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Transcript
Plant cell division: •Plant cells are surrounded by walls (cellulose microfibrils in matrix of pectins and hemicelluloses (1o) or lignin (2o)) •Lignin is hydrophobic and prevents cell expansion •little cell migration (cell sliding possible) •Cell expansion depends on orientation of microfibrils •Molecules fixed in cell wall might fix cell fate (and daughter cells’ fate) Figure 2-1 Cellulose microfibrils are laid down in different directions to increase strength Raven, Fig 3-30 If microfibrils are laid down perpendicular to long edge of cell, will cause increase in length as cell grows Raven Fig 3.35 •Interconnected cell walls create an apoplastic channel for cell-cell movement •10kDa - 50 kDa exclusion •Walls are negatively charged, therefore stop +ve charged molecule movement •Lignin (2o cell walls), Cutin (cuticle) or suberin (endodermis) blocks apoplast •Xylem is interconnected with apoplast - molecules secreted into apoplast can be transported through the xylem Vessel element (component of xylem) develops with ordered, helical secondary wall thickenings and eventual apoptosis Raven, fig 24-16, 17 A-c, Vessel elements Raven, Fig 24-16 Dtracheid Raven, Fig 24-13 Plasmodesmata: •Connections between neighbouring cells •Lined by plasma membrane, inner desmotubule (ER) •Symplastic channel for cell-cell movement •Movement of large nucleic acids and proteins may be facilitated by movement proteins Figure 2-2 Primary pit fields parenchyma sclerenchyma Raven, 3-33 Developing sieve tube element - plasmodesmata develop into pores = sieve plate Raven 24-22 Sieve tube elements and companion cells Raven, 24-19 Plant development is continuous, reiterative and plastic determinate = restricted in time and space (e.g. flower, leaf) indeterminate = unrestricted (e.g. shoot) totipotent Figure 2-3 Cell fate •Division, growth, differentiation Cell intrinsic, cell extrinsic control?