Final answer:
Microfilaments are polar due to structural asymmetry in monomeric actin which gives the filament distinct ends, whereas keratin filaments lack this polarity and form stable structures.
Step-by-step explanation:
A microfilament is polar because of the structural asymmetry of its components. In contrast, a filament of keratin does not display this polarity because it is composed of extended secondary structure proteins that align to form stable, unchanging secondary structures. This asymmetry in microfilaments arises due to the specific orientation of its protein monomers, the monomeric actin proteins, which have distinct plus and minus ends, leading to directional growth. Meanwhile, intermediate filaments, which include keratin, lack this polarity and are more stable, providing tensile strength rather than participating in dynamic cellular activities.