Реферат: Untitled Essay Research Paper Involvement of K

1 has also been isolated from Mimosa pudica, as has the molecule M-LMF 5 (Schildknecht).

The movement of the leaflets is effected by the swelling and shrinking

of cells on opposite sides of the pulvinus (Kim, et al.) In nyctinastic plants, cells that

take up water when a leaf rises and lose water when the leaf lowers are called extensor

cells. The opposite occurs in the flexor cells (Satter and Galston). When the extensor

cells on one side of the pulvinus take up water and swell, the flexor cells on the other

side release water and shrink. The opposite of this movement can also occur. However, the

terms extensor and flexor are not rigidly defined. Rather, the regions are defined

according to function, not position. Basically, the pulvini cells that are on the adaxial

(facing the light) side of the pulvinus are the flexor cells, and the cells on the abaxial

side are the extensor cells. Therefore, the terms can mean different cells in the same

pulvinus at varying times of the day. By coordinating these swellings and shrinkings, the

leaves are able to orient themselves perpendicular to the sunlight in diaheliotropic

plants.

Leaf movements are the result of changes in turgor pressure in the

pulvinus. The pulvinus is a small group of cells at the base of the lamina of each

leaflet. The reversible axial expansion and contraction of the extensor and flexor cells

take place by reversible changes in the volume of their motor cells. These result from

massive fluxes of osmotically active solutes across the cell membrane. K+ is the ion that

is usually implicated in this process, and is balanced by the co-transport of Cl- and

other organic and inorganic anions.

While the mechanisms of diaheliotropic leaf movements have not been

studied extensively, much data exists detailing nyctinastic movements. Several ions are

believed to be involved in leaf movment. These include K+, H+, Cl-, malate, and other

small organic anions. K+ is the most abundant ion in pulvini cells. Evidence suggests that

electrogenic ion secretion is responsible for K+ uptake in nyctinastic plants. The

transition from light to darkness activates the H+/ATPase in the flexor cells of the

pulvinus. This leads to the release of bound K+ from the apoplast and movement of the K+

into the cells by way of an ion channel. This increase in K+ in the cell decreases the