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In page Cellulose:

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In plants cellulose is synthesized at the plasma membrane by rosette terminal complexes (RTCs). The RTCs are hexameric protein structures, approximately 25 nm in diameter, that contain the cellulose synthase enzymes that synthesise the individual cellulose chains.[1] Each RTC floats in the cell's plasma membrane and "spins" a microfibril into the cell wall.[citation needed]