Transport of Water Entry point –Root hairs by osmosis Two pathways by which water moves toward the center of the root. 1.Apoplast (“nonliving” portion of the cells) –Water moves through cell walls from one cell to another without ever entering the cells. 2.Symplast (“living” portion of cells) –Water moves from the cytoplasm of one cell to the cytoplasm of the next through plasmodesmata.

Transport of Water When water reaches the endodermis, it can continue into the vascular cylinder (stele) only through the symplast pathway. –Apoplast is blocked by suberin that permeates the casparian strip. Endodermal cells allow water to enter the stele and are selective as to which minerals are allowed to enter.

Transport of Water Example –Potassium is allowed to pass (essantial mineral) –Sodium, common in soils but unused in plants, is blocked. Once through the endodermis, water and minerals continue by the apoplast pathway to the xylem. –Trachieds and Vessel elements.

Transport of Water Three mechanisms are involved in the movement of water. 1.Osmosis –Concentration gradient (soil to root) –Continuous movement of water out of the root by xylem –Higher mineral concentration inside the stele maintained by the selective passage of ions through the endodermis. 1.Root pressure (osmotic force)

Transport of Water Three mechanisms are involved in the movement of water. 2. Capillary action Rise of liquids in narrow tubes Contributes to movement of water up xylem Adhesion forces –Meniscus formation

Transport of Water Three mechanisms are involved in the movement of water. 3. Cohesion-tension theory Transpiration –Evaporation of water from plants –Removes water from leaves –Causing negative pressure or tension to develop within the leaves and xylem tissue Cohesion –Produces a single, polymerlike column of water from roots to leaves

Transport of Water Three mechanisms are involved in the movement of water. 3. Cohesion-tension theory Bulk flow –Occurs as water molecules evaporate from the leaf surface. –When a water molecule is lost from a leaf by transpiration, it pulls up behind it an entire column of water molecules. –In this way, water moves by bulk flow through the xylem by a pulling action generated by transpiration »Transpiration is caused by the heating action of the sun, therefore, the sun is the driving force for the ascent of water and minerals through the plants.

Control of Stomata Stomata opening and closing –Influences gas exchange, transpiration, the ascent of water and minerals (sap), and photosynthesis. Closed stomata –Water and carbon dioxide are not available, and photosynthesis cannot occur. Open stomata –Carbon dioxide can enter the leaf. –Water is delivered by the pulling action of transpiration –Problem: the plant risks desiccation from excessive transpiration.

Control of Stomata Mechanism of control –Guard cells Surround each stoma Cell walls are not uniform in thickness. –Thicker in the region that borders the stoma. When water enters the guard cell, it expands. –Due to the nonuniform construction of the cell wall, the expansion is distorted. »Bulging of the thinner wall »Produces two kidney-shaped guard cells »Creates an opening between them, the stoma.

Control of Stomata Mechanisms that control the opening and closing of stomata. 1.Close when temperatures are high. –Reduces loss of water –Shuts down photosynthesis 2.Open when carbon dioxide concentrations are low inside the leaf. –Allows photosynthesis to occur 3.Close at night and open during the day 4.Stomata opening occurs by a diffusion of potasium ions into guard cells, creating a gradient for the movement of water into guard cells.

Transport of Sugars Translocation –Movement of carbohydrates through the phloem from a source (leaves) to a sink (a site of carbohydrate utilization or storage). –Mechanism = Pressure-Flow hypothesis 1.Sugars enter sieve-tube members 2.Water enters sieve-tube members 3.Pressure in sieve-tube members at the source move water and sugars to sieve-tube members at the sink through sieve- tubes. 4.Pressure is reduced in sieve-tube members at the sink as sugars are removed for utilization by nearly cells.