Реферат: Untitled Essay Research Paper Involvement of K

their volume. In Samanea, K+ levels changed four-fold in flexor cells during the

transition from light to darkness. In a similar experiment, during hour four of a

photoperiod, the extensor apoplast of Samanea had 14mM and the flexor apoplast had 23 mM

of K+. After the lights were turned off, inducing nyctinastic movements, the K+ level in

the apoplast rose to 72 mM in the extensor cells and declined to 10mM in the flexor cells.

Therefore, it appears that swelling cells take up K+ from the apoplast and shrinking cells

release K+ into the apoplast.

In the pulvinus of Samanea saman, depolarization of the plasma membrane

opens K+ channels (Kim et al). The driving force for the transport of K+ across the cell

membranes is apparently derived from activity of an electrogenic proton pump. This creates

an electrochemical gradient that allows for K+ movement. From concentration measurements

in pulvini, K+ seems to be the most important ion involved in the volume changes of these

cells. How then, is K+ allowed to be at higher concentrations inside a cell than out of

it? Studies indicate that the K+ channels are not always open. In protoplasts of Samanea

saman, K+ channels were closed when the membrane potential was below -40mV and open when

the membrane potential was depolarized to above -40mV. A voltage-gated K+ channel that is

opened upon depolarization has been observed in every patch clamp study of the plasma

membranes of higher plants, including Samanea motor cells and Mimosa pulviner cells.

It is proposed that electrogenic H+ secretion results in a proton

motive force, a gradient in pH and in membrane potential, that facilitates the uptake of

K+, Cl-, sucrose, and other anions. External sodium acetate promotes closure and inhibits

opening in Albizzia. This effect could be caused by a decrease in transmembrane pH

gradients. The promotion of opening and inhibition of closure of leaves by fusicoccin and

auxin in Cassia, Mimosa, and Albizzia also implicate H+ in the solute uptake of motor

cells, since both chemicals are H+/ATPase activators, stimulating H+ secretion from the

plant cells into the apoplast. Vanadate, an H+/ATPase inhibitor, inhibits rhythmic leaflet

closure in Albizzia. Although this conflicts with the movement effected by fusicoccin and

auxin, it is believed that vanadate affects different cells, acting upon flexor rather

than extensor cells. The model indicates that there are two possible types of H+ pumps.