## 5 pH in Cellular Organelles  
The compartmentalization of the eukaryotic cell provides distinct environmental conditions for the optimal operation of individual metabolic pathways. The function of individual organelles depends on the stringently regulated pH (Figure 2) because protons dictate the charge of macromolecules and generate transmembrane electric potential [1, 3]. To study pH\({}_{i}\)_in vivo_, accurate pH determination is essential. In this section, we outline the genetically encoded sensors applied to monitor the pH of individual subcellular compartments (Table 4). Most organelles, except nucleus, have their own specific pH value associated with the processes that occur in these compartments. The nuclear membrane has an abundance of pores that are a weak barrier for protons; therefore, nuclear pH (pH\({}_{\mathrm{N}}\)) is similar to that of the surrounding cytosol. Both cytosolic pH (pH\({}_{\mathrm{C}}\)) and pH\({}_{\mathrm{N}}\) of various resting cells are slightly above neutral, between 7.2 and 7.7 (Figure 1, and see Tables 4-7 for references) [11, 12, 33, 34, 36, 39, 40, 44, 46, 48]. To specifically relocate the pH sensor to the nucleus, a nuclear localization signal NLS is fused to pHluorin [4, 49]. A cytosolic sublocation of pHluorin tethered with palmitoylation site, caveolin-1, \(\alpha\)1B-adrenooreceptor, or hCTN3 revealed that the pH of these cytosolic microdomains is not uniform and is 7.3, 7.3, 6.8, or 6.5-7.5, respectively [25, 43, 46, 50, 51]. The pH\({}_{\mathrm{C}}\) of yeasts _S. cerevisiae_, _Candida glabrata_, and _Schizosaccharomyces pombe_ is highly influenced by environmental changes, such as nutrients and external pH. Their pH\({}_{\mathrm{C}}\) differs from 6.4 when cells are starved to 7.5 when glucose is present in the media [31, 49, 52, 53, 54, 55]. The pH\({}_{\mathrm{C}}\) is also not uniform throughout plant tissues. The pH\({}_{\mathrm{C}}\) of _A. thaliana_ \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & **pK** & \(\lambda_{\mathrm{x1}}\) (**nm**) & \(\lambda_{\mathrm{x2}}\) (**nm**) & \(\lambda_{\mathrm{x1}}\) (**nm**) & \(\lambda_{\mathrm{x2}}\) (**nm**) & **Reference** \\ \hline GFpH GFPuv-EGFP & 6.1 & 380 & 480 & 510 & a & [43] \\ YfpH GFPuv-EYFP & 6.5 & 380 & 480 & 509/527 & a & [43] \\ pHERP & 6.5 & 397 & 515 & 520 & a & [44] \\ EYFP-GGGLEDPRVPVEK-GFPuv & & & & & & \\ ClopHensor & & & & & & 630 for & [45] \\ E\({}^{2}\)GFP-linker 20 aa-dsRedm & 6.8 & 458 and 488 & 543 for & & & \\ ClopHensor & 7.3 & 458 and 488 & & 535 & 630 for & [46] \\ (H148G,V224L) & & & & DsRed & & DsRed & [46] \\ pHusion & 5.8 & 488 & 558 or 585 & 500–550 & 600–630 & [47] \\ mRFP1-AVNAS-EGFP & & & & & & \\ \hline \hline \end{tabular}  
* not determined/not applicable.  
\end{table} 
Table 3: pH-sensitive fluorescent proteins. varies from 6.4 at the root cap to 7.3 in meristem cells, thereby proving the presence of significant pH gradients among different developmental regions in plant roots [4,15,47]. The pH of individual subcellular compartments in a prototypic eukaryotic cell. We show only pH values obtained using the genetically encoded pH-sensitive FPs as indicators. The pH values are collected from different sources and are referenced in Table 4.  
The pH sensors targeted to the lumen of endoplasmic reticulum via retrograde transport based on KDEL receptor show that the pH of endoplasmic reticulum (pH\({}_{\text{ER}}\)) is very close to its pH\({}_{\text{C}}\)[4,11,56]. The luminal pH throughout the secretory pathway becomes progressively more acidic (Figure 2, Table 5). The _cis_-Golgi with pH 6.8 is more acidic than the endoplasmic reticulum [4], and the acidification becomes more apparent when the pH changes from 6.1 to 6.6 in the _trans_-Golgi network. [PERSON] _et al_. [57] used GT-EGFP to show that the _medial/trans_-Golgi pH in cancer cells MCF-7 and HT-29, SW-48, is significantly more alkaline (pH 6.75) than in control cells (pH\({}_{\text{G}}\) 5.9-6.5), thereby clearly emphasizing the importance of stringent pH homeostasis.