The primary function of plant mitochondria is respiration, which is why they are often referred to as“powerhouses of the cell”. Besides their central role in energy metabolism, plant mitochondria arealso involved in the photorespiratory C2 cycle and in the provision of carbon skeletons to supportefficient nitrogen assimilation. All these functions are catalyzed by mitochondrial proteins. Theircomposition, abundance and interactions in plant mitochondria are the subject of this thesis. In yeast,Trypanosomes, and several mammalian cell types, mitochondria are organized as extensivemitochondrial networks, resulting in a situation where a cell only hosts few but large mitochondria. Inplants, hundreds of small mitochondria are only connected by fusion and fission over time but notphysically. Hence, the organelles form individual, functional units. Paradoxically, their biochemical andphysiological characterization focuses on large organelle populations and thereby disregards theproperties of the individual mitochondrion. This partially is based on the fact that cell biologicalapproaches capturing structural features of plant mitochondria often are of limited value forunderstanding their physiological properties. Chapter 2.1 of this thesis models the protein content ofa single mitochondrion by combining proteomics with classical cell biology. Besides other insights intothe function of a single plant mitochondrion, it could be shown that proteins involved in ATP synthesisand transport make up nearly half of the plant mitochondrial proteome. The five protein complexes ofthe OXPHOS system contribute most to this segment of the mitochondrial proteome, underlining theoverall importance of mitochondrial ATP synthesis for the entire plant cell. Despite the central functionof OXHPOS components in plants, certain unicellular parasites and yeasts apparently do not need acomplete OXPHOS system. Intriguingly, it recently has been reported that the mitochondrial genomeof the multicellular parasitic flowering plant Viscum album (European mistletoe) is reduced and lacksthe genes encoding the mitochondrially encoded subunits of complex I. This implies that thecorresponding genes either have been lost or, alternatively, were transferred to the nuclear genome.The consequences for the mitochondrial respiratory chain were so far unknown. Chapter 2.2 presentsdata suggesting that V. album indeed lacks mitochondrial complex I. The absence of complex I isaccompanied by a rearrangement of the respiratory chain including (i) stable supercomplexescomposed of complexes III2 and IV, and (ii) the occurrence of numerous alternative oxidoreductases.Mitochondria of V. album also possess less cristae than mitochondria from non-parasitic plants, whichcan be explained by low amounts of ATP synthase dimers. The mitochondrial proteome consists ofproteins encoded in the nucleus or in the rudimentary mitochondrial genome. The few proteinsencoded on the mitochondrial genome are translated by mitochondrial ribosomes. While structureand composition of these mitoribosomes are well established in yeast and mammals, the currentknowledge of plant mitoribosomes is negligible. Isolation of plant mitoribosomes is difficult due totheir sedimentation coefficient, which is very close to that of cytosolic ribosomes, their interactionwith the inner mitochondrial membrane, and the attachment of cytosolic ribosomes to themitochondrial surface. As part of this dissertation, plant mitoribosomes were analyzed via a novelcomplexome profiling strategy (chapter 2.3). This revealed an unconventional molecular mass of thesmall ribosomal subunit of plants. In addition, several pentatricopeptide repeat (PPR) proteins werediscovered to form part of both, the large and the small mitoribosomal subunit.