Passive mediated transport does not require assistance of other molecules to pass through the membrane.
Instead, there are 5 types of passive mediated transport protocols:
- Ion Channels
- Transport Proteins
Let’s begin with ionophores:
Ionophores are responsible for transporting ions across a membrane, something that isn’t very easy to do since the membrane contains hydrophobic regions. In other words, there is a high energy barrier that prevents ions from crossing.
Thus, some sort of system is needed to transport ions.
Two Types of Ionophores:
- Carrier ionophores
- Channel ionophores
How Carrier Ionophore Works:
The ion binds to a “carrier” on one side of the membrane and travels/diffuses through the membrane and releases the ion on the other side.
The free energy of the ion and equilibrium will decide how the ion transports (because it is passive transport so there will need to be some sort of “incentive” to travel to the other side)
Valinomycin – a cyclic peptide of D and L amino acids that have six carbonyl groups coordinate a potassium.
The geometry of coordination is specific for potassium and the exterior is hydrophobic.
- Span across the entire membrane allowing transport of ions
Example of a Channel Ionophore:
- Gramicidin A:
- Comprised of 15 amino acids (both L and D) that have a helical peptide structure that dimerizes to form a channel.
- This allows for potassium and sodium to travel through (depending on pore size)
- Key structural features: H-bonds, tryptophans, valines
- Beta barrels with a central cavity
- Selectivity depends on the size of the pore and the residues that line it.
Example of Porin:
- Maltoporin (a1->4 oligosacharides of glucose)
- Polar groups interact with the hydroxyls of glucose, while the hydrophobic groups interact with carbon rings of glucose.
Ion channels are needed to maintain osmotic balance. For example, cytosol has approximately 12 mM Na+ and 140 mM of K+, while the extracellular environment has 150 mM Na+ and 4 mM K+.
What this means is that the osmotic pressureneeds to be balanced at all times through ions moving back and forth.
Ion channels are often involved in signal transduction as well as neurotransmission.
- Comprized of a homotetramer
- Entrance is anionic to attract cations
- Has a selectivity coordination so that only K+ can travel through.
- K+ is coordinated by carbonyls
Ion Channels are Gated:
- Movement of ions across membranes is regulated by several methods to ensure appropriate ion concentration travels through.
Methods of Gating Channels:
- Mechanoselective: respon to local deformation in the lipid bilayer
- Ligand-gated = opens upon binding of extracellular messenger to a cell-surface receptor
- Signal-gated = opens upon binding of an intracellular messenger.
- voltage-gated = change in membrane potential causes gates to open/close
Potassium Channels in Nerve Cells are Voltage-gated
- Upon neuron stimulation, sodium channels open and sodium enters the cell
- Entrance of sodium induces potassium channels to open and potassium then leaves.
- Both channels quickly close
- Gate 1 = controls opening of K+ channels
- Gate 2 = controls closing of K+ channels
Nerve Action Potential
- Sodium channels open = deplolarization
- Potassium channels open = repolarization
- Sodim/Potassium pumps reestablish the resting potential
Aquaporins are found in our kidneys and salivary glands and function as water transporters (much faster than simple diffusion.
Aquaporins are specific for water, so hydronium ions (H3O+) cannot cross. It is contricted by size as well (2.8 A) with precise H-bond partners:
- Two helices and two asparagines (H-bond to water)
Transport proteins undergo conformational change in order to transport molecules.
So the transport protein can be conformationally “open” to the outside allowing for a molecule, such as glucose to bind.
It would then close as the glucose travels across the membrane and gets released.
Driven by electrochemical potentials
Types of Transport:
- Uniport: A molecule travels from one side of the membrane to the other.
- Symport: Two different molecules travel from one side of the membrane to the other.
- Antiport: Two molecules (on opposite sides of membrane) cross the membrane.