The mobile carrier connecting the first and second complexes to the third is:

1. Some carriers are fixed inside complexes and others shuttle between them. 2. One small lipid-soluble molecule links the early complexes to complex III. 3. The compound connecting the first and second complexes to the third is ubiquinone. 4. Complex I itself is composed of flavin mononucleotide and an iron-sulfur containing protein. _Source: OpenStax Biology for AP(R) Courses (CC BY 4.0), Ch 7 'Cellular Respiration', sections 7.1-7.7_