week 4: lecture 4 Flashcards
-Cytoskeleton: location cell (prokaryotes and archaea): structure/ function -Cytosol: structure/function
Cytoplasm (definition)
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
Cytoplasm consists of organelles such as
- The mitochondria
- golgi apparatus
- endoplasmic reticulum
- lysosome
What’s the cytosol?
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.
The cytosol is a complex mixture of?
- water (approx 70%)
- salts
- dissolved ions
- organic molecules.
The pH of the cytosol is?
7.2
pH of the cytosol is maintained in a range of 7.0 – 7.4 by?
Antiporter (Na+/H+) and Anion (Cl-/HCO3) exchanger membrane proteins
Antiporters and anions protect the cell against
Excessive cytoplasmic acidification
Cytosol is the site of numerous metabolic pathways
- glycolysis
- The pentose phosphate pathway
- Gluconeogenesis and synthesis of fatty acids
Functions of the cytosol
- 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
Cytoskeletal
The cytoskeleton is a network of fibers that extend through the cytoplasm in the cell.
3 cytoskeletal polymers
- Microtubules
- actin filaments
- intermediate filaments (a group of polymers)
are polymers of protein subunits held together by weak, non-covalent interactions
differences that distinguish architecture and function of the cytoskeletal polymer types and the networks they form are:
- mechanical stiffness
- dynamics of their assembly
- polarity
- type of molecular motors with which they associate
cytoskeletal function
- structure and support
- intracellular transport
- contractility and motility
- spatial organisation
What are microtubules?
- 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.
microtubules structure
-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.
Microtubules consist of
- Tubulin
- dimers of alpha
- beta-tubulin
Structural unit of microtubules
-Alpha and beta-tubulin heterodimer
dimers of alpha and beta tubulin
alpha and beta tubulins of microtubules
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.
how does tubulin heterodimers join in microtubule axis
join end to end to form protofilaments with alternating a & b subunits
What must happen for a heterodimer to associate with other heterodimers?
GTP must be ound to both a and b subunits
are microtubules polarised
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
microtubules function
- 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
breakdown of microtubules
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.
microtubules in flagella and cilia
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.
cytoplasmic dynein
The form of dynein associated with microtubules in the cytoplasm.
Space in between the protofilaments
seam
Microtubules can ____ and _____ in length
Grow; shrink
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
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.
Centrioles function
Organizes spindle fibers used during cell division
Helps in forming flagella and cilia
What does each centriole have
9 triplets of microtubules arranged in a ring
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)
microfilament functions
- maintenance of cell shape
- changes in cell shape
- muscle contraction
- cytoplasmic streaming
- cell motility
- cell diviison
Centrosome during interphase
the centrosome is usually located near the nucleus. Microtubules grow out from the MTOC forming a hub * spoke array.