A ribbon-like structure about 10-45 nm in width and visible only under the electron microscope. Microfibrils form the basic structural units of the cell wall. Each microfibril is formed from many long-chain cellulose molecules lying approximately parallel to each other. Hydrogen bonds between hydrogen and oxygen atoms in adjacent chains bind them together. The intermolecular spaces and the interfibrillar spaces are partly occupied by structural substances, e.g. pectic compounds and hemicellulose. As a result of imbibitional and capillary effects, these spaces are also filled with water so that the cell wall is normally heavily hydrated. In primary walls (see cell wall
) water may account for up to 70% of the volume. In secondary walls, more efficient packing of the microfibrils leaves less room for water between them.