Реферат: Untitled Essay Research Paper Involvement of K

channels. The other pump is a H+/K+ exchanger, in which K+ is pumped into the cell as H+

is pumped out of the cell in a type of antiport. The presence of this typ of pump is only

hypothetical, however, since at present there is no evidence to support it. Thus there are

two possible ways for K+ to enter the pulvini cells. The buildup of the pH gradient may

also promote Cl- entry into the cell via a H+/Cl- cotransporter as the H+ trickles back

into the cell. Cl- ions may also be driven by the electrochemical gradient for Cl- via Cl-

channels, as with K+. A large Cl- channel was observed in the membrane of Samanea flexor

protoplasts. The channel closed at membrane potentials above 50mV and opened at potentials

as low as -100mV.

Light-driven changes in membrane potential may be involved in the

activation of these proton pumps. This may be mediated by effects on cytoplasmic Ca2+.

Ca2+-chelators inhibit the nyctinastic folding as well as the photonastic unfolding

responses in Cassia. Thus Ca2+ may act as a second messenger in a calmodulin-dependent

reaction. The Ca2+ may be what turns on the electrogenic proton pumps, causing changes in

membrane potential. However, there is no direct evidence to support this hypothesis,

although chemicals that are known to change calcium levels have been shown to alter the

leaf movement of Cassia fasciculata and other nyctinastic plants. One study involving

Samanea postulates that Ca2+ channels are also present in the plasma membrane of pulvini

cells, and inositol triphoshate, a second messenger in the signal transduction pathway in

animals, stimulates the opening of these channels. This insinuates that some light signal

binds to a receptor on the outside of the cell and stimulates this transduction pathway.

However, whether this hypothesis is true is unclear. It has also been proposed that an

outwardly directed Ca2+ pump functions as a transport mechanism to restore homeostasis

after Ca2+ uptake through channels.

The changes in Cl- levels in the apoplast are less then that for K+.

The Cl- levels are 75% that of K+ in Albizzia, 40-80% in Samanea, and 40% in Phaseolus.

Therefore, other negatively charged ions must be used to compensate for the positive

charges of the K+. Malate concentrations vary, and it is lower in shrunken cells than in

swollen cells. It is believed that malate is synthesized when there is not enough Cl-