208C Membrane Transports Flashcards
Which solutes need transport in eukaryotes?
- Ions
- Glucose
- Amino Acids
Solutes such as ions need a transport system since these charged molecules have to cross the non-polar hydrophobic core of the membrane. Sugars and amino acids and waste products
What are the different types of transport systems?
PASSIVE:
1) Simple diffusion
2) Facilitated transports
ACTIVE:
1) Primary
2) Secondary
What is simple diffusion?
It is a passive transport system that allows non-polar compounds ONLY to be transported down a concentration gradient
Non-polar compounds can cross the hydrophobic core.
Which types of transport are concentration dependent?
Passive transport: simple diffusion and facilitated diffusion. B/c solutes migrate from a region of high concentration to low concentration (down the concentration gradient)
What is facilitated diffusion?
facilitated diffusion uses a pore to transport solutes. The transport is passive and transfer is concentration dependent (down its electrochemical gradient)
How does active transport differ from passive transport?
active transport requires energy b/c transport is against a concentration gradient whereas passive transport does not require energy b/c its down its concentration gradient
What is primary active transport?
free energy provided by the hydrolysis of ATP to ADP helps drive solute against its electrochemical gradient
What is secondary active transport?
movement again is against its gradient but the movement is coupled with the transport of an ion that is moving with the concentration gradient. In this transport method, the spontaneous process (ion transport down its concentration gradient) provides the energy
Which system of transport does not distinguish between passive and active transport?
Uniport = a single solute passes through a pore
CONTRANSPORT EXAMPLE:
Symport: more than 1 solute moving across but the movement of both solutes is in the same direction
Antimport= both solutes move in opposite direction
How can symport/antiport be an example of secondary active transport?
b/c there is movement of 1 solute w/the concentration gradient drives the movement of another solute in the same direction, even if the latter is against its concentration gradient.
Antiport can also be an example except that in this case one of the solutes is moving in the opposite direction
How is a membrane potential generated? What force does it favor and which does it oppose?
when ions of opposite charge are separated by a permeable membrane. This membrane potential produces a force that opposes ion movements that increase the membrane potential, while it favors ion movement that reduce the membrane potential
Movement across a membrane depends on what?
depends upon the concentration (chemical gradient) but also on the electrical (membrane potential) gradient aka electrochemical gradient
What is meant by electreoneutral?
when there is not net change during transport across the membrane
What is electrogenic?
when there is a change in the charge during transport across the membrane aka generates an electrical potential
Why is some passive transport non mediated?
b/c the concentration is the driving force with the solute moving down the gradient. There is no carrier needed. This works provided the solute is non-polar like Co2, O2 and other lipophilic molecules (such as some drugs) can pass right through the membrane.
Can cells regulate passive transport?
cells cannot regulate this type of transport’ it is non-specific and the transport is not saturable
NOTE: that lipophilic molecules in an aqueous solvent result in caging of water, i.e., result in an increase in entropy.
What is meant by mediated passive transport?
this means diffusion is facilitated and a transporter/carrier is required to transport solute although there is still no requirement for an external source of energy. THE DRIVING FORCE IS STILL THE CONCENTRATION GRADIENT
in this type of transport, the solute due to its chemical characteristics cannot easily pass through the membrane. For example, ions, amino acids, and sugars which are hydrophilic in nature will not pass through the hydrophobic core of the membrane. They will require a transporter such as a pore forming protein to facilitate their transport.
How is the free energy requirement of a simple diffusion of a hydrophilic solute in the absence of a transporter? What does the transporter do to this free energy?
The free energy is high. The transporter protein lowers the that activation barrier by forming non-covalent bonds w/the dehydrated solute, thus replacing the hydrogen bonds that the solute would normally form w/water.
In addition, transporters are very specific. Kinetics of facilitated diffusion are similar to enzyme‐catalyzed reactions, including saturation of the transporter‐protein at high solute concentrations.
What are the two types of mediators?
1) pore-facilitated
2) carrier facilitated
What is the difference between pore facilitated and carrier facilitated mediators?
Pore-facilitated = is faster and less specific than carrier facilitated
Pore-facilitated = is relatively temperature-INDEPENDENT, while carrier-facilitated is HIGHLY temperature-DEPENDENT.
Why is carrier-facilitated mediators temperature dependent?
b/c as the temperature increases the membranes become more fluid allow the mediator through
What are two examples of pore-facilitated proteins/channel formers?
1) Porins (beta barrel proteins that re quite large and allow transport of amino acid into the cell
2) Gramicidin A, which is an antibiotic from bacillus brevis, it is helical in shape and can act as an ion channel.
What is the structure of Gramicidin A?
it is a 15mer peptide with both D and L amino acids. It has hydrophobic residues (especially trp) that face towards the outside. It forms a head to head dimer as the foundation of the helical pore and it transports K+ and Na+ but it is blocked by Ca++ b/c it is too big and blocks the pore
Why is Gramicidin A not an alpha helix although it has a helical structure?
b/c an alpha helix does not have a pore down its center and Gramicidin A is a pore
What are examples of carrier facilitated passive transport mediators?
1) Ionophores: e.g., monensin and valinomycin
How do ionophores carry solutes across a membrane?
Ionophores like monensin and valinomycin have polar residues in the core of the protein which helps chelae the ion. The outside of the carrier is hydrophobic which interacts favorably w/the lipid tails in the core of the membrane
Apart from behaving as a carrier facilitator, what other role does monensin play? What is the structure?
Monensin is also an antibiotic produced by a specific Streptomyces strain.
there are several polar oxygen atoms in the center of the carrier, while there are several methyl groups on the exterior. oxygen helps hold the ions (binds sodium Na+) in the center and the carbons in the methyl and methylene groups which point outward bind to the hydrophobic core of the membrane
Why is active transport not a spontaneous reaction and why does it require energy? Where does it get energy from?
b/c transport is agains the gradient.
In primary active transport, energy is provided by hydrolysis of ATP to ADP and inorganic phosphate to drive the non-spontaneous reaction.
In secondary active transport, transport of one solute (X in this case) is coupled with transport of a second solute (Y in this case) moving down its gradient; the Y movement is spontaneous and provides the free energy to drive the transport of solute X
The sodium potassium pump is an example of what type of transport? How does this pump work?
Example of primary active transport.
For every 3 NA+ pumped from the inside of the cell to the outside, there are 2 K+ pumped into the cell.
Explain membrane potential in terms of sodium/potassium concentration
Concentration of sodium inside the cell is (12nM) and outside the cell (145nM) so sodium moving out is movement against its concentration gradient.
The membrane potential ranges from -50 to -70 V. With charge separation, the outside of the membrane is + charged and the inside is negative charged. Thus the transport of the positively charged Na+ is also agains the electrical gradient. AKA electrochemical gradient is not favorable. Energy needs to be provided to and this energy comes from the hydrolysis of ATP in the case of the Na+ K+ ATPase (in animal systems)
In the case of bacteriorhodopsin, how is energy provided to pump protons from inside of the cell to the outside?
the energy is provided by light
In electron transport/redox reactions how is energy provided to pump electrons?
In the electron transport chain, the electrons pass from one complex to the next, in the process providing free energy to run the proton pump.
What is the F0F1 ATP synthases pump?
A pump that pumps protons and can work in a reversible manner. (ATP formation from H+ pump and ATP hydrolysis to pump H+)
Protons moving from low to high conc. flow against the proton gradient and energy is required in the form of ATP. However, in the reverse direction, protons are moving from high to low conc. This is a spontaneous process and the energy produced actually condenses ADP and inorganic phosphate to form ATP.
How does lactose permease in e.coli move across the membrane?
there is a proton gradient with a high conc. of protons outside the cell compared to the inside. So the protons move down their concentration gradient. In addition, the electrical potential favors the movement of postive charges into the negatively‐charged cell. Hence proton transport is spontaneous and thus can provide free energy to help transport the lactose against its conc. gradient.
Transporters just like enzymes undergo what when they bind the ligand?
they undergo a changes in conformation
