Transport in Vascular Plants

Transport in Plants Overview  movement of materials from one part of a plant to another  involves 2 specialized tissues: 1. Xylem 2. Phloem

Xylem Tissue Structure:  made of thick cell walls  most cells are dead  cells are stacked up on top of another to form a hollow tube through the plant Function:  provides structural support to the plant  carries water and minerals from roots to leaves

Phloem Tissue Structure  cells are living  cell walls are porous, allowing exchange of materials with neighbouring cells Function  carries nutrients (sugars and hormones) throughout the plant

Water Transport in Xylem (1 of 2)  Roots take in water through the root hairs & epidermal cells by osmosis  Water flows through the cell walls, into the intercellular spaces within the root, and enter the xylem  Water is then transported in the xylem tissue up through the root into the stem  Within the stem, water and minerals move by diffusion (and, to a lesser extent, active transport) into the other tissues of the plant

Water Transport in Xylem (2 of 2)  As the xylem tissue carrying water and other minerals enters the leaf, the conducting vessels branch and rebranch into the numerous veins visible in the leaf  From the end of each vein, water and minerals can diffuse into the cells of the leaf  About 99% of the water that reaches the leaf is lost through transpiration water vaporizes from the leaf through the stomata  this draws (pulls) water up the rest of the plant

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 1. Root pressure: because of a concentration gradient through osmosis, water enters the root 2. Capillary action: water is drawn into the tiny diameter of the root 3. Cohesion-tension: water likes to stick together and travels up the xylem

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

2. Capillary Action  takes place because of the adhesion of water molecules and walls of xylem turn upwards  the finer the capillary or vessel, the higher water will climb up

3. Cohesion-tension  Also known as “transpiration pull”  &feature=related  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  Scientists still unsure of how exactly transport happens in 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