The regulation of axonemal dynein activity is critical for flagellar beat frequency and cilia waveform. Similarly, flagella grow by exporting flagellin through the flagellar machinery. The pelta is a sheet of microtubules that curve around the flagellar bases. The gradient is usually used to drive ATP synthase, flagellar rotation, or transport of metabolites. Some of the proteins participating in T3SS share amino-acid sequence homology to flagellar proteins. The entire flagellar structure consists of 200 proteins. Technically speaking, type III secretion is used both for secreting infection-related proteins and flagellar components. The middle residues make up the outer surface of the flagellar filament. The helical nature of the individual flagellar filament is critical for this movement to occur. Species are not classified morphologically anymore but historically have been by flagellar shape. The flagellar movement produces forward propulsion and also a turning force. There is a single nucleus which is near the flagellar root. The nucleus is centrally located and is parallel with the flagellar depression and pyrenoid. In other words, the flagellar apparatus is "irreducibly complex". It bears some similarity to the flagellar motors discussed below.