Chloroplasts and mitochondria are central to crucial cellular processes in plants and contribute to a whole range of metabolic pathways. The use of calcium ions as a secondary messenger in and around organelles is increasingly appreciated as an important mediator of plant cell signaling, enabling plants to develop or to acclimatize to changing environmental conditions. Here, we have studied the four calcium-dependent mitochondrial carriers that are encoded in the Arabidopsis genome. An unknown substrate carrier, which was previously found to localize to chloroplasts, is proposed to present a calcium-dependent S-adenosyl methionine carrier. For three predicted ATP/phosphate carriers, we present experimental evidence that they can function as mitochondrial ATP-importers.