The Cytoskeleton

Question 1

What is the diameter of the micro filaments ?

Answer 1

7 nm

Question 2

What is the diameter of the microtubules ?

Answer 2

25 nm

Question 3

What is the diameter of the intermediate filaments ?

Answer 3

10 nm

Question 4

What is the structure of microfilaments ?

Answer 4

It is made of globular actin (G actin) monomers which form filamentous polymers (F actin)

Question 5

How is F actin formed?

Answer 5

It is assembled in a directional way. G actin adds to the plus end of the F actin polymer and is removed from the minus end.

Question 6

Where are microfilaments found in cells ?

Answer 6

They are distributed throughout the cytosol but are very concentrated at the edge of the cell in the cell cortex which is found underneath the cell membrane.

Question 7

What are the functions of microfilaments ?

Answer 7

1) stabilisation of the cell surface
2) pulling of the cell membrane
3) pushing of the cell membrane

Question 8

Give an example of when microfilaments carry out stabilisation

Answer 8

In the microvilli - actin building proteins, lateral cross links and spectrin cross links allow actin filament bundles to form which allow microvilli to be static structures

Question 9

Give an example when microfilaments carry out pulling forces on the cell membrane

Answer 9

When a cell divides during mitosis, each cell needs a nucleus and so a ring of actin and myosin forms which divides the cytoplasm of the cell to form 2 cells. This ring is known as a contractile ring.

Question 10

Give an example when microfilaments carry out pushing forces on the cell membrane

Answer 10

Microfilaments carry out pushing forces when extending the cell membrane. The cell membrane can extend to form micro spikes or broad flat sheets. Actin subunits are formed at the nucleation site and this causes the membrane to extend. As more subunits are added, they are pushed back until they detach. This is known as actin treadmilling.

Question 11

What are microtubules ?

Answer 11

Microtubules are formed from tubulin subunits which are globular proteins. The tubulin subunits join in a spiral method to form protofilaments. Microtubule associated proteins (MAPS) stabilise the microtubule and provide motile forces along it.

Question 12

Where are microtubules found ?

Answer 12

They are found near the centrioles of cells. The minus end is located at the centriole.

Question 13

What are the functions of microtubules ?

Answer 13

Intracellular transport and organisation e.g. In fast axonal transport they help neurotransmitters to be released at synapses and recycled back in

Question 14

What is the structure of the intermediate filaments ?

Answer 14

They are made up of subunits of big fibrous proteins to form filaments which extend from the nuclear envelope to the plasma membrane

• when phosphorylated, the filaments disintegrate
• when dephosphorylated, the filaments assemble again

Question 15

What are the different types of intermediate filaments ?

Answer 15

In all cells, intermediate filaments make up nuclear lamina which are found on the inside of the nuclear envelope.
In nerve cells, they make up neurofilaments
In muscle cells, they make up desmin
In epithelia, they make up cytokeratins

Question 16

What are the functions of intermediate filaments ?

Answer 16

1) Strength - different cell types have different requirements hence the different intermediate filament types
2) Control of the nucleus and assembly - they support the nuclear envelope and dismantle the envelope in cell division
3) Clinical uses - able to identify the tissue of origin of epithelium derived tumours