Membrane Transport and Membrane Potential

Question 1

How does the permeability of the lipid membrane differ for different types of molecules

Answer 1

Small hydrophobic molecules (O2, CO2, N2, steroid hormones) can pass through the bilayer no problem

Small uncharged polar molecules (H2O, urea, glycerol) will not typically cross unassisted (exception: aquaporins)

Large uncharged polar molecules (glucose, sucrose) need help crossing

Ions cannot pass through the membrane unassisted at all

Question 2

What are the different major types of transport proteins?

Answer 2

Channel mediated proteins and transporter mediated proteins

Question 3

What percentage of all membrane proteins are transport proteins?

Answer 3

15-30%

Question 4

How do passive transport and active transport differ?

Answer 4

Passive transport relies on diffusion of molecules along their concentration gradients

Active transport requires energy input in order to move a molecule against its gradient

Question 5

What factors contribute to the combinatorial gradient across the membrane?

Answer 5

The concentration gradient and the electrical gradient

Question 6

What are the different types of ion channels?

Answer 6

Voltage gated

Ligand gated (can be extracellular or intracellular)

Mechanically gated

Question 7

True or false: If the membrane potential is above a specified threshold, all channels will be open.

Answer 7

Fale

Channels are not simply all open or all closed, the percentage of open channels increases with membrane voltage

Question 8

What mechanism do transporter proteins utilize to transport solutes into/out of the cell?

Answer 8

Conformational changes allow the exchange of material between the sides of the lipid membrane

Question 9

What is the difference between primary and secondary active transport?

Answer 9

Primary: uses ATP directly

Secondary: uses energy stored in electrochemical gradients to power movement of molecules

Question 10

What are the different types of transport molecules?

Answer 10

Uniport

Symport

Antiport

Question 11

What is the most commonly used ion for coupled transport?

Answer 11

Sodium (Na+)

Question 12

How is glucose transported into the cell?

Answer 12

Glucose is actively transported into the cell against its own gradient by a transporter protein powered by the sodium electrochemical gradient

Question 13

What are the three classes of ATP-driven pumps and how do they differ?

Answer 13

P-type pumps: phosphorylated during ion transport

F-type/V-type pumps: F make ATP from the gradient, V hydrolyze ATP to pump H+

ABC transporters: pump molecules while hydrolyzing ATP

Question 14

How is the Na+ gradient established and maintained?

Answer 14

The Na+/K+ pump hydrolyzes ATP to move 3Na+ out of the cell in exchange for 2K+ into the cell

Question 15

How much of our energy is devoted to the Na+/K+ pump?

Answer 15

About 1/3

Question 16

Describe the pumping cycle of the Na+/K+ pump.

Answer 16

1) Na+ binds, ATP hydrolysis, phosphorylation of cytoplasmic residues
2) Conformational change, Na+ transfer
3) K+ binds on extracellular face, dephosphorylation
4) K+ transfer with dephosphorylation induced conformational change

Question 17

How do ABC transporters function?

Answer 17

ABC = ATP Binding Cassettes

These transporters require the hydrolysis of ATP in order to power a conformational change that allows non-ionic small molecules to pass through the membrane

Question 18

What are some examples of ABC transporters in disease?

Answer 18

Multi-drug resistance proteins (MDR) pump numberous drugs out of the cytosol. Increased expression is responsible for drug resistance in many cases

CFTR: a Cl- channel that is defective in cystic fibrosis

Flasmodium falciparum: agent of malaria that uses ABC transporter to pump out antimalarial drugs

Question 19

What is cystinuria?

Answer 19

A genetic defect in an amino acid transporter responsible for removing cystine from urine and the intestine

Individuals devlop cystine stones in their bladders and kidneys

Question 20

How does glucose get from the intestinal lumen into the extracellular fluid?

Answer 20

It passes through a sodium-glucose symport on the apical side of the epithelium (against its concentration gradient)

On the basal side, glucose passively diffuses out of the cell into the extracellular fluid

Question 21

True or False: Inside the cell, [Na+] is low and [K+] is high

Answer 21

True

Intracellular [Na+] = 5-15 mM

Intracellular [K+] = 140 mM

Question 22

True or false: extracellular [Na+] is low and [K+] is high

Answer 22

False.

extracellular [Na+] is high (145mM)

[K+] is low (5mM)

Question 23

What is the Nernst equation?

Answer 23

For a given ion, here potassium

Vk = (RT/zF)*ln(Ko/Ki)

RT/zF=61.5mV for K

Question 24

What is the average resting membrane potential for a cell?

Answer 24

-70 mV

Question 25

True or false: A positive voltage means that a +ion is leaving the cell or a - ion is entering the cell.

Answer 25

True

Positive voltage means positie current and positive current is defined as either a +ion leaging the cell of a -ion entering the cell

Question 26

If Vk >Vm, a positive ion will ____ the cell and/or a negative ion will _____ the cell

Answer 26

If Vk >Vm, a positive ion will enter the cell and/or a negative ion will leave the cell

Question 27

What are the Nernst potentials for calcium, sodium, chlorine and potassium?

Answer 27

Calcium = 125 mV

Sodium = 70 mV

Chlorine = -65 mV

Potassium = -90 mV