Tension and Toughness: Intermediate Filaments Function

If microfilaments are 7 nm and microtubules are 25 nm, then fibers with a diameter of 8-12 nm fall in the middle range. If they fall in this middle range, then they are appropriately termed "intermediate."

If a cell needs to keep its nucleus in a specific location rather than letting it float freely, then it requires a sturdy structural cage. If intermediate filaments are the most permanent fibers, then they are the ones that provide this anchoring function.

If an axon is very long and thin, then it needs internal reinforcement to prevent breaking or collapsing. If neurofilaments (a type of intermediate filament) provide this support, then they are essential for neuron structure.

If a cell is treated with high-salt solutions or detergents and the only fibers left behind are intermediate filaments, then they must be the most stable and least soluble part of the cytoskeleton.

If the plasma membrane needs to be attached to the rest of the cell to prevent it from peeling away, then it must be anchored to the cytoskeleton. If intermediate filaments link to membrane proteins in desmosomes, then they provide that anchor.

If microtubules and microfilaments constantly assemble and disassemble to help the cell move, but intermediate filaments remain intact even after a cell dies, then intermediate filaments are considered more permanent.

If microfilaments and microtubules have distinct plus and minus ends for directional growth, then they are polar. If intermediate filaments are symmetrical and do not have these distinct directional ends, then they lack structural polarity.

If microfilaments use ATP and microtubules use GTP to remain stable, then they are energy-dependent. If intermediate filaments are more stable and do not require nucleotide hydrolysis to maintain their structure, then they do not depend on those energy sources.

If keratin provides the strength that holds skin cells together and anchors them to the basement membrane, then a defect in keratin means the cells will tear apart easily. If they tear apart, then fluid builds up and causes blisters.

If microtubules resist compression (pushing) and intermediate filaments resist tension (pulling/stretching), then the intermediate filaments are the primary defense against the cell tearing under stress.

If the nuclear envelope requires a lining to maintain its shape and protect the DNA, then it needs a protein meshwork. If intermediate filaments form the nuclear lamina, then they provide that internal structural support.

If intermediate filaments are a defining feature of complex eukaryotic cells, especially in animals, then they are not typically found in prokaryotes like bacteria. Bacteria use different protein systems for their cell shape.

If the outer skin needs to be tough and waterproof, then it must be composed of a durable fibrous protein. If keratin is the protein used for these structures, then it is a member of the intermediate filament family.

If intermediate filaments are designed to resist pulling forces and link cells together, then they bear tension and form desmosomes; however, they are made of fibrous proteins, not tubulin, and do not act as tracks for kinesin.

If plants use cellulose walls for strength, then animal cells must use internal proteins. If intermediate filaments are the toughest and most permanent internal fibers, then they provide that framework.

If intermediate filaments act as a "cage" or anchor for internal structures, then they secure the nucleus, mitochondria, and ER; ribosomes are too small/numerous and chloroplasts are not found in the animal cells where these filaments are most prominent.

If different tissues require specific types of structural support, then the body must produce many variations of the same filament type. If the keratin family has the most variants, then it provides the most diverse structural functions.

If tissues like skin need to stay attached under physical stress, then they must be "riveted" together. If intermediate filaments provide the internal strength for these rivets, then they are part of desmosomes.

If intermediate filaments are categorized by their fibrous structure and specific diameter, then keratin, lamin, and vimentin qualify; actin and myosin are categorized as microfilaments and motor proteins, respectively.

If the subunits of a filament are long and thread-like rather than round or globular, then they are called fibrous proteins. If intermediate filaments are built from these threads, then they are fibrous cables.