Plant Transport Chapter 36

What you need to know! The function of xylem and phloem tissue The specific functions of tracheids, vessels, sieve-tube elements, and companion cells. The role of passive transport, active transport, and cotransport in plant transport The role of diffusion, active transport, and bulk flow in the movement of water and nutrients in plants. How the transpiration cohesion-tension mechanism explains water movement in plants. How pressure flow explains translocation.

Transport in Plants Solutes diffuse down their electrochemical gradient –Passive transport –Active transport If solutes do not fit across the cell membrane a transport protein is required (i.e. aquaporin for water) Plants most commonly use cotransport with proton pumps –i.e. hydrogen (proton pump) and sucrose Water moves from high to low water potential Bulk Flow is the massive movement of water and solutes through the xylem and phloem

Path of Transport - Transpiration Water and Minerals 1.Soil  root (via root hairs) –Mycorrhizae often help (fungi – mutualism) 2.Roots  Xylem (tracheids and vessels) 3.Xylem (bulk flow)  stem and leaves 4.Leaves  exit plant (transpiration)

Transpiration Mechanism 1.Root Pressure –Positive pressure that forces fluid up through the xylem (caused by water diffusing in through the root) –Not strong enough to push water to the tops of trees 2.Transpiration-cohesion-tension mechanism –Water evaporates from the leaves (caused by low water potential of the air) –Water is cohesive (hydrogen bonds); as it leaves the leaf it pulls the water behind it –Water, stuck to the xylem (adhesion), is pulled into the leaves (cohesion) –This pulls the water below it…

Transport in Roots

Stoma Pores in the leaves that take in carbon dioxide and lose water (transpiration) Guard cells open and close stomata to regulate CO 2 and water levels Opening and Closing: Potassium ions (K+) enter guard cells which lowers water potential Water enters the guard cells, swelling them and opening the stomata The opposite reaction closes stomata

Transpiration

Phloem - Translocation Sieve tube cells move sugar from it’s source to a sugar sink –Sink’s either use (metabolism) or store (roots and fruits) the sugar The mechanism is pressure flow: 1.Sucrose is moved into sieve tubes using proton pumps which builds pressure* 2.The sugar moves through the sieve tube through plasmodesmata and is unloaded to the sugar sink which releases pressure* * The details of the pressure build-up and release are not required

Translocation