1-cell memb. transp. 2- 11 oct
- function of memb: barrier or compartmentalisation
simple passive diffu.:
Through the membrane
Through a channel/pore: proteins
solute not in contact with hydropho. part of the memb.
eg: aquaporin
carrier proteins
binding of the solute to protein, shuffling or flipping of the protein
to shuffle that molecule across memb.
passive transp. mechanis.
down conc. gradient
couple the carrier of one solute
by the mov. of other solute.
Secondary active transport processes. ⚠
harnessing the energy of one conc. gradient
to mov. another solute against conc. gradient
symport (same direction)
antiport (opposite direction)
pumps: primary active transport
Secondary active transport processes. : ⚠repeating
harnessing the energy of one conc. gradient
to mov. another solute against conc. gradient
symport (same direction)
antiport (opposite direction)
input of energy required: comes from ATP
no conc. grad. needed
important in establishing conc. grad.
fundamental for electrophysiological properties of cell
2- pores and channels
aquaporin
specialised channel:
selective diffus. of w across memb.
accelerates the rate of transport
swelling and bursting of cell as a consequence of cell volume
benefit
rapidly accelerate the transfer of solute across the memb.
types:
non gated channels= pores
gated pores=channels
always open, always there
allowing the mov. of solutes
non directional=can go in both directions
net mov. of solutes based on conc. gradient
benefit of channels: we can regulate the opening and closing under different conditions
influence the movement of solutes
gating: mechanisms by which we can open the cap
or open up the ion channel
no of gating mechanism related to ion channels
voltage gated channels
electrical charge across membrane
net electrical negativity inside the cell
and positive charge outside of the cell
charged components of that ion channel
if change in the voltage
activation of ion channel
allows the movement of ion through those channels
ligand gated channels
req. binding of a ligand
often from the extra cellular side:
extra cellular surface of the protein
causes a conformational change
oppening up the ion channel
allowing the diffusion of the solutes through the membrane
mechanically gated channels
req. mechanical stimulus
mediated through the cytoskeleton or strech of the memb.
causing the physical opening of
that ion channel
and the diffusion of that solute across the memb.
they all can regulate when they open and close
under different types of stimulus
4-eg. very selective potassium channel
3- Channel proteins
highly specific
the rate of transport high
eg. aquaporin can increase
the transfere of w molec. by 10 fold
very selective for that molec.
has 4 identical transmembrane components
provide pore through membrane
allow selective diffusion across memb.
and also maintains the very rapid transfer of solute
negatively charged amino acid: attractive to cations
non selectively attracts cations to the opening pore
only selectivly allows the trasfere of potassium across the channel
10.000 time more permeable to K than Na
different sizes of Na+ ion and K+ ion
Na+ << K+
to transfer ion needs to shed (get rid of) its water molec.
K+ can do that because it enters into selectivity filer
K+ can shed its w channels and
binds with the oxygen molecules
because of the size and proximity of those
it is energetically favourable for K+
to be able to make those connections and transfer through the selectivity filter
Na+ is too small for chemical bonding process to occur
5-rate of transport
trans. rate for simple diffu. through the memb. or specific ion channels depends on 2 things:
passive process: because it is really governed by conc. gradie.
5- facilitated diffusion:
carrier mediated transport
specialised proteins embeded in the membrane
specific binding of the solute to the binding domain on the protein
cause conformational change on the structure of the protein
protein flipps around and allows the release of solute on the other side of the memb.
random flopping from configuration A to B
sometimes the solute binding triggers the conformational change
in some cases it is random fluctuation- back and forwards
end up with: net transfer of solute from region of high conc to region of low conc. net rate is always down its conc. grad. but solute might also go backwards
difu. down conc. gradient
facilitated by the carrier protein
diagram of carrier mediated transport
important feature
the carrier is only open to one side of the memb. at any one time
1-if it is open to extra cellular side, the solute can bind to the binding domain
2-get a conformational change
3-the closing of extra cellular components
4-both sides closed, then the sequential opening of the intra cellular side which allows the movement of solutes
it in never like a pore where it is open to either side of the memb. at any one time: so it is not the same as simple water filled pore
rate of transport
influence of conc. grad.
flux 🚩 conc. grad 🚩
but we reach a point that called saturation
=maximum transfer rate
=maximum flux for the solute
if we increase the conc. grad. we cant get greater rate of transport ax=cross the membrane
saturation is an important feature
relates to the:
no. of binding sites
the no. of carrier proteins that are present in relation to conc. gard.
another important feature:
how well those solutes can
actually bind to the carrier protein
similar to enzyme substrate reaction
the transport of the solute across the memb. not result in any change in that solute characteristics
just a physical transfer of one side of the memb. to another
quantify that by Km=affinity constant : concentration of the solute required to achieve half the maximum transfer rate
relates to the binding affinity
solutes that bind with
higher affinity to that carrier protein 🚩 lower Km 🏴 means better binding
more quickly to saturate
limitation of transport rate
by carrier protein: all of these factors
will slow down the rate of the transport of the molec.
and limit the transfer rate or the flux through carrier mediated transport
how many binding sites?
how strong is the binding affinity between the
solute and carrier protein
binding process takes certain amount of time
to bind and release of the other site
how quickly this protein can change its configuration
the sequence: binding to the solute- conformational change- release of the solute across the other side
final points:
read the slide:
Points about carriers (facilitated diffusion)
work bidirectional:
depending on the conc. grad.
get saturated at high conc.
transfere rate can be reduced by compatitive binding
bind molecules with similar structure
limits the rate of transport of particular solute
they are temperature dependent
can be blocked by some pharmacological agents and toxins
difference between transfer mediated by pores and those mediated by carrier proteins
Carriers/transporters are NOT water-filled pores/channels
6- sofar uniport process: uniporter
Transport a single solute
across the memb. down its conc. gradient
7-coupled transport: Co-transporters
Transfer of one solute depends
on the simultaneous transport of
a second solute
utilising the conc. grad. of one solute to pump or cotransport the other solute against its conc. gradient
symport:
both solutes transfere in the same direction
might be from extra cellular site to the intracellular site
antiport:
the transfer of one solute into the cell is coupled
to the transfer of another solute out of the cell
example of cotransport mechanism: process is referred to as
secondary active transport ⚠
sodium - Glucose transporter SGLT1
Na is one of the most utilised ions in the body
in terms of the co-transport process -
part of the reason:
we have very high extracellular conc. gradient for Na
but very low inside of the cell
so very strong conc. gradient and strong electrical gradient
combined electrochemical gradient is very favourable
for the diffusion of sodium ions into the cell
very strong potential energy we have there
utilise and harness that to transport other solutes to the cell against their concentration gradient
diagram
blue=sodium
electrochemical gradient for Na going into the cell
specific transporter protein
cotransport mechanism=symporter
binding of one solute
increases the affinity of other solute to bind to that molecule
by Na binding to the carrier protein:
increases the affinity for glucose binding to that carrier protein
both of those bind to the perspective binding sites
then we get the conformational change of those proteins
Na moving down conc. grad. into the cell
allows the pumping or transport of Glucose against its conc. grad to accumulate inside the cell
transport solutes in the same direction
other example
hydrogen ions based transported processes
very impo for maintaining the PH in cells and
different organelles within the cell as well
relies on the presence of conc. grad:
need to have that to trans. other solute against its conc. grad
an active transport
because we need energy of one conc. gradien to
drive the transfer of another solute against its conc. grad.
NOT establish a conc. grad. itself
inorder to do that we have the process of the active transport
Water filled pores and channels increase permeability
Pores and channels can be selective:
eg aquaporin for water.
JX = Flux of X across the membrane (moles/cm2.s)
Permeability of membrane to X: depend on the permeability of the memb. to that particular solute
Px: proportional relationship
[x] concentration grad. across the memb. for that particular solute: proportional relationship ([X]o - [X]i)
carriers are gated by 2 doors : never open at the same time