biological structure (noun, pl. biological structures) – any part of an organism including the entire body of the organism whether large and easily observed or microscopically small and only examined or analyzed with special scientific instruments (optical, analytical, etc.). Biological structures are typically organized within a modular natural hierarchy. Thus, organs are composed of tissues, or (if tissues are not formed) of cell aggregates like fungal hyphae. Tissues or cell aggregates are formed of cells and cells are composed of cell organelles. Very small structures of an organism that can only be observed with the light microscope are typically referred to as the anatomy. Structural parts of cells that are even smaller and can only be studied with the transmission electron microscope (TEM) are referred to as fine structure or ultrastructure.