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Essay On Calcium Channels

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Calcium Channels
There is back and forth conversation between cytoplasm and cell wall which is coordinated by Ca+(Hepler and Winship 2010). Ca+ influx in the cytosol from apoplast, vacuoles, and ER is mediated by Ca+-channels found in all plant the membranes. Those channels are classified based on their voltage dependence such as depolarization-activated calcium channels (DACCs), hyperpolarization-activated calcium channels (HACCs), and voltage-insensitive calcium channels (VICCs) (Miedema et al. 2001; Sanders et al. 2002; White 2000). It is believed that DACCs are encoded by a homologue of AtTPC1 gene; VICCs by cyclic nucleotide gated channel (CNGC) and glutamate receptor (GLR) gene families and HACCs are plant specific annexins (K
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Cell system uses active transporters such as Ca+-ATPases and H+/Ca+-antiporters (also known as cation exchanger (CAX)) to restore to basal cytosolic Ca+(White and Broadley 2003). This is required for appropriate cytoplasmic metabolism, replenishment of intracellular and extra cellular Ca+-stores (Harper 2001; Klüsener et al. 1995), removal of divalent cations (Hirschi 2001; Wu et al. 2002) and supply of Ca+ to the ER to keep functioning of secretory systems (Blatt 2000; Ritchie, Swanson, and Gilroy 2002). Ca+-ATPases are thought to be high affinity, Km= 1-10 µM; (Evans and Williams 1998) but low capacity and H+/Ca+-antiporters are low affinity, Km= 10-15 µM but high capacity Ca+ -transporters. Among different types of Ca+-ATPases, autoinhibited calcium ATPases (ACAs) are located in tonoplast (ACA4, ACA11) (Geisler et al. 2000; Li et al. 2006), plasma membrane (ACA8, ACA9 and ACA10) (Bonza et al. 2000; George et al. 2008; Schiøtt et al. 2004), ER membrane (ACA2)(Harper et al. 1998) and plastid membranes (ACA1) (Huang et al. 1994) and ER-type calcium ATPases (ECAs) such as ECA1 are located only in ER (ECA1) (Liang et al. 1997), the Golgi (ECA3) (Mills et al. 2008), and endosomes (ECA 3)(Li et al. 2008).

H+/Ca+-antiporters are located in tonoplast (CAX1 to CAX4)(Cheng et al. 2005; Hirschi et al. 2000), and plasma membrane (Evans and Williams 1998;
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