What are the many functions of the cytoskeleton?

• Aid in the dividing of cells
• Aids the intracellular traffic of materials in organelles
• Supports the plasma membrane frame and allows it to bear stresses and form their shapes
• Allows cells to swim or crawl
• Allows contraction within muscles and the extension of axons and dendrites
• Guides the growth of plant cell wall

What are the 3 classes of cytoskeletal filaments?

Microtubules (25nm), Intermediate filaments (10nm), Microfilaments (5-8nm)

Where are microtubules usually located and what are their functions?

• They come from the centrosome by the nucleus (looks like a spider)
• They determine positions of organelles and direct intracellular transport of materials

Where are intermediate filaments located and what are their functions?

• All all throughout the cytoplasm to form a lamina around the nuclear membrane (confetti filler in the "gift bag")
• They provide strength and resistance to shear (sliding) stress

Where are microfilaments located and what are their functions?

• They usually line the edges of the cell membrane on the cell periphery (bubble wrap of the intermediate filaments)
• They determine the shape of the cell's surface and are necessary for whole-cell locomotion

T/F The cytoskeletal filaments can function on their own without the need of accessory proteins

False. Accessory proteins are what bind them to each other as well as to other cell components

What cytoskeletal functions do microtubules play a part in?

• Acting as train tracts for material movements and in axons
• Provide the force for movement \
• Are mitotic spindles
• (Think of them as octopus arms managing things on the inside!)

What are actin filaments a part of?

a) microtubules

b) intermediate filaments

c) mircofilaments

b) intermediate filaments

T/F Actin filaments are only in one shape

False. They can come in a variety of linear bundles like 2D sheets or 3D gels in the cell cortex

What do a ctin filaments do?

• They push and pull to mold the cell into it's shape
• They also generate force during muscle contractions

T/F All species have intermediate filaments

False. Not all species have intermediate filaments because they evolved later down the phylogeny tree

How do intermediate filaments affect the nucleus?

They organize the chromatin inside the nucleus to control nuclear disassembly when undergoing mitosis

T/F The cytoskeleton structures are always stable

False. They can be both stable or dynamic depending on the cell. A highway or ant trail

Give an example of a dynamic cytoskeletal structure and a stable cytoskeletal structure

Dynamic: actin filaments and microtubules when a cell divides

Stable: Skin cells I'm assuming

T/F Once cytoskeletal filaments are made they cannot be broken down

False. They can assemble and disassemble depending on what the cell needs

Fill in the blanks

• Assembly/disassembly happens at the _______ (end/middle) of the filaments.
• The process occurs _______ (slowly/rapidly)
• The nucleation step is rate limiting and energetically _________ (favorable/unfavorable)

• ends
• rapidly
• unfavorable

What is critical concentration (Cc)?

It is the amount of free subunits allowed in the cell

Fill in the blanks

• The rate at which subunits add is ________ (proportional/ not proportional) to the concentration of free subunits
• The rate at which subunits leave is _________(dependent/ independent) to the concentration of free subunits
• At equilibrium the rates are _______ (equal/not equal)

• Proportional
• independent
• equal

When the current concentration is above the Cc there will be net ________ (assembly/disassembly)

Assembly. They need to use up the subunits so the concentration will lower

When the current concentration is below the Cc there will be net ________ (assembly/disassembly)

Disassembly. They need more free subunits to increase the concentration

T/F Microtubules and microfilaments have the same assembly rates at both ends

False. It grows faster at the plus end and slower at the minus end (probably because it is already in the right shape to attach. The minus end needs to mold its way in into that shape)

T/F The plus and minus ends are structurally distinct

True. The plus end has the indented arrow shape for subunits to easily attach. The minus end is the outwards arrow shape which doesn't match with the subunit's straight edge

Multiple protofilaments with lateral bonds provide stability

True. Single ones are unstable (for reasons I don't quite get yet)

T/F Microtubule assembly is a heterodimer

True. Alpha and beta subunits stick together and to each other to make long strands. Then the strands are wrapped around in a tube like shape

T/F Microtubules are stabilized

True. There are multiple lateral and longitudinal bonds holding them together

T/F Microfilaments are subunits for the protein actin

True. Actin is made up of microfilaments

Search up Nucleotide Hydrolysis in microtubule assembly. Dynamic instability and treadmilling

Go do it!

T/F Intermediate filament assembly is not accompanied by nucleotide hydrolysis and they do not have intrinsic polarity

True. They are not and do not