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Absorption of water and minerals Water and minerals enter the plant through the epidermis of the root, through the cortex, and into the stele (vascular xylem) Diagram on pg. 755 Endodermis is very selective in what gets to the xylem. The minerals are “screened” by the symplast when they enter at the epidermis Those that enter the endodermis through the apoplast, are stopped by the Casparian strip – a waxy barrier However, the water can cross the membrane and enter with symplast water. Then the water enters the tracheid and vessels elements of the xylem – these are apoplastic – solutes are transferred from symplasts to apoplast. Now, the water and minerals can be transported throughout the plant. Transport of xylem sap Water is being “pulled” by transpiration, so must be replenished by soil water. At night, root cells push mineral ions into the xylem This lowers the water potential causing water to flow in and force water up the xylem – root pressure This cause guttation fluid in the morning Root pressure cannot compete with transpiration at sunrise. Guttation Cohesion and Adhesion The cohesive and adhesive nature of water contribute to transpiration Guard cells In dicots, guard cells take on water, become turgid, and expand due to the position of the cellulose microfibrils. This opens the stomata Influx of K ions also causes water to enter the guard cells and the stomata to open. Light triggers this. Stomatal opening also correlates with H ion being transported out of the cell Blue light receptors in the guard cells are triggered at dawn to power proton pumps and promote K ion uptake Also, guard cells begin to photosynthesize, making ATP for proton pumps. Guard cells also contain an internal clock that open and close stomata C4 Plants C4 plants minimize the cost of photorespiration by incorporating CO2 into four-carbon compounds in mesophyll cells This step requires the enzyme PEP carboxylase PEP carboxylase has a higher affinity for CO2 than rubisco does; it can fix CO2 even when CO2 concentrations are low These four-carbon compounds are exported to bundle-sheath cells, where they release CO2 that is then used in the Calvin cycle © 2011 Pearson Education, Inc. Figure 10.20 The C4 pathway C4 leaf anatomy Photosynthetic cells of C4 plant leaf Mesophyll cell PEP carboxylase Mesophyll cell Bundlesheath cell Oxaloacetate (4C) Vein (vascular tissue) PEP (3C) ADP Malate (4C) Stoma Bundlesheath cell CO2 ATP Pyruvate (3C) CO2 Calvin Cycle Sugar Vascular tissue CAM Plants Some plants, including succulents, use crassulacean acid metabolism (CAM) to fix carbon CAM plants open their stomata at night, incorporating CO2 into organic acids Stomata close during the day, and CO2 is released from organic acids and used in the Calvin cycle © 2011 Pearson Education, Inc. Figure 10.21 Sugarcane Pineapple C4 Mesophyll cell CAM CO2 Organic acid 1 CO2 incorporated (carbon fixation) Calvin Cycle Sugar (a) Spatial separation of steps Organic acid Night CO2 CO2 Bundlesheath cell CO2 2 CO2 released to the Calvin cycle Calvin Cycle Day Sugar (b) Temporal separation of steps Transport of Sugars Translocation is the movement of carbohydrates through the phloem from a source to a sink. The source is leaves, the sink is where the carbohydrate will be used. Pressure-flow hypothesis 1. 2. 3. 4. 5. Soluble sugars like fructose and sucrose move from palisade mesophyll to sieve tube members by active transport Water then diffuses into the cells Pressure in sieve tube causes water and sugar to flow toward sink Sugars are moved by active transport into neighboring cells Water diffuses back to xylem Starch Any cell can act as a sink if they convert soluble sugar into starch. Any cell can act as a source if they break down starch into glucose.
