Downside to compartmentalization of the cell
While membranes provide the requisite barrier function to allow separation of constituents, proteins and other molecules must traverse this barrier
Relative Permeability of a synthetic bilayer
Which ions are highly concentrated in the cytosol?
Which ions are highly concentrated outside the cell?
Na+, Mg2+, Ca2+, Cl-
What are the two types of membrane transport proteins?
- Transport proteins (Transporters)
- Channel proteins (Channels)
- Permeases, carriers, or pumps
- Active or passive transport
Always passive transport
Passive transport or facilitated diffusion
- Movement of a solute down a concentration or electrochemical gradient
- All channel proteins and many carrier proteins use this method of transport
- Use Transporters (pumps) to move solutes up a concentration of electrochemical gradient
- Coupled to an energy source
What's the difference between a concentration gradient and an electrochemical gradient?
An electrochemical gradient factors in membrane potential, not just concentrations
What are three types of active transport pumps?
Name two types of coupled transporters
- antiport-coupled solutes move in opposing directions
- symport- coupled solutes move to same side
- uniport- not coupled, solute moves in one direction
Sodium driven glucose uptake is an example of ___
an ion-driven symporter
SGLT is an___
Na-glucose co-transporter in epithelial cells of the proximal tubule of the kidney. SGLT inhibitors are new drug for diabetes.
• Built from inverted repeats
• LeuT- a bacterial leucine tranporter
– First transporter of this class to be characterized structurally
• DAT-Dopamine Transporter
• SERT-Serotonin transporter
• NET-Norepinephrine Transporter
What are the three types of ATP-driven pumps?
P-type, V-type, and ABC transporters
– Phosphorylate themselves during their reaction cycle
F-type (V-type) pumps
– ATP synthetase-uses proton gradient to make ATP
– V-type-use ATP to pump protons
ABC transporters (ATP Binding Cassette)
– Pump small molecules rather than ions
– Largest family of membrane transport proteins
What type of pump is the sarcoplasmic reticulum calcium pump?
A P-type pump
• Moves calcium from the cytoplasm back into the SR following muscle contraction
• Plasma membrane protein that exchanges sodium (3-going out) for potassium (2- coming in)
• ATP driven antiporter
• P-type ATPase
• Responsible for generation the large Na+ gradient across the plasma membrane
Describe the Na-K pumping cycle
What's the difference between ABC transporters in bacterial cells vs eukaryotic cells
Bacteria-solute pumped into cytosol
Eukaryotic-solute pumped out of cells
• Form hydrophilic pores across membranes
• Most plasma membrane channels in animal transport inorganic ions, hence they are called “ion channels”
• Ion selectivity – Highly selective and narrow pores
• Gated – Not continuously open, rather open in response to specific stimuli
• Much faster than carrier proteins – ~108 ions/sec vs. ~1000
• Cannot be coupled to an energy source and must rely on electrochemical gradients
Types of gated ion channels
- mechanically gated
Voltage-gated ion channels
open in response to a change in voltage across the membrane
Mechanically-gated ion channels
open/close in response to mechanical stress
Ligand-gated ion channels
open/close in response to a ligand binding
- A membrane voltage generated by small differences in electrical charges across a membrane
- Most all cells have a membrane potential with negative charge inside the cell
- Neurons (nerve cell) in particular utilize membrane potential to communicate information
Generating a membrane potential
• The electrogenic Na+-K+-ATPase has a minor role in membrane potential
• In animals, the K+-leak channel is the ion channel primarily responsible for generating a membrane potential
– A plasma membrane ion channel selective for potassium (conducts K+ 10,000 better than Na+)
K+ Channel Selectivity Filter
The amino acids lining the pore of the K+ channel are arranged in such a way the energetically favorable interactions with a K+ ion devoid of if its hydration shell will “fit”.
How do mechanosensitive filters work?
Pressure from the membrane, like with swelling from osmosis, can trigger the gate to open or close.