lecture 3 transport mechanism across membrane.ppt

Chapter 2
Plant nutrients uptake and transport Mechanisms
nutrient
(plasmodesmata )
THE CELL WALL
Before traversing the plasma membrane (.), ions must first cross the cell wall and contact the . The bulk flow of soil solution could theoretically carry anic ions into the cell wall and through the apoplastic pathway. The thickness of a cell wall is in the range of 500 to 1000 nm. It is porous.
表1-13一些物质的颗粒直径(毫微米nm)
颗粒
分子或离子晶体直径
水化离子直径
颗粒
分子或离子晶体直径
水化离子直径
葡萄糖

Mg2+


Na+


Ca2+


K+


CI-


NH4+


NO3-

Compared to cell walls(500-1000nm),
the nutrient ions or molecular are very small.
Cell wall (细胞壁)
The cell wall of parenchyma cells(薄壁细胞) is made up of a middle lamella (.)(胞间薄层), which separated the daughter cells after cell division, and the primary wall(初生壁) which was deposited on to the
While the . is made up of pectic substances(果胶物质), the primary wall also contains hemicelluloses and cellulose. Pectins(果胶质) are based on branched chains of a sugar polymer made up of galacturonic acid(半乳糖醛酸). This is galactose (半乳糖)in which the alcoholic group(醇羟基) at C6 is replaced by a carboxyl group(羰基).
Cell wall(细胞壁)
Free space
Clearly, ionization of carboxyl groups creates a negative charge which makes the cell wall a cation exchange resin. Indeed much Ca2+ is associated with the pectins. In soils with high quantities of free Al3+, this toxic ion is bound in large quantities to the cell wall.
Free space
Thus when ions enter the cell wall, cations can exchange with the cell wall according to their binding affinity :typically
trivalents>divalents>monovalents.
Cation exchange capacity (CEC)
Anions should not be bound at all.
Cell membranes(生物膜)
Allow for controlled intracellular environment
Overton in 1890 demonstrated that ration of many solutes across the membrane was a function of their lipid solubility. He proposed therefore, that the membrane itself was l