membrane Ultra Structure And Function Flashcards
what is a cell
A the fundamental functional unit of a tissue
within that it has cell specific functions and can grow and divide
what does the plasma membrane do
compartmentalise the cell
keeps stuff in or out
Selectively permeable
what do micrtotubules do
give structure to the cell
what do membrane vesicles do
intra cellular transport
endo/exocytosis
what components make up the phospholipid bilayer
hydrophobic fatty acid tails
hydrophilic head
integral proteins
peripheral proteins
cholesterol
sugar side chain
describe fatty acid tails
non polar
Hydrophobic
saturated and non saturated bonds
describe phospholipid head
polar
hydrophilic
what are the 3 possible phospholipids
serine (phosphatidyl-serine)
choline (phosphatidyl-choline)
inositol (phosphatidyl-inositol)
what modifies the fluidity of the bilayer
cholesterol and temperature
what is the membrane permeable to
water (aquaporins)
gases
small uncharged polar molecules eg urea and etanol
what is the membrane impermeable to
ions
charged polar molecules eg ATP
large uncharged polar molecules eg glucose
what are 6 methods of membrane transport
simple diffusion
facilitated diffusion
primary active transport
secondary active transport
ion channels
pino/phago cytosis
what undergoes simple diffusion (5)
blood gases
water
Urea
fatty acids
ketone bodies
what undergoes facilitated diffusion
glucose
GLUT family
what undergoes primary active transport
Ions
water soluble vitamins
energy direct froM ATP
what undergoes secondary active transport
glucose
symporters (Na+ + X) (anions)
energy from ion gradient
co transport
what goes through ion channels
there are lots of types
voltage gated ions.
“leak” channels
what undergoes pino/phagocytosis
vesicles
it’s a way of moving larger molecules into and outside of the cell
why are membranes and membrane proteins needed
cell polarisation
compartmentalisation
ionic gradients - allows diffusion and membrane potential
these processes are tightly regulated and will be disrupted by disease
define membrane potential Em
potential difference across 4e cell membrane generated by differential ion concentrations of key ions
(K+, Na+, Ca2+, Cl-)
what does ion permeability depend on
channel numbers
channel gating
changing the ion permeability will change membrane potential which will change function
what happens if you have too much k+
hyperkalaemia
membrane potential is less negative lading to depolarisation
reaches threshold more easily
cell depolarisation more likely
heart decreases SAN firing
what happens if you have too less K+
hypokalaemia
membrane potential more negative leading to hyper polarisation
disrupts various K+ channels
abnormal heart rhythm so (arrhythmias)
what do epithelia do (in relation to membranes )
require polarisation of plasma membrane - apical vs basolateral surfaces
permits cell specific function - secretion. / absorption
strongly adhere to neighbours - tight junctions
what are the 6 types of signal transduction receptors
ion channels
membrane bound steroid receptors
neurotransmission
growth factors
nuclear steroid receptors
G protein coupled receptors
where are signal transduction receptors found
In the cytosol
except for nuclear steroid receptors which are in the nucleus
what do membrane bound steroid receptors do
they have an indirect effect on gene expression
what do nuclear steroid receptors do
they have a direct effect on gene expression
what are the 6 parts of G protein coupled receptors
Receptor - gives primary specificity
three G proteins- a;b,y
Enzyme to modulate second messenger
enzyme to terminate signal - phosphodiesterase
how does pH affect membranes
damage proteins
inhibits cell function
has a critical role for acid base homeostasis
how does temperature affect membranes (if it’s too cold)
too cold - proteins slow down membranes less fluid
why is intra cellular usually negative
because there are lots of ions there
typical resting membrane potential value
lies between -50 and -75 mV
what causes resting membrane potential
the differences in concentration gradient and electrochemical gradient across the cell membrane
where are greater concentrations of Sodium (Na+) and chloride (Cl–) ions
extracellulary
where are greater concentrations of potassium (K+) ions
intraceullularly
where are organic anions
these negatively charged molecules are most prevalent intracellularly
what does the Na/K ATP-ase pump do
plays an essential role in maintaining the sodium and potassium concentrations by actively transporting these ions against their concentration gradients.
3 sodium ions exit the cell in return for 2 potassium ions
overall is the intracellular environment more negative than extracellular
the intracellular environment is negatively charged compared to the extracellular environment, hence the resting potential of ~-50 to -75mV
when is the membrane depolarised
If the membrane potential becomes more positive than the resting potential
when is the membrane hyperpolarised
if it becomes more negative than the resting potential the membrane is said to be hyperpolarised
what does passage of a specific substance (charged) across the membrane depend on (3)
- electric charge
- molar mass
- polarity of the molecule
examples of uncharged substances
O2, CO2, urea, alcohol and glucose
what does movement of uncharged substances depend on
depends only on their concentration gradient because the membrane is permeable to these molecules
so they can move freely as their concentration gradients allow
why can’t charged substances diffuse freely through the cell membrane
due to its internal hydrophobic structure
how do charged substances diffuse through the cell membrane
they use specialised, water-filled pores known as ion channels.
are ion channels selective
yes
three factors that can induce the movement of the ions through ion channels
- concentration gradient
- electric gradient
- active transport
how does the concentration gradient affect movement of ions
ions cross the membrane from a compartment with a higher concentration to the compartment with a lower concentration
how does the electric gradient affect movement of ions
an electrical potential difference across the membrane defined as the electrical potential value inside the cell relative to the extracellular environment.
Positive ions will be attracted to negative electrical potential and repelled from positive electric potential, and vice versa
what is the normal movement of potassium ions regarding concentration gradient
The intracellular concentration of potassium > extracellular concentration (~130mmol/L vs ~4mmol/L).
so potassium ions will tend to exit the cell according to the concentration gradient
what is the normal movement of potassium ions regarding electric gradient
as positively charged K+ ions are released, the charge of the intracellular space becomes relatively negative.
so some K+ ions are attracted back towards the intracellular space, despite the concentration gradient leading them in the opposite direction.
overall movement of potassium ions
two “streams” containing K+ ions are created
- one that expels potassium as per its concentration gradient
- one which attracts potassium as per the increasing negative intracellular electrical environment
what happens at equilibrium potential
the rate at which ions leave by concentration gradient is equal to the rate at which ions enter via the electrochemical gradient
in a cell where only one type of ion can cross the membrane, the resting membrane potential will equal the equilibrium potential for that particular ion
what is the Nernst equation used for
to calculate the value of the equilibrium potential of a particular cell for a particular ion
overall direction of movement of potassium ions
extracellular
overall impact on resting potential by potassium ions
makes it more negative
overall direction of movement of sodium ions
intracellular
overall impact on resting potential by sodium ions
makes it more positive
overall direction of movement of chloride ions
intracellular
overall impact on resting potential by chloride ions
makes it more negative (small impact)
overall direction of movement of organic anions
cannot cross the membrane
overall impact on resting potential by organic anions
makes it more negative (small impact)
which ions have the greatest impact on membrane potential
sodium and potassium ions
because
the cell is most permeable to them
how are action potentials generated
- change in the permeability of the cell membrane to ions
- (via channels opening or closing) t
- then the membrane potential would be altered
- ap is generated
what would happen if there was nothing to maintain the ionic concentration gradients
the resting membrane would dissipate, and so therefore would the membrane potential.
what is Hyperkalaemia
a potassium (K+) level in the blood that is higher than normal.
what is normal blood potassium level
3.6 to 5.2 millimoles per litre (mmol/L).
what happens to membrane potential in hyperkalaemia
- gets more positive
- as the concentration gradient driving the movement of K+ ions out of the cell is reduced
- moves the resting membrane potential closer to the threshold for action potential generation
- the neurone enters into a state of heightened excitability
- so smaller deviations from this new resting potential are needed to promote action potential generation
- can significantly interfere with the physiological functions of nerve cells or muscles.
5 main functions of cell membranes
- Forming a continuous, highly selectively permeable barrier – both around cells and intracellular compartments.
- Allowing the control of an enclosed chemical environment – important to maintain ion gradients
- Communication – both with the extracellular and extra-organelle space
- Recognition – including recognition of signalling molecules, adhesion proteins and other host cells (very important in the immune system)
- Signal generation – in response to a stimulus creating a change in membrane potential.
examples of membrane proteins (4)
- Catalysts – enzymes.
- Transporters, pumps and ion channels.
- Receptors for hormones, local mediators and neurotransmitters.
- Energy transducers.
how much of a cell membrane is protein
usually 60%
what are integral proteins
deeply embedded within the bilayer
what are peripheral proteins
associated with the surface of the cell
structure of cholesterol
consists of a polar head, a planar steroid ring and a non-polar hydrocarbon tail.
why is cholesterol important in the membrane
helps to maintain cell membrane stability and fluidity at varying temperatures
how is cholesterol bound to neighbouring phospholipid molecules
via hydrogen bonds
what happens to cholesterol at low temperatures
- reduces their packing
- rate of movement is lowest
- a fluid phase is maintained
what happens to cholesterol at high temperatures
- helps to stop the formation of crystalline structures
- the rigid planar steroid ring prevents intrachain vibration
- making the membrane less fluid.
describe dry weight of membrane
40% lipid
60% protein
1-10% carbohydrate
how many carbons in fatty acid tails
normally consisting of between 14-24 carbons (but the most common carbon lengths are 16 and 18)
what does it mean if fatty acid tail contains a cis double bond
the chain is kinked – therefore reducing the tight packing of the membrane and so increasing its movement
why are phospholipid molcules amphipathic
they are both hydrophilic and hydrophobic.
they spontaneously form bilayers in the water with the head groups facing out and the tail groups facing in.
which bonds are there between the fatty acid tails of the phospholipid
van der Waal forces
which bonds are there between the hydrophilic groups and water
electrostatic and hydrogen bonds
