week 4: lecture 4 Flashcards
-Cytoskeleton: location cell (prokaryotes and archaea): structure/ function -Cytosol: structure/function (87 cards)
1
Q
Cytoplasm (definition)
A
Consists of all the contents that we can find within the plasma membrane excluding the cell nucleus (separated from the cytoplasm by the nuclear envelope).
Located outside the nucleus and inside the plasma membrane
2
Q
Cytoplasm consists of organelles such as
A
- The mitochondria
- golgi apparatus
- endoplasmic reticulum
- lysosome
3
Q
What’s the cytosol?
A
Cytosol is an intracellular fluid present within the cytoplasm.
- Gel-like, semifluid material that surrounds the membranous organelles and particles of the cytoplasm
- Surrounds membranous organelles
- Takes up most of the cell volume.
4
Q
The cytosol is a complex mixture of?
A
- water (approx 70%)
- salts
- dissolved ions
- organic molecules.
5
Q
The pH of the cytosol is?
A
7.2
6
Q
pH of the cytosol is maintained in a range of 7.0 – 7.4 by?
A
Antiporter (Na+/H+) and Anion (Cl-/HCO3) exchanger membrane proteins
7
Q
Antiporters and anions protect the cell against
A
Excessive cytoplasmic acidification
8
Q
Cytosol is the site of numerous metabolic pathways
A
- glycolysis
- The pentose phosphate pathway
- Gluconeogenesis and synthesis of fatty acids
9
Q
Functions of the cytosol
A
- Protects the cell against excessive cytoplasmic acidification and establishment of a steady-state pH permissive for growth and cell development
- transport of metabolites
- Site for numerous metabolic pathways
-site for protein biosynthesis
(translation)
- involved in singling transduction
- forms the basis of cytoskeleton
10
Q
Cytoskeletal
A
The cytoskeleton is a network of fibers that extend through the cytoplasm in the cell.
11
Q
3 cytoskeletal polymers
A
- Microtubules
- actin filaments
- intermediate filaments (a group of polymers)
are polymers of protein subunits held together by weak, non-covalent interactions
12
Q
differences that distinguish architecture and function of the cytoskeletal polymer types and the networks they form are:
A
- mechanical stiffness
- dynamics of their assembly
- polarity
- type of molecular motors with which they associate
13
Q
cytoskeletal function
A
- structure and support
- intracellular transport
- contractility and motility
- spatial organisation
14
Q
What are microtubules?
A
- Microtubules are hollow cylinder rods made of protofilaments (13, side by side) about 25 nm in diameter and about 200 nm to 25 μm long
- most rigid of the 3 polymers (approx. >100 times that of actin filaments).
- have the most complex assembly and disassembly dynamics.
- have filament flexibility that increases with length.
- Can form almost linear tracks that can span the length of a typical animal cell.
15
Q
microtubules structure
A
-Composed of a protein called tubulin, refers to a group of globular proteins.
A microtubule are made of 13 protofilaments arranged side-by side into a cylinder:
• Its a polymer
• The basic structural unit is dimers of (alpha) α and (beta) β-tubulin heterodimer.
-The heterodimer does not
come apart, once formed.
16
Q
Microtubules consist of
A
- Tubulin
- dimers of alpha
- beta-tubulin
17
Q
Structural unit of microtubules
A
-Alpha and beta-tubulin heterodimer
dimers of alpha and beta tubulin
18
Q
alpha and beta tubulins of microtubules
A
Homologous but not identical.
Each has a nucleotide binding site.
a-tubulin has a bound GTP, that doesn’t hydrolyse.
B-tubulin may have bound GTP or GDP.
B-tubulin can hydrolyse its bound GTP to GDP plus Pi release Pi, and exchange the GDP for GTP.
19
Q
how does tubulin heterodimers join in microtubule axis
A
join end to end to form protofilaments with alternating a & b subunits
20
Q
What must happen for a heterodimer to associate with other heterodimers?
A
GTP must be ound to both a and b subunits
21
Q
are microtubules polarised
A
Yes.
microtubules are so considered polarized
polymers, as their subunits are structurally
asymmetrical at the molecular level.
• There is a ‘tip’ of the microtubule that is
GTP-bound, while beta-tubulin deeper down
the microtubule, added longer ago, are
GDP-bound
22
Q
microtubules function
A
- shaping the cell
- give strength and rigidity to the cell by resisting compression pressure
- separating chromosomes during cell division
- guiding movements of organelles
- cell motility in cilia and flagella
23
Q
breakdown of microtubules
A
If something happens to sever the microtubule, the less stable, GDP bound beta-tubulin becomes exposed and the protofilaments will start to peel away.
IF this happens, the microtubule will disassemble until it hits an island of GTP - bound beta - tubulin.
Can be rescued by the addition of new GTP - bound tubulin dimers.
24
Q
microtubules in flagella and cilia
A
have 9 + 2 arrangement of microtubules through their cross section. Typically, cells posses one or two long flagella, whereas ciliated cells have many short cilia.
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cytoplasmic dynein
The form of dynein associated with microtubules in the cytoplasm.
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Space in between the protofilaments
seam
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Microtubules can ____ and _____ in length
Grow; shrink
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Centrosomes
-Microtubules grow out from a
centrosome region near the nucleus.
-The centrosome region is defined as the
“microtubule-organizing centre” and is not
surrounded by a membrane.
-In animal cells, the centrosome has a pair of
centrioles, each with nine triplets of
microtubules arranged in a ring
29
Centrioles strcture
Cylindrical structure, found in pairs orientated at right angles to one another. The wall of each centriole cylinder is made of nine interconnected triplet microtubules. Centriolar microtubules are more stable than most cytoplasmic microtubules. The centriole, duplicates once each cell cycle.
30
Centrioles function
Organizes spindle fibers used during cell division
| Helps in forming flagella and cilia
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What does each centriole have
9 triplets of microtubules arranged in a ring
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What is the outer wall of the centriole cylinder typically comprised of?
nine triplet MTs, which are formed by one complete MT (the a- tubule) fused with two incomplete MTs (the B- and C-tubules)
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microfilament functions
- maintenance of cell shape
- changes in cell shape
- muscle contraction
- cytoplasmic streaming
- cell motility
- cell diviison
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Centrosome during interphase
the centrosome is usually located near the nucleus. Microtubules grow out from the MTOC forming a hub * spoke array.
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What happens to centrioles prior to mitosis
they're duplicated
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importance of microtubules in the spindle apparatus
microtubules form the spindle apparatus which is vital for organisation of the cellular contents during cell division.
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mitotic spindle
structure that forms during cell division and separates duplicated chromosomes to ensure that they are equally divided among the two daughter cells.
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centrosome during cell division
duplicated centrosome helps to organize the mitotic spindle. In dividing cells, centrosomes organise MTs into the bipolar spindle apparatus upon which chromosomes segregate at mitosis.
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motor proteins
A protein that interacts with cytoskeletal elements and other cell components, producing movement of the whole cell or parts of the cell. They associate with their filament tracks through a "head" region, or motor domain, that binds and hydrolyzes AtP.
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intermediate filaments functions
- maintenance of cell shape
- anchorage of nucleus and certain other organelles
- formation of nuclear lamina
- link to the extracellular matrix
41
Microfilaments (actin filaments)
- Most dynamic (they grow, shrink and change conformation most swiftly).
- Provide mechanical support to cells.
- connect the contents of the cytoplasm with cell membrane in order to support signal transduction.
- Vesicles and organelles are moved along microfilament tracks in a controlled manner cargo transport myosin.
42
How do we know about the cytoskeleton?
biochemistry
-identified and characterized basic building blocks
Molecular biology:
-characterise the
genes involved in the processes and protein
expression
advanced microscopy-
determine, the locations and dynamics of these cytoskeletal proteins during processes such as:
-cell division
-motility
43
13, longitudinally
each microtubule contains an arrangement of ___ protofilaments aligned _____ along its length
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protofilaments defination
assembled from heterodimers of non-identical alpha and beta tubulin subunits
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protofilaments
each microtubule contains an arrangement of 13 ____ aligned longitudinally along its length.
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heterodimers, non-identical, alpha, beta, tubulin
each protofilament is assembled from ____ of ____ ___ and ___ ____ subunits
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non-covalent
alpha and beta tubulin subunits assemble into heterodimers of protofilaments which are held together by _____ interactions
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polarity
the tubulin dimers give ____ to the microtubules
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plus-end
one end of the microtubule where it terminates with beta subunits
one end of the microtubule that is the dynamic end where growth and disassembly occurs
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minus-end
one end of the microtubule where it terminates with alpha subunits
one end of the microtubule where it does not experience growth or disassembly
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Microtubules structure: (α & β tubulins)
```
The α & β tubulins, each
about 55 kDa, are homologous
but not identical.
• Each has a nucleotide binding
site.
• α-Tubulin has a bound GTP,
that does not hydrolyze.
• β-Tubulin may have bound
GTP or GDP.
-β-tubulin can hydrolyze its bound GTP to GDP plus Pi,
release Pi, and exchange the GDP for GTP
```
52
Once formed, the microtubule
| protofilaments,
associate laterally to form
| the hollow microtubule.
53
A key characteristic of microtubules (dynamic)
dynamic nature, microtubules dynamically alter their organization in response to the needs of the cell.
54
Periods of rapid microtubule polymerization
| alternate
with periods of shrinkage in a
| process known as dynamic instability.
55
Actin filaments
Made up of identical actin proteins, arranged in a long spiral chain.
Actin filaments considered to have polarity, that is plus and minus ends, with more ATP - powered growth occuring at a filament's plus end.
Has tight binding sites that mediate head to tail interactions with two other actin monomers, so actin monomers polymerize to form filaments.
56
actin filaments in motility
Actin filaments are the primary cytoskeletal component to drive cell motility. Essential in cell migration and are predominantly found at the leading edge of a moving cell.
57
movement of vesicles:
Material can also be moved using vesicles
loaded with cargo that can move along the
cytoskeleton.
58
All eukaryotic cells must
move materials
| from one place to another in the cytoplasm.
59
Example of Molecular motors movement
```
in a nerve cell with an axon
that may extend far from the cell body,
vesicles can be moved along tracks of
microtubules from the cell body to the end
of the axon.
```
60
Actin-Myosin
The motor protein associated with the actin
microfilaments is myosin.
• Actin-associated myosin motor proteins use
ATP hydrolysis to exert forces against the
stress fibers during muscle contraction
61
Actin filaments in motility
Actin filaments are the primary cytoskeletal
component to drive cell motility.
• Here, actin filaments found in membrane
protrusions such as filopodia and
lamellipodia rapidly assemble and
disassemble.
• These cellular structures are essential in cell
migration and are predominately found at
the leading edge of a moving cell.
62
Microfilaments (Actin Filaments)
Microfilaments are solid rods about 7 nm in
diameter, built as a twisted double chain of
actin subunits. The structural role of
microfilaments is to bear tension, resisting
pulling forces within the cell.
• They form a 3-D network called the cortex
just inside the plasma membrane to help
support the cell’s shape
• Bundles of microfilaments make up the core
of microvilli of intestinal cells
63
Each monomer is rotated by
-by 166o in the filaments, which therefore have the appearance of a double-stranded
helix.
-2 strands wrapped around each other
64
location of intermediate filaments
- found in cytoplasm and then nucleus
65
nuclear lamins
Intermediate filaments that line the inner face of nuclear envelop - fibrous meshwork
- provides structural support
- provides attachment sites for binding proteins and chromosomes
- breaks down during cell division - mains phosphorylated
66
what are the protein subunits in intermediate filaments?
lamins - in the nucleus
desmin- in muscle
keratin - in hair and epithelial
67
what are the protein subunits of microtubules?
tubulin
68
what are the protein subunits of actin filaments ?
G-actin
69
what are the motor proteins of actin filaments?
myosin I and myosin II
70
what are the motor proteins of microtubules?
kinesics (+) and dynes (-)
71
what does different types of intermediate filaments for different cells depend on?
the protein subunits
72
when are nuclear lamins broken down and phosphorylated?
during mitosis
73
the protein subunits are lamins in nuclear lamins
LMNA LMNB LMNC
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what is required cell shape and movement in a cell?
actin filaments
75
intracellular transport in a cell is provided by what?
microtubules
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mechanical strength in a cell is provided by what?
intermediate filaments
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which proteins form intermediate filaments?
Fibrillar proteins
78
spectrin
- cytoskeleton protein
- inner plasma membrane
- mechanical strength stability, shape
- link membranes to motor proteins and all filament systems
79
actin filament motor proteins
- myosins
- globular heads binds ATP
- heads bind to actin filaments; hydrolysis of ATP drives movement
- tails bind structures such as plasma membrane vesicles
80
formation of actin filaments
-filament polarised
-monomoners globular (G-actin) add to either end
- more rapidly at the plus end
-polymerisation associated with ATP
hydrolysis
-filament assembly and organisation regulated by interactions with a variety of actin binding proteins
81
actin filament function
- flexibility
- cell motility and contraction
- adhesion and mechanosensing
82
location of actin filaments in cell
cytoplasm
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structure of actin filaments
- actin monomers
| - use ATP to build
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Dynein movement
- produces microtubule sliding
| - dynein causes microtubule to bend
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microvilli
localised on the villi
- increase surface area for absorption
- contain actin filament
- extension of a plasma membrane
86
flip back and forth to push material over the surface
- can get primary cilia (sensory organs)
| - mobile cilia (in trachea move mucus away from the lungs)
87
microtubule assembly
- assemble from organising centres
- centrosome (normal cell)
- spindle poles (mitotic spindle)
- basal body (cilia/flagella)
- assemble from dimers of alpha and beta tubular
- form cylinder of 13 protofilaments
