Complex I (CI) is a large membranous mitochondrial enzyme that serves as the major entry point for electrons from NADH into the respiratory chain. The CI enzyme is considered to be conserved between different organisms, but plant CI includes an extra ‘egg-shaped’ module – the γ-carbonic anhydrase domain – and a paper by Cordoba et al. in this issue of Journal of Experimental Botany (pages 1589–1603) indicates that its functions are required during Arabidopsis embryogenesis. The respiratory machinery of the mitochondrion is usually made up of four major protein complexes, CI to IV, embedded within the inner-mitochondrial membrane. Electron transfer through CI (NADH:ubiquinone oxidoreductase), CIII and CIV mediates the pumping of protons (H+) across the inner membrane, forming the chemical potential (ΔpH), which is utilized by ATP-synthases to generate ATP. CI is present in many prokaryotes, including α-Proteobacteria, the proposed progenitors of mitochondria, and is thus considered to have arisen early in evolution. The bacterial and mammalian enzymes show an L-shaped structure composed of two major fragments, including an integral membrane-domain and a soluble arm (Baradaran et al., 2013; Shimada et al., 2014). The plant CI consists of ≥50 different subunits, encoded by both nuclear and mitochondrial loci, forming a ≥1.0 MDa structure that has several distinguishing features (Fig. 1) (Meyer, 2012; Braun et al., 2014). These include the presence of additional Nad subunits that are encoded in the mtDNA. The nad pre-RNAs in plant mitochondria undergo extensive maturation processes, including numerous RNA-editing events and the splicing of many group II-type introns that are removed posttranscriptionally from the coding region they interrupt (Brown et al., 2014). Fig. 1. A hypothetical 3D-structural model of Arabidopsis CI. The proton-pumping CI enzyme is a large bi-partite membranous protein assembly, which has a central role in the production of cellular energy in bacteria and mitochondria. CI has a characteristic L-shaped ... Another unique feature of plant CI, compared with those in animals and fungi, is the presence of many additional subunits (Meyer, 2012; Braun et al., 2014). These include several γ-carbonic anhydrase (CA)-related proteins, which form a matrix-exposed ‘egg-shaped’ module in the membrane arm (Fig. 1) (Sunderhaus et al., 2006; Braun and Zabaleta, 2007). However, the function of this CI-related module remains unclear, and this is where the data presented by Cordoba et al. (2016) are novel and important.