208C Membrane Transports

Question 1

Which solutes need transport in eukaryotes?

Answer 1

• 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

Question 2

What are the different types of transport systems?

Answer 2

PASSIVE:

1) Simple diffusion
2) Facilitated transports

ACTIVE:

1) Primary
2) Secondary

Question 3

What is simple diffusion?

Answer 3

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.

Question 4

Which types of transport are concentration dependent?

Answer 4

Passive transport: simple diffusion and facilitated diffusion. B/c solutes migrate from a region of high concentration to low concentration (down the concentration gradient)

Question 5

What is facilitated diffusion?

Answer 5

facilitated diffusion uses a pore to transport solutes. The transport is passive and transfer is concentration dependent (down its electrochemical gradient)

Question 6

How does active transport differ from passive transport?

Answer 6

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

Question 7

What is primary active transport?

Answer 7

free energy provided by the hydrolysis of ATP to ADP helps drive solute against its electrochemical gradient

Question 8

What is secondary active transport?

Answer 8

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

Question 9

Which system of transport does not distinguish between passive and active transport?

Answer 9

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

Question 10

How can symport/antiport be an example of secondary active transport?

Answer 10

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

Question 11

How is a membrane potential generated? What force does it favor and which does it oppose?

Answer 11

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

Question 12

Movement across a membrane depends on what?

Answer 12

depends upon the concentration (chemical gradient) but also on the electrical (membrane potential) gradient aka electrochemical gradient

Question 13

What is meant by electreoneutral?

Answer 13

when there is not net change during transport across the membrane

Question 14

What is electrogenic?

Answer 14

when there is a change in the charge during transport across the membrane aka generates an electrical potential

Question 15

Why is some passive transport non mediated?

Answer 15

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.

Question 16

Can cells regulate passive transport?

Answer 16

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.

Question 17

What is meant by mediated passive transport?

Answer 17

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.

Question 18

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?

Answer 18

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.

Question 19

What are the two types of mediators?

Answer 19

1) pore-facilitated

2) carrier facilitated

Question 20

What is the difference between pore facilitated and carrier facilitated mediators?

Answer 20

Pore-facilitated = is faster and less specific than carrier facilitated

Pore-facilitated = is relatively temperature-INDEPENDENT, while carrier-facilitated is HIGHLY temperature-DEPENDENT.

Question 21

Why is carrier-facilitated mediators temperature dependent?

Answer 21

b/c as the temperature increases the membranes become more fluid allow the mediator through

Question 22

What are two examples of pore-facilitated proteins/channel formers?

Answer 22

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.

Question 23

What is the structure of Gramicidin A?

Answer 23

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

Question 24

Why is Gramicidin A not an alpha helix although it has a helical structure?

Answer 24

b/c an alpha helix does not have a pore down its center and Gramicidin A is a pore

Question 25

What are examples of carrier facilitated passive transport mediators?

Answer 25

1) Ionophores: e.g., monensin and valinomycin

Question 26

How do ionophores carry solutes across a membrane?

Answer 26

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

Question 27

Apart from behaving as a carrier facilitator, what other role does monensin play? What is the structure?

Answer 27

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

Question 28

Why is active transport not a spontaneous reaction and why does it require energy? Where does it get energy from?

Answer 28

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

Question 29

The sodium potassium pump is an example of what type of transport? How does this pump work?

Answer 29

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.

Question 30

Explain membrane potential in terms of sodium/potassium concentration

Answer 30

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)

Question 31

In the case of bacteriorhodopsin, how is energy provided to pump protons from inside of the cell to the outside?

Answer 31

the energy is provided by light

Question 32

In electron transport/redox reactions how is energy provided to pump electrons?

Answer 32

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.

Question 33

What is the F0F1 ATP synthases pump?

Answer 33

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.

Question 34

How does lactose permease in e.coli move across the membrane?

Answer 34

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.

Question 35

Transporters just like enzymes undergo what when they bind the ligand?

Answer 35

they undergo a changes in conformation