Transport in Plants

Need for Transport Systems in Plants

Vasculature of Plants Water and minerals are passed up the xylem, ultimately to be transpired out of the plant Sugar, amino acids, and other organic molecules are passed down phloem to the stem and roots

What causes water to move up xylem tubes? Step 1: Absorption at the root—both active transport of minerals and diffusion of H2O are involved.

Step 2: Capillary action Water moves up a narrow xylem tubes because of cohesion (water sticks to itself) and adhesion (water sticks to other surfaces)

Capillary action Cohesion and adhesion cause water to “crawl” up narrow tubes. The narrower the tube the higher the same mass of water can climb. Maximum height: 32 feet.

Cohesion-tension theory Cohesion between water molecules creates a “water chain” effect. As molecules are removed from the column by evaporation in the leaf, more are drawn up.

Step 3: Evaporation Evaporation at the surface of the leaf keeps the water column moving. This is the strongest force involved in transpiration.

Leaf Cross Section

Stomata When stomata are open, evaporation draws water out of the leaf. Gas exchange can also occur to keep photosynthesis and respiration running. When stomata are closed, evaporation cannot occur, nor can gas exchange. What happens to photosynthesis and transpiration?

Summary What is the source of sugar in a plant? How is it transported down? What’s the source of water in plant? How is it transported up?