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Fundamentals of the living cell > The cytoskeleton > Flashcards

The cytoskeleton Flashcards

1
Q

The cytoskeleton is made of 3 components, what are these and what do they do? (3)

A
  • Microtubules – organelles positioning and intracellular support
  • Intermediate filaments – mechanical strength
  • Acting filaments – cell shape, organelles shape and cell migration
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2
Q

What is the function of the cytoskeleton? (4)

A

Shape of cell

Intracellular movement/ location of organelles

Modify cells in response to environmental cues

Cell Movement

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3
Q

How does the cytoskeleton work? (6)

A

Cytoskeleton is dynamic.

· The various filaments are made of monomers that continually polymerise and depolymerise.

  1. The cell receives a signal (via receptors on cell membrane etc.).
  2. Existing filaments in the cell depolymerise to form free monomers.
  3. The monomers rapidly diffuse.
  4. The monomers reassemble at a new site.
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4
Q

What do accessory pigments regulate? (3)

A
  1. Nucleation: The site and rate of filament formation
  2. (de)polymerisation
  3. Function
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5
Q

Name 4 general points about actin filaments and microfilaments

A

Helical polymers (made of actin)

Flexible structure: 2D networks, 3D gels

Gives shape to cell and organelles.

Involved in cell migration.

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6
Q

Describe the structure of the actin filaments/ microfilaments (9)

A

Twisted chain of Globular Actin (G-

actin, a protein monomer, aprox.43 KDa).

· When the G-Actin join together to form a filament/ polymer, it is called; F-actin.

· Thinnest form of cytoskeleton filaments (7nm).

· Structural polarity.

· Large number of ABP (actin binding proteins) in F-actin.

· Three isoforms of G-actin with different isoelectric points:

  • α-actin found mainly in muscle cells
  • β-actin in non-muscle cells
  • γ-actin in non-muscle cells
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7
Q

Describe actin polymerisation

A
  • Actin filaments (F-actin) can grow by addition of actin monomers (G-actin) at either end.
  • The length of the filament is determined by: concentration of G-actin and presence of actin binding proteins (ABPs.)
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8
Q

How are G-actin levels regulated

A
  • Profilin: facilitates actin polymerisation.

* Thymosin b4: prevents the addition of actin monomers to F-actin

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9
Q

Describe actin-bundling proteins and cross-linking filaments and give examples in each case

A
  • Actin Bundling Proteins: keep F-actin in parallel bundles (as in the microvilli observed in epithelial cells)
  • Cross-linking proteins: maintain F-actin in a gel-like meshwork (as seen in the cell cortex, underneath the plasma membrane)
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10
Q

Describe F-actin severing proteins and motor proteins

A
  • F-actin severing proteins: break F-actin into smaller filaments.
  • Motor proteins (Myosin): transport of vesicles and/or organelles through actin filaments
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11
Q

Describe the function of actin filaments in skeletal muscle and non-muscle cells

A
  • Interaction with Myosin allows for muscle contractions.
  • Arranged in a para-crystaline display integrated with different ABP
  • Cell cortex: form a thin sheath beneath the plasma membrane.
  • Associated with myosin: cleavage of mitotic cells (D) (cytokinesis).
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12
Q

Describe cell migration

A
    1. The cell pushes out protrusions at its front (lamellipodia & filopodi).
    1. Actin polymerisation.
    1. These protrusions adhere to the surface.
    1. Integrins (link the actin filaments to the extracellular matrix surrounding the cell).
    1. Cell contraction and retraction of the rear part of the cell.
    1. Interaction between actin filaments and myosin.
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13
Q

Describe intermediate filaments

A
  • Toughest filaments: resistant to detergent and high salt
  • Rope structure with many long strands twisted together and made up of different subunits.
  • Intermediate size (8-12nm) between actin and microtubules.
  • Form a network:
  • Throughout the cytoplasm, joining up to cell-cell junctions (desmosomes). This withstands mechanical stress when cells are stressed
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14
Q

Describe the function of the intermediate filaments

A
  • Tensile strength: this enables the cells to withstand mechanical stress (to stretch)
  • Structural support by: creating a deformable 3D structural framework and reinforcing cell shape and fix organelle localisation
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15
Q

Describe the structure of the intermediate filaments

A 
•	Each unit is made of:
1.	N-terminal globular head
2.	C-terminal globular tail
•	Central elongated rod-like domain
•	Units form stable dimers
•	Every 2 dimers form a tetramer
•	Tetramers bind to each other and twist to constitute a rope-like filament
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16
Q

Describe the types of intermediate filaments

A

As shown on image

17
Q

Describe intermediate filaments binding proteins generally and in the nucleus

A
  • IFBP stabilise and reinforce IF into 3D networks.
  • An example is Fillagrin: Binds to keratin filaments into bundles.
  • Another example: Synamin and Plectin: bind desmin and vimentin. This links IF to other compounds (e.g. actin and microtubules) as well as to cell-cell contact structures (desmosomes).
  • Final example: Plakins: Keep contact between desmosomes of epithelial cells.
  • Present in all nucleated eukaryotic cells.
  • Forms ‘mesh’ structure rather than “rope-like” structure.
  • They line the inner face of nuclear envelope to strengthen it and provide attachment site for chromatin.
  • Disassemble and reform at each cell division as nuclear envelope disintegrates.
  • i.e. very different from the stable cytoplasmic Intermediate filaments.
  • Process controlled by post-translational modifications (mainly phosphorylation and dephosphorylation).
18
Q

Describe the structure of microtubules

A
  • Hollow tubes made up of protein: tubulin
  • Stiff and thick
  • Relatively stiff (25nm), is the thicker of the filaments
  • Each filament is polarised (i.e. has direction – head/tail or +/-)
  • Rigid, long straight
  • Organelle positioning
  • Intracellular transport
  • Cell movement
  • Dynamic structure relating to the next 3 points below:
  • (dis)assembles in response to cell needs
  • Tubulin in cell is roughly 50:50 as free or in filament
  • i.e. very different from the stable cytoplasmic intermediate filaments
19
Q

Describe the polymerisation of microtubules

A
  1. Microtubules organising centre (MTOC): specialised protein complexes where microtubule assembly of tubulin units starts.
  2. Centrosome: In perinuclear region: the MTOC of most cells
  3. Contains gamma(YYYY) -tubulin ring that initiates the microtubule growth
  4. Heterodimers of a and b tubulin constitute the microtubule.
  5. It is a polarised growth (i.e. there is an end that grows faster (+end) than the other (- end).
20
Q

Describe the function of microtubules

A

cell shape and intracellular transport of vesicles and organelles

Fundamentals of the living cell (28 decks)

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