cytoskeleton

Question 1

structure and function of cytoskeleton

Answer 1

intricate network of protein filaments extending throughout the cytoplasm

SUPPORT function (eukaryotic cells have no cell wall)
cells can adopt a variety of shapes, organise compartments, interact mechanically with the environment, movement

Question 2

three types of protein filament found in the cytoskeleton

Answer 2

intermediate filaments
microtubules
actin filaments

Question 3

why is the cytoskeleton described as dynamic?

Answer 3

it is continuously reorganised, changing with cell division

can quickly assemble / disassemble networks (eg. neutrophil creating a pseudopod to help movement in chasing bacteria)

Question 4

describe the structure of cytoskeleton filaments in polarised epithelial cells

Answer 4

apical surface: bundled actin filaments form microvilli for absorption

band of actin filaments for the adherens junctions
intermediate filaments anchored to desmosomes / hemidesmosomes

microtubules run vertically from top to bottom of the cell

Question 5

how does the cell assemble / disassemble cytoskeleton filaments?

Answer 5

individual protein monomers (subunits) can easily diffuse around the cytosol

hence filaments (polymer) can be reassembled at another site - joined both end to end and side to side

Question 6

name the two classes and four types of intermediate filaments

Answer 6

Cytoplasmic:
keratin filaments (epithelial cells)
vimentin / vimenten-related filaments (CT, muscle, glial cells)
neurofilaments (nerve cells)

Nuclear:
nuclear lamins (all animal cells)

Question 7

describe the structure of actin filaments

Answer 7

thin, flexible protein threads
made up of actin monomers in a two-stranded helix, with clefts in each = binding site for ATP/ADP

Question 8

what are activated carrier molecules? give examples

Answer 8

molecules that can be split to release free energy
eg. ATP, NADH

Question 9

describe treadmilling of actin filaments

Answer 9

a process of dynamic disassembly/reassembly
actin monomer hydrolysis its bound ATP to ADP when it is incorporated into the filament.
ATP-actin added to plus end as ADP-actin lost at minus end

Question 10

where does the mechanical strength of actin come from?

Answer 10

actin forms bundles, strength from bundling/crosslinking
withstands tension rather than compression

Question 11

what are filopodia?

Answer 11

finger-like cytoplasmic projections of the plasma membrane rich in actin filaments
often helps cell migrate

Question 12

how does actin help a cell move?

Answer 12

forces can be generated in actin-filament-rich cortex
actin polymerisation –> protrusion in actin cortex = forward movement
contraction at the rear of the cell

Question 13

describe the structure of microtubules

Answer 13

hollow tubes made of globular tubules subunits (dimers)
dimer orientation –> structural polarity (+ and - end)

Question 14

describe the functions of microtubules

Answer 14

organise the cytoplasm of eukaryotic cells
makes up the mitotic spindle in dividing cells
bundled to form cilia

Question 15

what is the arrangement of microtubules determined by?

Answer 15

the location / orientation of microtubule arrays are controlled by microtubule-organising centres (centrosomes in fibroblasts)

Question 16

what controls the dynamic instability of microtubules?

Answer 16

GTP hydrolysis.
Tubulin dimers carrying GTP bind more tightly than GDP-bound dimers
hydrolysis–>peels away, release of dimers–> microtubules shrink

Question 17

function of dynamic instability of microtubules

Answer 17

controls polymer length / cell movement
when GTP-tubulin addition is faster than hydrolysis, GTP cap forms
when rate slower, cap lost, filament shrink

Question 18

role of microtubules in transport in nerve cells

Answer 18

guides transport of organelles, vesicles, macromolecules alone a NERVE CELL AXON
outward (to axon terminal)
backward (to cell body)

Question 19

what is the role of motor proteins and how do they work?

Answer 19

drives cell movement using energy from ATP hydrolysis to travel along microtubule / actin filament

Question 20

describe actin based motor proteins

Answer 20

eg. myosin
two coiled helices with head and tail domain
–> muscle contraction

Question 21

name two microtubule based motor proteins

Answer 21

kinesin moves along microtubule, outward from cell body
dynein moves in the opposite direction