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Review: • Three regions – Meristematic – Elongation – Maturation Root Anatomy • Root hairs and branching greatly increase the surface through which water and dissolved minerals can be absorbed. Functions of Roots • Absorption TODAY • Anchorage • Storage • Conduction of water • Hormone synthesis TOMORROW Uptake of Ions by the Roots – WHAT? Root hairs bring nitrates, phosphates, sulphates, and a variety of other essential inorganic nutrients into the plant Why are these nutrients needed? Uptake of Ions by the Roots – HOW? • Root interception – Root grows and intercepts ions • Simple diffusion – Ions move down their concentration gradient – No energy expense by plant – Example: K+ • Mass flow – Bulk flow of water into the root “carries” ions to root – Delivers N, Ca, Mg, S • Active transport – Ions move against their concentration gradient – Requires a specific protein “pump” in the cell membrane – Energy expense by plant Water Transport into the Roots Water flows from Epidermis Root Cortex Endodermis Xylem SOIL via osmosis Water Transport into the Roots • Cells are usually interconnected via plasmodesmata. • Their plasma forms a continuum (a “symplast”). Water Transport into the Roots • The transport of substances from cell to cell via plasmodesmata is called symplastic transport. Water Transport into the Roots • Apoplastic transport is when water is transported between cells through the extra-cellular space. Water Transport into the Roots • The endodermis functions as a last checkpoint for the passage of dissolved minerals into the vascular tissue Water Transport into the Roots • Minerals already in the symplast when they reach the endodermis continue through the endodermis and pass into the xylem Water Transport into the Roots • Minerals that reach the endodermis via the apoplast encounter a waxy layer on the endodermis that blocks their entry into the xylem called the Casparian strip. Water Transport into the Roots • The only way to get through the endodermis is through the symplastic route. Water Transport into the Roots • Water and minerals following the apoplastic route must first cross the plasma membrane of the endodermis cells and enter the symplast system. This ensures that no minerals can enter the vascular system with out first being “checked” by the plasma membrane Water Transport into the Roots • Once through the endodermis layer, the water and minerals are now free to enter the xylem tracheids and vessels. The Casparian Strip blocks Uptake of water through passage of water unless it root epidermis by osmosis is moving via the symplast Water that does not cross the Water that crosses the root hair plasma membrane hair plasma enters the root apoplast system membrane enters the symplast system Water moves into the xylem for transport up the plant