Lecture 5: Principles of active an passive membrane transport

Question 1

Downside to compartmentalization of the cell

Answer 1

While membranes provide the requisite barrier function to allow separation of constituents, proteins and other molecules must traverse this barrier

Question 2

Relative Permeability of a synthetic bilayer

Answer 2

Question 3

Which ions are highly concentrated in the cytosol?

Answer 3

K+

Question 4

Which ions are highly concentrated outside the cell?

Answer 4

Na+, Mg2+, Ca2+, Cl-

Question 5

What are the two types of membrane transport proteins?

Answer 5

• Transport proteins (Transporters)
• Channel proteins (Channels)

Question 6

Transporters

Answer 6

• Permeases, carriers, or pumps
• Active or passive transport

Question 7

Channels

Answer 7

Always passive transport

Question 8

Passive transport or facilitated diffusion

Answer 8

• Movement of a solute down a concentration or electrochemical gradient
• All channel proteins and many carrier proteins use this method of transport

Question 9

Active transport

Answer 9

• Use Transporters (pumps) to move solutes up a concentration of electrochemical gradient
• Coupled to an energy source

Question 10

What’s the difference between a concentration gradient and an electrochemical gradient?

Answer 10

An electrochemical gradient factors in membrane potential, not just concentrations

Question 11

What are three types of active transport pumps?

Answer 11

Question 12

Name two types of coupled transporters

Answer 12

• antiport-coupled solutes move in opposing directions
• symport- coupled solutes move to same side
• uniport- not coupled, solute moves in one direction

Question 13

Sodium driven glucose uptake is an example of ___

Answer 13

an ion-driven symporter

Question 14

SGLT is an___

Answer 14

Na-glucose co-transporter in epithelial cells of the proximal tubule of the kidney. SGLT inhibitors are new drug for diabetes.

Question 15

Neurotransmitter Transporters

Answer 15

• 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

Question 16

What are the three types of ATP-driven pumps?

Answer 16

P-type, V-type, and ABC transporters

Question 17

P-Type pumps

Answer 17

– Phosphorylate themselves during their reaction cycle

Question 18

F-type (V-type) pumps

Answer 18

– ATP synthetase-uses proton gradient to make ATP

– V-type-use ATP to pump protons

Question 19

ABC transporters (ATP Binding Cassette)

Answer 19

– Pump small molecules rather than ions

– Largest family of membrane transport proteins

Question 20

What type of pump is the sarcoplasmic reticulum calcium pump?

Answer 20

A P-type pump

• Moves calcium from the cytoplasm back into the SR following muscle contraction

Question 21

Na+-K+-ATPase

Answer 21

• Plasma membrane protein that exchanges sodium (3-going out) for potassium (2- coming in)
• ATP driven antiporter
• Electrogenic
• P-type ATPase
• Responsible for generation the large Na+ gradient across the plasma membrane

Question 22

Describe the Na-K pumping cycle

Answer 22

Question 23

What’s the difference between ABC transporters in bacterial cells vs eukaryotic cells

Answer 23

Bacteria-solute pumped into cytosol

Eukaryotic-solute pumped out of cells

Question 24

Channel proteins

Answer 24

• Form hydrophilic pores across membranes
• Most plasma membrane channels in animal transport inorganic ions, hence they are called “ion channels”

Question 25

Ion Channels

Answer 25

• 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

Question 26

Types of gated ion channels

Answer 26

• Voltage-gated
• mechanically gated
• ligand-gated

Question 27

Voltage-gated ion channels

Answer 27

open in response to a change in voltage across the membrane

Question 28

Mechanically-gated ion channels

Answer 28

open/close in response to mechanical stress

Question 29

Ligand-gated ion channels

Answer 29

open/close in response to a ligand binding

Question 30

Membrane potential

Answer 30

• 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

Question 31

Generating a membrane potential

Answer 31

• 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+)

Question 32

K+ Channel Selectivity Filter

Answer 32

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”.

Question 33

How do mechanosensitive filters work?

Answer 33

Pressure from the membrane, like with swelling from osmosis, can trigger the gate to open or close.