IPA: //ˌʌl.trəˈstrʌk.tʃər//
KK: /ˈʌltrəˌstrʌkʧər/
The very small and detailed structure of a biological specimen, like a cell or tissue, that can only be seen with special tools like electron microscopes.
Scientists studied the ultrastructure of the cell to understand its functions better.
Ultrastructure is formed from "ultra-" (meaning beyond) and "structure" (from Latin "structura", meaning arrangement or building). The term refers to the detailed structure of a biological specimen at a scale beyond what can be seen with a light microscope, emphasizing the complexity of its arrangement.
Think of 'beyond' ('ultra-') the usual 'arrangement' ('structura') to remember that 'ultrastructure' refers to the intricate details of a structure.
No commonly confused words.