What structure is formed by spiral strands of protein molecules in a cell?

The structure formed by spiral strands of protein molecules in a cell is microtubules. These are components of the cytoskeleton, which provides structural support and shape to the cell. Microtubules are made up of tubulin proteins arranged in a helical fashion, and their spiral structure allows for flexibility and dynamic stability. They play a crucial role in various cellular processes, including cell division, intracellular transport, and maintaining cell shape.

In contrast, microfilaments are composed of actin proteins and are involved in muscle contraction and maintaining cell structure, but they do not specifically form spiral strands. Intermediate filaments provide mechanical support and help maintain cell integrity but are made from a variety of proteins that form a more rope-like structure rather than a spiral. The cell membrane, on the other hand, is a lipid bilayer that serves as a barrier and protects the cell, but it is not formed from protein strands at all.