Cytoskeleton

Question 1

Cytoskeleton (three functions)

Answer 1

A network of filaments and tubules that:

1. Provide mechanical support to the cell and maintains its shape
2. Provides the cell with movement ability (cell motility)
3. Mediates the movement of organelles and individual molecules (intracellular transport)
4. Regulates biochemical activities of the cell by transmitting mechanical forces.

Question 2

What are Microtubules made of?

Answer 2

Microtubules are hollow tubes made of the globular protein tubulin.

Question 3

Tubulin

Answer 3

Two types: alpha-subunits and beta-subunits. Alpha-subunits and Beta-subunits polymerize into alpha-beta dimers which then polymerize into chains that form the walls of a hollow tube = micotubule.

Question 4

How do they grow?

Answer 4

Microtubules grow by adding dimers of alpha and beta subunits to their ends. Tubulin proteins arrange in a helix to form around the hollow core.

Question 5

Function of a Microtubule

Answer 5

1. Maintains cell shape by resisting compression.
2. Motility:
   - Movement of cells
   - Movement of organelles (move vesicles from Golgi to the plasma membrane for secretion)
   - Movement of chromosomes

Question 6

How does the cytoskeleton move?

Answer 6

The cytoskeleton does not contract and extend for movement; instead, it assembles and disassembles.

Question 7

Centrosomes

Answer 7

• In animal cells, microtubules often grow from structures called centrosomes, which are made of two centrioles.
• Each centriole is 250 nm, arranged perpendicular, and is made of 9 sets of 3 microtubules each.
• When cells divide, centrioles divide too
• Plant cells do NOT have centrioles

Question 8

Similarities between Cilia and Flagella

Answer 8

• Apparent on cells that physically move

- The locomotive organs move due to a special arrangement of microtubules

Question 9

Flagellum

Answer 9

• Usually a single flagellum
• Same diameter as cilium, but longer
• Instead of beating, it undulates

Question 10

Ultrastructure of cilia and flagella

Answer 10

• The microtubule core is sheathed in the plasma membrane
• The basal body anchors the cilium/flagellum to the cell
• The core structure consists of 9 doublets of microtubules arranged in a circle, and 2 single microtubules in the center = 9+2 structure

Question 11

Cilia and Flagella Microtubule doublet

Answer 11

• The 9 doublets are connected by dynein (a large protien which acts as motor responsible for the movement of cilia and flagella) arms
• Dynein + ATP = change in conformation (=movement)
• Each doublet is connected to the two central microtubules by radial spokes
• Flagella and Cilia are anchored to the cell by a basal body
• The basal body has a structure identical to a centriole

Question 12

Dynein-mediated movement

Answer 12

1. Dynein arms of one doublet attach to the neighboring doublet
2. They pull, and doublets slide against each other in opposite directions
3. Dynein arms detach from the doublet
4. They reattach further up and pull again.
   - Requires ATP

Question 13

Microfilaments

Answer 13

• Two intertwined strands of protein called actin
• 7nm in diameter
• Polymers of globular monomers of actin
• Present in the center of microvilli (cellular projections that increase surface area to help absorb material from outside the cell)

Question 14

Functions of microfilaments

Answer 14

• Maintains cell shape by resisting tension

- Motility (muscle contraction, cell division, cytoplasmic streaming)

Question 15

Muscle Contraction and Microfilaments

Answer 15

In muscles: actin filaments are arranged parallel to myosin filaments. When actin and myosin filaments pass each other, the cell becomes shorter (muscle contraction)
- Similiar actin-myosin sliding is involved in amoeboid movement of some cells. Cell moves where there is less gel (amoebas, white blood cells)

Question 16

Microfilaments’ effect on cytoplasm

Answer 16

• The presence of microfilaments makes the cytoplasm more “rigid” = gel state
• Less microfilaments = sol state
• Gel state to Sol state transition, due to actin myosin interactions, causes cytoplasmic streaming (mainly in plants)
• Cytoplasmic streaming is the circular motion of the cytoplasm within large cells to help distribute materials inside cells.

Question 17

Intermediate Filaments

Answer 17

• Diameter = 8-12 nm
• Made of fibrous protein, super-coiled into thicker strands (MT and MF made of globular subunits)
• Proteins comprising IF belong to the keratin family
• MF and MT are often disassembled, but IF are more permanent (they support more constant features of cell shape and structure)

Question 18

Functions of Intermediate Filaments

Answer 18

• Maintain cell shape by resisting tension
• Anchor the nucleus (in a filament “cage) and other organelles
• Form nuclear lamina
• Maintain permanent cell shape in neurons

Question 19

Components of the Cytoskeleton

Answer 19

1. Microfilament: 2 strands, actin monomer
2. Intermediate Filaments: Fibrous subunit, variable number of subunits
3. Microtubules: 13 columns, alpha-beta tubulin dimer forming helix around hollow core

Question 20

Intercellular connections

Answer 20

Animal: 
- Gap Junctions
- TNTs
Plant:
- Plasmodesmata
All critical for cell-cell communication and transport, and are permeable channels.

Question 21

Similarity between Plasmodesmata and TNTs, and difference with Gap Junction.

Answer 21

Larger and can open further to move large molecules (proteins, RNA, viruses). Gap junctions are smaller, and cannot open more than they normally are.

Question 22

Other Animal Intercellular connections

Answer 22

1. Desmosome: anchor adjacent cells together (cardiac muscle)
2. Tight junctions: areas where membranes of adjacent cells are fused (intestinal epithelium)
   - all not permeable.

Question 23

Gap Junctions

Answer 23

1. Gap junction protein = connexin
2. 6 connexin subunits make up a connexon
3. 2 connexons (one from each cell) connected to each other make up a gap junction

Question 24

TNTs (tunneling nanotubes)

Answer 24

1. One cell forms an actin-driven protrusion directed towards the target cell. Fusion of the cell protrusion with the membrane of the target cell results in TNT formation
2. TNTs may form between adjacent cells, which subsequently diverge.
   - Cargo carried: lysosomal, ER, golgi vesicles transported with the help of molecular motors, proteins, organelles (mitochondria), pathogens (virus, bacteria, prion proteins).