Transport in Plants

a) Xylem Dead, hollow cells stacked on top of one another forming a hollow tube. Conducts water and minerals from roots of plants to stem and leaves.

Transport in Xylem Water moves into the plant by osmosis through the root hairs into the epidermal cells. Water is then transported from the root up the plant stem through the xylem: water and minerals move by diffusion into tissues of the plant.

How does water move against the force of gravity? Root Pressure Pressure exerted on root when soil moisture is high

2. Capillary Action The tendency of a liquid to rise in narrow tubes result of the intermolecular attraction within the liquid

3. Cohesion Tension Theory Water is a polar molecule (bond electrons pulled closer to oxygen)  When water molecules approach they are attracted to each other and form a Hydrogen Bond.  Water is constantly lost by transpiration in the leaf through the stoma. When one water molecule is lost another is pulled along by the cohesive force of waters hydrogen bonds.

Terminology Adhesion: Attractive force between unlike molecules Water may cling to other groups of molecules such as cellulose Water clings to walls of xylem helping it to move upward against gravity

Terminology Continued Cohesion: Attractive force between like molecules Water is polar which means one end of the water molecule is attracted to the other end of a different molecule thus the molecules stick together. Water molecules use cohesion to tug each other from root to leaf.

How is transpiration controlled? Controlled by opening and closing of stomates Guard cells control the movement of stomates Turgid (water filled)  forces cells to open Flaccid (water lost)  forces cells to close

What about minerals? - Minerals move by Active Transport in the form of mineral ions from the soil into the roots; these ions are incorporated into various compounds and then released into the xylem where they move with water.

Lets Review Cohesion Theory 1. Transpiration Water evaporates through the stomata of the leaves 2. Result Tension which provides the force needed for the water to move up the xylem, against gravity.

3. Cohesion: Within the xylem, hydrogen bonds hold nearby water molecules together firmly, like a chain (strongly attracted) 4. Cohesion: Water molecules exiting the xylem in the leaf, tug on the rest of the chain which is pulled up the xylem.

5. Adhesion Between water molecules and the xylem wall, aids in the ability of the water column to move upward (cling to wall of xylem due to attraction) Water enters the root hairs by osmosis to replace, water which has been pulled by the xylem.

b) Phloem Living cells containing cytoplasm that allow exchange of materials with the surrounding cells. Transports organic materials such as nutrients and hormones throughout the plant.

Transport of Carbohydrates in Phloem Experiments with radioactive isotopes have shown that carbohydrates travel in the phloem Carbohydrates (glucose, sucrose, starch) are produced in the leaves (source) The stem, root tip, flowers and fruits need carbs for growth (sink) Movement of carbohydrates from source to sink is called translocation

Translocation Cells of the phloem form a continuous pipeline called a sieve tube. Sieve tube cells are perforated at the end living cells that have lost their nuclei Companion cells therefore take over control of sieve cells activities

Pressure Flow Model Most accepted theory for translocation Result of differences in pressure

Main Points of Model Active transport moves sugar molecules from source into sieve tube Solution inside tube is hypertonic  water enters tubes by osmosis from neighbouring cells of xylem  increases hydrostatic pressure inside sieve tubes Increased pressure pushes sap down into next sieve tube…and so on Active transport moves sugar from sieve tubes into sink cells moving water with it by osmosis This relieves pressure in tube and leaves sieve cells hypotonic

Continued 6. Difference in pressure moves sap from source to sink 7. Sugar in sink cells is stored as starch or used for metabolism 8. Xylem recycles water from sink to source

Videos http://www.biol.unizh.ch/filme/life_on_earth/Chapter_24/Present/Activities/24_4/24_4_1a.swf http://nortonbooks.com/college/biology/animations/ch31a06.swf

Homework Read pages 320-321 Answer questions page 326 #1-7