vascular cylinder which contains the xylem. Water is then transported in the xylem tissue up through the root into the stem and then branches into the veins of the leaf. It then moves into various tissues of the leaf such as the palisade and spongy mesophyll.

Water Transport in Xylem (2 of 2) Most water exits the plant through a process called transpiration The water diffuses into air spaces in the spongy cells and then diffuses out of the leaf through the tiny pores called stomata. As each water molecule exits the leaf, another one enters the plant through the roots

How does a plant move water up from its roots to its leaves? 3 main theories that have been developed to account for the movement of water in plants these mechanisms are not mutually exclusive; any or all of these may be at work in a single plant at any one time Root pressure Capillary action Cohesion-tension

1. Root Pressure Plant roots build up pressure that forces water upward Occurs normally in shorter plants Pressure built in roots of tall trees won’t really push the water that far up (approx. 1 meter at most)

2. Capillary Action (in narrow tubes) takes place because of the adhesion of water molecules and walls of xylem Adhesion: intermolecular force between water and xylem vessel wall the finer the capillary or vessel, the higher water will climb up

3. Cohesion-tension Also known as “transpiration pull” Transpiration is the evaporation of water vapour from the leaves through the stomata. depends on the cohesive forces between water molecules and adhesive forces between water and the walls of the vessel

Mineral Transport By testing the concentration of minerals in the roots, scientists know that the amount exceeds that of diffusion Thus roots are actively pumping minerals and ions from the surroundings These minerals are dissolved in the water in the xylem or roots, which makes it more concentrated This increases the uptake of water into the plant

Translocation: Transport in Phloem Translocation occurs through the phloem Phloem transport is bi-directional Most accepted theory: mass-flow theory Moves from a sugar source (a place where sugar is produced by photosynthesis or by the breakdown of sugars) to a sugar sink (an organ which consumes or stores sugar) “borrows” water from xylem to help transport sugars

Transport in Xylem vs. Phloem (P. 569—copy into your notes)

Homework: 1) Copy the translocation diagram from P. 569 onto your handout  2) Do P. 570 #4, 6