Chapter 29 Phloem and Stomata

You Must Know How phloem sap moves through plants. How and why stomata open and close.

Concept 29.7: Sugars are transported from sources to sinks via the phloem The products of photosynthesis are transported through phloem by the process of translocation. The translocation of phloem sap through sieve tubes by bulk flow is driven by positive pressure. Phloem In angiosperms, sieve-tube elements are the conduits for translocation. © 2014 Pearson Education, Inc. 3

Bulk flow, the movement of a fluid driven by pressure. Bulk flow differs from diffusion It is driven by differences in pressure potential, not solute potential. It moves the entire solution, not just water or solutes. It is much faster. © 2014 Pearson Education, Inc. 4

Loading of sucrose into phloem Mesophyll cells have lots of chloroplasts Apoplast Symplast Mesophyll cell Mesophyll cell Phloem parenchyma cell Sieve-tube Element (part of the phloem) (b) A chemiosmotic mechanism is responsible for the active transport of sucrose. High H+ concentration Low H+ concentration Proton pump Cotransporter Sucrose S H+ Sucrose manufactured in mesophyll cells can travel via the symplast (blue arrows) to sieve-tube elements. In some species, sucrose exists the symplast near sieve tubes and travels through the apoplast (red arrows). It is then actively accumulated from the apoplast by sieve-tube elements and their companion cells. A storage organ can be both a sugar sink in summer and sugar source in winter. Sugar must be loaded into sieve-tube elements before being exported to sinks. Depending on the species, sugar may move by symplastic or both symplastic and apoplastic pathways. Companion cells enhance solute movement between the apoplast and symplast. In most plants, phloem loading requires active transport. Proton pumping and cotransport of sucrose and H+ enable the cells to accumulate sucrose. At the sink, sugar molecules diffuse from the phloem to sink tissues and are followed by water. Loading of sucrose into phloem 5

Sieve tube (phloem) Vessel (xylem) Figure 29.22 Sieve tube (phloem) Bulk flow by positive pressure 1 2 Loading of sugar H2O Sucrose Source cell (leaf) 1 Vessel (xylem) 2 Uptake of water Bulk flow by negative pressure Unloading of sugar Sink cell (storage root) Sucrose 3 Figure 29.22 Bulk flow by positive pressure (pressure flow) in a sieve tube Recycling of water H2O Sucrose 4 3 6

http://bcs.whfreeman.com/webpub/Ektron/Hillis%20Principles%20of%20Life2e/Animated%20Tutorials/pol2e_at_2504_The_Pressure_Flow_Model/pol2e_at_2504_The_Pressure_Flow_Model.html

Stomata: Major Pathways for Water Loss About 95% of the water a plant loses escapes through stomata. Guard cell Leaves generally have broad surface areas and high surface-to-volume ratios. These characteristics increase photosynthesis and increase water loss through stomata. Guard cells help balance water conservation with gas exchange for photosynthesis. Each stoma is flanked by a pair of guard cells, which control the diameter of the stoma by changing shape. Stomatal density is under genetic and environmental control. 8

cellulose microfibrils Figure 29.19a Radially oriented cellulose microfibrils Cell wall Vacuole Guard cell Guard cells turgid/Stoma open Guard cells flaccid/Stoma closed Changes in turgor pressure open and close stomata . When turgid, guard cells bow outward and the pore between them opens. When flaccid, guard cells become less bowed and the pore closes. The radial orientation of cellulose microfibrils in the cell walls cause the guard cells to increase more in length than width when turgor increases. Since the two guard cells are tightly joined at their tips, they bow outward when turgid, causing the stomatal pore to open. (a) Changes in guard cell shape and stomatal opening and closing (surface view) 9

Guard cells turgid/Stoma open Guard cells flaccid/Stoma closed CYTOPLASM EXTRACELLULAR FLUID Proton pump Hydrogen ion (a) H+ and membrane potential H+ Guard cells turgid/Stoma open Guard cells flaccid/Stoma closed H2O H2O H2O H2O H2O K+ K+ H2O Changes in turgor pressure result primarily from the reversible uptake and loss of potassium ions (K) by the guard cells. Proton pump actively pumps hydrogen ions out of the guard cell. The resulting voltage (membrane potential) drives K+ into the cell through specific membrane channels. The absorption of K+ causes the water potential to become more negative within the guard cells, and the cells become more turgid as water enters by osmosis. Stomatal closing results from a loss of K+ from guard cells to neighboring cells, which leads to an osmotic loss of water. H2O H2O H2O H2O (b) Role of potassium ions (K+) in stomatal opening and closing 10

Stimuli for Stomatal Opening and Closing Generally, stomata open during the day and close at night to minimize water loss. Stomatal opening at dawn is triggered by Light CO2 depletion An internal “clock” in guard cells All eukaryotic organisms have internal clocks; circadian rhythms are 24-hour cycles 11