Excitable cells and Cytoskeleton Flashcards
What is the concentration of Na+, K+ and Cl- inside and outside of cells?
Na+ - higher outside than inside
inside = 15 mM
outside = 150mM
K+ - higher inside than outside
inside = 140 mM
outside = 5 mM
Cl-
inside = 9 mM
outside = 125 mM
What creates the chemical and electrical gradient in cells?
chemical gradient
- created by the Na-K+ pump
electrical gradient
- created by K+ leakage via ion channels
= creates potential difference due to higher negative charge within the cell
How does the Na-K+ pump work?
3 Na+ and ATP bind to the pump/transport protein
ATP is hydrolyses into ADP and Pi
- catalyses movement of Na+ outside
2 K+ bind and are moved in using the Pi from ADP
creates a chemical gradient
How does potassium leak from the cell?
potassium is higher in the cell than outside
potassium moves down the concentration down out of the cell
- high to low concentration
potassium leaves the cell via the K+ ion channel
leaves large inorganic anions in the cell
creates a separation of charge = means a voltage has been established
- more negative inside the cell than outside
creates a electrical gradient
- there is potential difference
What is membrane potential? What is resting potential?
membrane potential
- voltage established across the membrane
- can be used to send and receive signals
resting potential
- membrane potential when the cell is not sending signals
- normally -70mV in cells
What is an equilibrium potential of an ion, and how does it determine the membrane potential?
equilibrium potential
- membrane potential at which the electrical and chemical gradients of a specific ion are balanced
can be used to calculate the resting potential of a cell taking into account the multiple ion permeabilities
What does a change in membrane potential occur? What are the steps in change of membrane potential?
membrane potential changes because of gated ion channels
- open and close in response to stimuli
- depolarisation
- membrane potential is more positive then resting potential
- Na+ influx only if the stimuli is strong enough to meet the threshold limit
repolarisation
- membrane potential returns to resting potential
- K+ moves out
hyperpolarisation
- membrane potential is more negative than resting potential
- excess K+ moves out
What is the purpose of hyperpolarisation? What is hyperpolarisation also known as?
hyperpolarisation
- allows recovery
- stops new action potentiel from being generated as the previous one is still present
also known as the refractory period
How does an action potential affect cardiac cell? How do they change in membrane potential?
stimuli causes depolarisation
depolarisation causes sodium channels to open
- sodium influx, membrane potential rises = more positive
repolarisation begins
- potassium channels open
- calcium channels are activated = causes a plateau as membrane potential is not able to drop as much, no overall change in membrane potential
repolarisation
- calcium channels close
- potassium channels are open = potassium ions move out of the cell
resting state
- most sodium and potassium channels are closed
controls rhythm and synchronicity of the contractions of the heart
What is the difference between neuronal and cardiac depolarisation?
membrane potential?
resting potential?
shape of action potential?
speed/duration of action potential?
neuronal
- resting potential is -70 mV
- has rapid depolarisation and repolarisation = angled peak
- action potential last 5 milliseconds
- threshold for sodium channels to open is -55mV
cardiac
- resting potential is -90 mV = more permeable to K+
- threshold for sodium channels to open is -40 mV
- has rapid depolarisation, plateau then rapid depolarisation
= due to calcium influx causing no overall change in membrane potential
- action potential lasts 300 milliseconds
What is the cytoskeleton? Why is it needed?
the cytoskeleton is the skeleton of the cell
is needed to keep the cells shape and modify it in response to environmental cues/changes
is dynamic
What are the different components of the cytoskeleton? What is their function?
microtubules
intermediate filaments
actin filaments
= microfilaments
needed for
- shaping of the cell
- intracellular movement of organelles
- cell movement
What is the structure of microfilaments/actin? What are they made up of?
are twisted chains of monomers
- monomers of actin
are the thinnest of the cytoskeleton filaments
are polarised
G-actin = are the actin monomers
F -actin = filamentous form formed by G-actin (monomers) joining up
What are the different forms of actin?
there are three isoforms of actin
they are polarised
- have a positive end where addition of monomers is favoured and a negative end where addition is not favoured
alpha actin
- found in muscle cells
beta actin
- found in non-muscle cells
gamma actin
- found in non-muscle cells
How does actin polymerise? What does the length of filament depend on?
G actin join up to form F-actin
- G-actin monomers add on at the positive end of the monomers (growth end)
length of the filament depends on concentration of G-actin and presence of actin binding proteins
What are ABPs? What are the different types?
ABPs are actin binding proteins
profilin
- facilitates actin polymerisation
thymosin beta 4
- prevents the addition of actin monomers to F-actin
What are the 4 different actions of ABPs?
Actin Bundling Proteins
- keep F-actin in parallel bundles
Cross-linking proteins
- maintain F-actin in a gel-like meshwork
F-actin severing proteins
- break F-actin into smaller filaments = controls filament length
Motor proteins (Myosin) - transport of vesicles and/or organelles through actin filaments
What are the different functions of actin filaments?
skeletal muscle?
non-muscle cells?
cytokinesis?
cell migration?
skeletal muscle
- arranged in a para-crystalline array integrated with different ABPs
- interaction with myosin motors allow muscle contraction
non-muscle cells
- cell cortex : form a thin sheath beneath the plasma membrane
- associated with myosin form a purse string ring result in cleavage of mitotic cells
cytokinesis
- involvement of an actin-myosin ring = form contractile ring, contraction splits ring into two
cell migration
- the cell pushes out protrusions at its front (lamellipodia and filopodia)
= actin polymerisation
What are the intermediate filaments?
toughest of the cytoskeletal filaments
- resistance to destruction
rope-like structure with many long strands twisted together and made up of different subunits
What are intermediate filaments made up of?
rope-like structure with many long strands twisted together and made up of different subunits
Each unit is made of:
- N-terminal globular head
- C-terminal globular tail
- Central elongated rod-like domain
units form stable dimers
ervery 2 dimers form a tetramear
tetramers bind to each other and twist to constitute a rope-like filament
What are the different types of intermediate filament?What is the function of intermediate filaments?
cytoplasmic and nuclear filaments
cytoplasm
- tensile strength: this enable the cells to withstand mechanical stress (stretch)
structural support by:
- creating a deformable 3D structural framework
- reinforcing cell shape and fix organelle localisation
nucleus
- form mesh rather than “rope-like” structure
- line in the inner face of the nuclear envelope to:
= strengthen it
= provide attachment sites for chromatin
What are microtubules?
Hollow tubes made up from the protein tubulin
Each filament is polarized (i.e. has direction – head/tail or +/-)
It is a dynamic structure
- can assemble and disassemble in response to cell needs
How do microtubules polymerise?
assembly starts at the microtubule organism centre
= MTOC
centrosome in the perinuclear region is the MTOC of most cells
MTOC contains gamma tubular ring that initiates microtubule growth
heterodimers of alpha and beta tubulin constitute the microtubule
- it is a polarized growth (i.e. there is an end that grows faster (+end) than the other (- end).
What are the different forms of tubulin?
alpha tubulin
beta tubulin
gamma tubulin - initiates polymerisation
What are the functions of microtubules?
intracellular transport
- act like railway tracks on which molecular motors run
different motors for different cargoes
- directionality of filaments is vital (each motor only moves in one direction)
organises position of organelles
- therefore provides polarisation of cells
- directionality of filaments is vital
motor proteins = use ATP
- dynein = only moves towards the negative/minus end
- kinesin = only moves towards the positive/plus end
