14 - the cytoskeleton Flashcards
what cellular compartments are inside the cell that are part of the cytoskeleton?
nucleus
cytoplasm
- cytosol (gel-like fluid containing proteins, RNA, sugars, ions etc.)
- cell organelles
what cellular compartments are outside the cell?
extracellular matrix (ECM)
interstitial fluid
what are four membranous organelles?
- endoplasmic reticulum (ER)
- mitochondria
- Golgi apparatus
- lysosomes
membrane organelles are surrounded by lipid membrane.
what are four non-membranous organelles?
- centrioles
- ribosomes
- microvilli
- cytoskeleton
what is the cytoskeleton?
an organised network of protein filaments within the cytosol.
what does the cytoskeleton do?
provides scaffolding and structure to the cell.
an intracellular highway which allows transport of organelles and key cellular proteins.
involved in key cellular functions, e.g. movement (motility) and cell division.
helps to maintain a stable environment within a cell - homeostasis.
- cell level - enables coordinated action of many internal proteins and organelles
- tissue level - enables multiple cells to coordinate and attain homeostasis.
what are the three main function of the cytoskeleton?
- spatially organises cell contents.
- connects the cell to its environment.
- generates coordinated forces (e.g. for cell motility)
these functions require coordination between numerous cytoplasmic proteins and organelles.
the cytoskeletal mesh incudes three main classes of filaments/tubes, what are these?
- microtubules.
- intermediate filaments.
- microfilaments or actin filaments.
each are made of different building blocks (subunits) and perform different roles within the cell.
what do microtubules do?
determine the positioning of membrane bound organelles.
what are microtubules formed of?
tubulin subunits (α-tubulin, β-tubulin dimer).
in what cells do 13 strands of linear tubulin wind together to form a rigid hollow tube?
mammalian cells
in microtubules, what direction do the subunits point?
subunits all point in the same direction - structural polarity.
where do microtubules originate from?
centromeres near the nucleus
what do intermediate filaments do?
provide mechanical strength to the cell - resists tensile forces, not compressive forces.
it is the least stiff of the three fibres.
what are three examples of intermediate filaments?
vimentin, lamin and keratin.
what do actin microfilaments do?
determine the cells outer shape (e.g. protrusions) and enables locomotion.
what are actin microfilaments formed from?
actin subunits and two single filaments twisted in a right hand helix.
what do polarised actin microfilaments allow?
allows fibres to expand and move the cell in one direction.
how are tensional forces produced?
flexible and multiple fibres can crosslink into bundles to produce tensional forces.
where are actin microfilaments found in highest concentrations?
beneath the cell membrane (actin cortex)
what type of cells are tubulin and actin found in?
eukaryotic cells
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Genetic sequences are highly conserved between species compared to other proteins (e.g. Ifs, regulatory proteins).
These proteins must be able to interact with dozens of other proteins - changes would disrupt their function.
how is the cytoskeleton constructed by filament assembly?
generation of a nucleation site (where cytoskeletal proteins can bind)
formation of a linear chain of protein subunits (profilament) - weal non-covalent bonding.
protofliaments associate laterally to form the three cytoskeletal filament types - provides stability.
accessory proteins are often involved in this process, e.g. motor proteins.
how is the cytoskeleton dynamic?
filaments are rapidly assembles (polymerised) and dissembled as required by the cell.
assemble can occur in the space of seconds, e.g. neutrophils chasing bacteria.
reorganisation allows processes such as mitosis and cell motility.
what does disassembly allow subunits to do?
disassembly allows subunits to diffuse rapidly through the cytoplasm.
what are five examples of regulatory proteins?
- capping proteins
- polymerases
- depolymerising and severing factors
- cross-linkers and stabilising proteins
- nucleation-promoting factors
what is the main microtubule-organising centre (MTOC)?
the centrosome is the main MTOC located near the nucleaus.
how many microtubules may grow from one centrosome?
hundreds
what do MTOCs control about microtubules?
length, distribution and polarity.
what are centrioles?
centrioles (perpendicular tube-like structures) act as construction points of microtubules.
what does polarity effect in microtubules?
polarity affects the polymerisation rate of microtubules:
- minus ends are slow polymerising and connects to MTOCs
- positive ends are fast polymerising
what is an example of stable structures often required?
actin bundles within hair cells (inner ear) are maintained for the cells lifetime.
…
cytoskeletal structures can span from one side of the cell to another.
how often are individual actin fibres replaced?
frequently.
what are accessory proteins involved with?
crosslinking individual filaments together or to other cell components.
filaments can enable cell polarisation, give an example of this
filaments can enable cell polarisation - e.g. endothelial cells lining blood vessels align with the direction of flow.
how can filaments mechanically connect to cellular membranes?
via accessory proteins
what proteins form the linker of nucleoskeleton and cytoskeleton (LINC) complex?
proteins such as nesprin
what connects actin to the external cell environment?
focal adhesion proteins connect actin to the external cell environment (extracellular matrix)
actin filaments can also evert outward forces on the cellular membrane to generate protrusions.
what are three of these?
lamellipodia - sheet like protrusions
filopodia - long, thin protrusions
podosome - larger protrusions which interact with the environment/ extracellular matrix.
what are two appendages present on some cell types?
Flagella - enable cell motility
Cilia - move particles over the cell surface
what do Microvilli do?
increase cell surface area for nutrient absorption in the gut lining.
what do hair cells do in the inner ear?
sensory protrusions within the inner ear detect passing sound waves and enable hearing.
what are these hair cell, sensory protrusions made from?
protrusions involve actin bundles and motor proteins.
what does loss of sensory protrusion structure in the inner ear result in?
damaged hearing
what is the cytoskeletons role in cell division?
microtubules start arranged rapidly from the centrosome.
during mitosis, microtubules form a bipolar mitotic spindle.
microtubules allow separation of duplicated chromosomes.
actin filaments form a contractile ring which pinches the cell into two new cells.
what are molecular motors?
accessory proteins which convert energy (from ATP hydrolysis) into mechanical force/motion.
filaments act as tracks
can move organelles along the filaments or can move the filaments themselves.
e.g. myosin as an accessory protein to actin.
why is microtubule popularity important?
it is thought to be important for interaction with motor proteins
retrograde and anterograde motors can pull opposite directions. give an example of each.
kinesin-1 is an anterograde motor protein. it ‘walks’ along a single protofilament.
cytoplasmic dynein is retrograde motor protein. it uses two adjacent protofliaments.
what disease can involve reduced transport of mitochondria along axons?
motor neuron disease.
what are neuronal growth cones?
growth cones are cytoskeletal structures that lead axonal growth.
they allow neutral cells to span long distances and interact with their environment.
what does movement of cells involve?
movement of cells involves increased actin polymerisation at the leading edge of the cell.
what does actomyosin concentration do to the actin filaments?
actomyosin concentration pull the actin filaments along each other.
e.g. neutrophils chasing bacteria.
what is cytoskeletal tension?
actin bundles act as stress fibres within the adherent cells. assembly can nucleate from focal adhesions.
actomyosin concentration generates cellular tension and cell stiffness increases.
this initiates chemical signalling and functional changes within the cell.
how do muscles contract?
muscle contraction is also a result of actomyosin concentration.
muscle fibres are multinuclear structures with coherent cytoskeletal complexes.
muscle fibres are divided into sarcomeres with overlapping actin-myosin filaments.
what is endocytosis?
the process of external material being internalised by the cell.
external material interacts with receptors on the cell membrane and as a result becomes surrounded by cell membrane.
this pocket of material becomes contained with cellular membrane and drawn into the cell.
actin filaments assemble to bud off this contained packed of material into the cytoplasm (a vesicle).
