(B) Sieve plate P-protein Sieve tube Sieve plate element Lateral sieve Modified plastid Sieve tube 日日日 element Companion Smooth ell endoplasmic reticulum Branched Cytoplasm plasmodesma Plasma Vacuole membrane Chloroplast Figure 6-1 Thickened一 primary wall Nucleus Sieve plate pore Mitochondria Sieve plate
Figure 6-1
Middle lamella Figure 6-2 lasma membrane Protein particles on outer leaflet of er ell wall ooplast cytoplasm Endoplasmic reticulum acute plasmodesma Protein particles on Desmotubule Protein particles on inner leaflet of Er with appressed ER inner leaflet of plasma membrane
Figure 6-2
Plasma membrane Cytoplasm Endoplasmic reticulum Desmotubule Central rod Desmotubule St Cell wall Plasma membrane Cell wall Middle lamella Central cavit Cytoplasmic sieeve Cross sections I FIGURE 1.27 Plasmodesmata between cells. (A) Electron micrograph of a wall separating two adjacent cells, showing the plasmodesmata. (B)Schematic view of a cell wall with Central cavity two plasmodesmata with different shapes. The desmotubule is continuous with the ER of the adjoining cells. Proteins line Central rod the outer surface of the desmotubule and the inner surface of Cytoplasmic Spokelike the plasma membrane; the two surfaces are thought to be sleeve filamentous connected by filamentous proteins. The gap between the pro proteins teins lining the two membranes apparently controls the mol- ecular sieving properties of plasmodesmata. (A from Tilney et al. 1991: B after Buchanan et al. 2000.) igure 6-2-1
Figure 6-2-1
Acquiring phloem sap. Aphids are small insects that remove nutrients from phloem by means of a needlelike mouthpart called a stylet a Aphid with stylet in place. b When the experimenter removes the aphids body, phloem sap is available for 50 collection and analysis. stylet Figure 6-4
Figure 6-4
Mouthpart Sieve the Mouthpart 25m Aphids used to study translocation in phloem. (a)Mature aphid, a tiny insect about 3 to 6 mm in length, feeding on a stem. (b) LM of phloem cells, showing a sieve tube member that has been penetrated by the aphid mouthpart (a, Dwight Kuhn; b, M.H. Zimmerman, Science, Vol. 133, pp. 73-79 (Fig 4), 13 Jan. Figure 6-4-1 1961. Copyright 2002 by the American Association for the Advancement of Science)
Figure 6-4-1
Small vein Cell wall-apoplast Sugar athway cells Sugar e loading Cytoma p Phloem parenchyma cell Bundle sheath cell Mesophyll cell Figure 6-5 Pla embi FIGURE 10.14 Schematic diagram of pathways of phloem complex, but they could also enter the apoplast earlier in loading in source leaves. In the totally symplastic pathway, the path and then move to the small veins. In any case, the sugars move from one cell to another in the plasmodes- sugars are actively loaded into the companion cells and mata,all the way from the mesophyll to the sieve elements. sieve elements from the apoplast. Sugars loaded into the In the partly apoplastic pathway, sugars enter the apoplast companion cells are thought to move through plasmodes at some point. For simplicity, sugars are shown here enter. mata into the sieve elements ing the apoplast near the sieve element-companion cell
Figure 6-5
TABLE 10.4 Figure 6-5-1 Patterns in apoplastic and symplastic loading Apoplastic loading Symplastic loading Transport sugar Sucrose Oligosaccharides in addition to sucrose Type of companion Ordinary companion cells Intermediary cells cell in the minor veins or transfer cells Number of plasmodesmata W Abundant connecting the sieve elements and companion cells to surrounding cells Xylem vessel Plasmodesmata Companion Intermediary ce cell Phloem parenchyma- Sieve element Source: Drawings after van Bel et al. 1992 Note: Some species may load both apoplastically and symplastically, since different types of companion cells can be found within the veins of a single species
Figure 6-5-1
sieve element-companion cell complex Figure 6-6 H+-ATPase TP H CADP+(PD Sucrose-H+ symporter H sucrose Sucros∈ High H+ Low H+ concentration concentrati。n FIGURE 10.16 ATP-dependent sucrose transport in sieve element loading. In the cotransport model of sucrose load ing into the symplast of the sieve element-companion cell complex, the plasma membrane ATPase puy Proton con of the cell into the apoplast, establishing a hig centration there. The energy in this proton gradient is then used to drive the transport of sucrose into the symplast of the sieve element-companion cell complex through a sucrose symporter
Figure 6-6
Bundle sheath cel Intermediary cell Sieve element O Glucose ●. Galactose FRuctose Sucrose 0→005 acrose o→o Raffinose Plasmodesma Sucrose, synthesized in the phyll, diffuses from In the intermediary cells, Raffinose and stachyose raffinose(and stachyose) the bundle sheath cells are able to diffuse into are synthesized from into the intermediary cells the sieve elements. As a sucrose and galactose e through the abundant thus maintaining the result, the concentration plasmodesmata of transport sugar rises in diffusion gradient for the intermediary cells and sucrose. Because of their the sieve elements larger sizes, they are not Figure 6-7 able to diffuse back into the mesophyll
Figure 6-7
lem vessel elements Phloem sieve elements Companion cell H2O Source cell H2O Active phloem Sugar at the source illustrated here by sucrose Hw=-08 MP loading into =-1.1MPa (red spheres)is actively =-0.7MPa sieve elements Mo= 0.6 MP decreases the loaded into the sieve Ps=-01 MP solute potential element-companion cell Water enters complex and high turgor pressure results Pressure-driven bulk flow of water and solute from source to sink Transpiration Sink cell strean H2O At the sink, sugars are unloaded Active phloem unloading W=-0 6 MP increases the HE-0 4 MP H2O 中p=-0.5MP solute potential Pp= 0.3 MPa Ye=-0.1 MPa water flows out Ye=-0.7 MPa and a lower Sucrose turgor pressure results FIGURE 10.10 Pressure-flow model of translocation in Figure 6-9 the phloem. Possible values for Y, Pn, and y, in the xylem and phloem are illustrated. (After Nobel 1991
Figure 6-9
