Thursday, January 8, 2008
Slide 1 - What is covered in the exam is what’s covered in the lectures.
- For the next lectures, there will be fill-ins.
- Tutorials will start the following Monday (not next Monday) – schedule is
posted on Blackboard.
Slide 2 Membrane transport of small molecules
Slide 3 - In the 1 lecture we looked at the membrane that surrounds the cell &
surrounds the organelles of the cell. Went through the features of this
membrane & 1 of the features of these membranes is that they isolate those
membranes from many other parts of the cell.
- Now we’re going to cover how small molecules are transported across these
membranes b/c these organelles & the cell itself need many of the molecules
that the membranes are impermeable to so there needs to be transport of small
molecules across these membranes in order for the cell to survive. So we’re
going to cover how small molecules are transported across the membranes.
Slide 5 - The lipid bilayer is not permeable to all molecules. So the lipid bilayer is
permeable to hydrophobic molecules such as O, carbon dioxide, N & benzene
& the movement of these molecules across the membranes will be by simple
diffusion across the membrane & these molecules will move across the
membranes from a high concentration to a low concentration & this is
referred to as going down the concentration gradient so hydrophobic
molecules will do this readily.
- Small uncharged polar molecules will also cross the membrane so the
membrane is permeable to these molecules such as water, urea & glycerol but
to a lesser degree than hydrophobic molecules.
- Can conclude that more hydrophobic or nonpolar molecules will diffuse
faster across the lipid bilayer, down its concentration gradient.
Slide 6 - The lipid bilayer is relatively impermeable to large uncharged polar
molecules like glucose & sucrose – there is a conundrum there right away as
the cell needs this glucose in order to survive – so there has to be an active
mechanism present to be able to transport this glucose across the membrane
since the membrane itself is relatively impermeable to glucose which is a
large uncharged polar molecule.
- Also ions which are charged molecules & the membrane is impermeable,
almost completely impermeable, to ions so the membrane requires proteins in
order to transport ions & large uncharged polar molecules like glucose across
Slide 7 - Membrane transport proteins are what transport these molecules across the
membrane. These are multipass transmembrane proteins – that means they
transverse the membrane multiple times.
- There’s different cell membranes & the different cell membranes such as
the membranes that will surround an organelle or the plasma membrane will
each have a different set of transport proteins which are selective for specific
molecules. So you’ll have transport proteins that can transport glucose, for
example, & then you’ll have other transport proteins that can transport ions,
such as Ca or Na ions across the membrane. Na transport proteins can’t
transport a glucose molecule.
Slide 8 - There are 2 different types of transport that can occur across the membrane
by transport proteins:
1) Passive transport – the transport of molecules down their concentration
gradient – this is a process that is also called facilitated diffusion so diffusion
will also occur down the concentration gradient. Transport proteins that partake in passive transport will transport molecules that are normally
impermeable in the plasma membrane & they’ll transport these molecules
down their concentration gradients also.
2) Active transport – the transport of molecules against the concentration
gradient – so it will transport the molecule from a low concentration to a high
concentration – since this is against the concentration gradient this active
transport requires an input of energy.
- Membranes have an electrical potential difference across them – this means
that there is an uneven distribution of charges on each side of the membrane.
Look at the plasma membrane: the outside of the plasma membrane is
positively charged relative to the inside of the plasma membrane. So the
inside is negatively charged & the outside is positively charged.
Slide 9 - So for a charged molecule, the driving force for the transport of a small
charged molecule will be a combination of both the concentration gradient &
the membrane potential & this adds up to produce the driving force which is
called the electrochemical gradient – so this is for a charged molecule. The
movement of a charged molecule is influenced by both its concentration
gradient & its membrane potential. For an uncharged molecule, it’s only the
concentration gradient that will influence its transport.
- Left diagram: This is a charged molecule with a high concentration outside
& a low concentration inside – there is no membrane potential here so then
the movement will only be influenced by the concentration gradient.
- Middle diagram: If we add a membrane potential which is positive outside
relative to inside there will be additive effect onto the concentration gradient
b/c the positive charges tend to be repelled by the positive charges on this
side of the membrane & so the positive charges tend to move towards
negative charge on the inside of membrane – that additive effect will be the
electrical gradient & the driving force of the 2 combined is stronger than in
this situation where only the concentration gradient will be driving the
movement of these molecules. In this situation, both the membrane potential
& concentration gradient are driving the molecule in the same direction,
having an additive effect.
- Right diagram: in this example, the charges are reversed so you have a
positive charge on the inside of the membrane & a negative charge on the
outside of membrane – so the positive charges inside will tend to repel the
positive charges as they are going in & the negative charges will tend to
attract them & will want to maintain them on this side – so in this case, the
concentration gradient & membrane potential are working against each other
to present this electrochemical gradient.
- For a charged molecule, it’s the both of these that will contribute to this
electrochemical gradient which is the driving force that will influence the
movement of these molecules across the membrane.
Slide 10 - Passive transport: will transport a charged molecule down the
electrochemical gradient. Remember for an uncharged molecule, it will
transport it down its concentration gradient.
- And for a charged molecule, active transport will be against the
electrochemical gradient & again this requires energy.
Slide 11 - Active transport is against concentration gradient & it requires energy since
you’re driving a molecule against the gradient, you need energy. This energy
can come from coupled transport – this means that the energy of driving 1
molecule down its concentration gradient is used to move a 2 nd molecule
against its gradient so this is energetically favourable so you would be
moving 1 molecule down its electrochemical gradient in this case & the
energy that this would release would be used to move the 2 molecule against its electrochemical gradient – these would be 2 different molecules
butndne transport protein would be carrying out both of these actions.
- 2 type of transport is driven by ATP: hydrolysis of ATP is used to produce
energy to drive the movement of molecules against their electrochemical
- Some bacteria can harness light energy & use the energy from light in order
to move molecules against their concentration gradient also – not going to go
through any detailed examples of this kind, only focus on the previous 2.
Slide 12 - What is passive transport? – Because it’s “electrochemical gradient”, we
can assume it is a charged molecule. If it was an uncharged molecule, could
also say it was down the concentration gradient of that solute.
- What is active transport? – Again, if it was uncharged molecule, it would be
against the concentration gradient of that solute.
Slide 13 - This animation shows the movement of molecules by simple diffusion,
different types of passive transport, channel-mediated, carrier-mediated
passive transport & also active transport against the electrochemical gradient
of a molecule.
- Simple diffusion: there is no animation – just the movement of molecules
from a high concentration to a low concentration for an uncharged molecule
& down the electrochemical gradient for a charged molecule.
- Channel-mediated passive transport – this is down the electrochemical
gradient – the yellow molecules are moving down their electrochemical
gradient to eventually lead to an equal concentration if this is an uncharged
- Carrier-mediated passive transport – this is actually a protein that is
involved in mediating passive transport across the membrane so again, it goes
down the electrochemical gradient so you can see that the molecule is
actually transporting the molecule across the membrane but this does not
require any source of energy since the molecules are moving down the
electrochemical gradient. Can see in this case something that is rather
important is that the protein, once it binds to the molecule undergoes a
- Active transport – carried out by transport proteins is against the
electrochemical gradient so we can see that this one would be at a low
concentration at the inside of the membrane & this carrier would be actively
transporting out using the hydrolysis of ATP as a source of energy to move
this molecule against its electrochemical gradient.
Slide 15 - They bind to a specific molecule. Ex: A glucose transporter will bind to
glucose but not to ions. They undergo a conformational change once they
bind to their molecule of interest & then what happens they will transport this
molecule across the lipid bilayer.
- A uniporter means that it is transporting one molecule at a time.
- The direction of transport is reversible – so depending on the direction of
the electrochemical gradient, if the electrochemical gradient was going down,
it would transport this molecule in this direction (as indicated in the diagram).
If the electrochemical gradient was going up, it could reverse its direction &
move this molecule down its concentration gradient – depending on the
direction of the electrochemical/concentration gradient, the direction of
transport by uniporters can be reversed.
- Animation: In the process known as facilitated diffusion, a special carrier
protein with a central channel acts as a selective corridor which helps
molecules move across the membrane. These special carrier molecules that form the protein channel bind only to a specific molecule, such as a particular
sugar or amino acid. Once the molecule binds to the carrier protein, this
protein helps or facilitates the diffusion process by changing shape & moving
the molecule down its concentration gradient, through the membrane into the
cell where it is released. Facilitated diffusion is similar to simple diffusion in
that both involve movement of molecules down their concentration gradient
& this movement is carried out without any input of energy. However, in
facilitated diffusion, the movement of molecules will only take place if it is
facilitated or helped by a special protein carrier in the membrane. Facilitated
diffusion can occur in either direction depending on the concentration
gradient. If there is a higher concentration of the particular molecule inside
the cell, the same carrier protein would then transport the molecules out of
- You can see how the uniporters can transport the molecules in either
direction but again, the important part to recognize is that the molecules will
only move across if the proteins are there to transport them across. In that
way, these transporters can be thought of as enzymes – the reaction that
they’re carrying out is the transport of the molecule across the membrane &
their substrate is that small molecule
Slide 18 - Like enzymes, they have characteristic Michalis-Menten kinetics where the
molecule or uniporter will have a maximum velocity where the carrier is
saturated & this is the maximum speed at which it can carry that small
molecule across the membrane. At this concentration, that carrier is saturated.
- It also has a Michalis constant which is half of v so if we look at the
maximum velocity on this curve, half of that maximum velocity is the solute
concentration – this Michalis constan