Figure 1: Major Ca2+ efflux systems in a leaf cell, and structure of the ER spanning two cells. CAX1 is the major cation exchanger in leaf cells, but CAX3 can compensate if CAX1 activity is compromised. Not shown is a vacuolar proton ATPase that uses ATP to pump protons into the vacuole. The energy from the proton gradient is used to pump Ca2+ into the vacuole. There are also two Ca2+ pumps on the tonoplast membrane, ACA4 and ACA11. The ER and plasma membrane also have Ca2+ pumps (lower right). Ca2+ pumps are also found on the nuclear envelope and chloroplast (not shown). The reticulate nature of the ER is modeled next to the plasma membrane but reticulation (and the ER) is found throughout the cell. Cortical ER is found near the cell wall and is less dynamic than ER in the interior. A desmotubule spans a single plasmodesma between the upper cell and a partial cell on the bottom, but of course there are multiple plasmodesmatal connections between most cells (except guard cells and between the epidermis and mesophyll). Both the cytosol and the ER lumen are continuous across the plasmodesmata.