Passive and Active transport

Question 1

What do substance try to do in regards to movement across membranes?

Answer 1

Substances tend to equilibrate their concentrations across membranes
The difference in chemical potential depends on the ratio of concentration of a substance on both sides of the membrane

Question 2

What are the types of membrane transport?

Answer 2

Passive transport - simple diffusion and facilitated diffusion (requires a protein)

Active transport - this is ATP driven or ion-driven and requires a protein

Question 3

What are the differences in permeability of substances?

Answer 3

Highly permeable - gases and hydrophobic molecules (benzene)
Partially permeable - small, polar molecules (H2O and ethanol)
Not permeable - large polar molecules and charged molecules (glucose, ions and amino acids)

If the substance isn’t permeable they may require a protein to aid the transport

Question 4

What is passive-mediate transport carried out by?

Answer 4

Ionophores
Porins
Ion channels
Aquaporins
Transport proteins

Question 5

What are ionophores?

Answer 5

Small hydrophobic organic molecules that dissolve inlipidbilayers and increase their permeability to specific inorganic ions

Question 6

What are the two types of ionophores?

Answer 6

Carrier ionophore - increases the permeabilities of membranes by binding an ion, diffusing through the membrane, and releasing it on the other side
Their ionic complexes are soluble in nonpolar solvents

Channel-forming ionophore - form transmembrane channels or pores through which their selected ions can diffuse

Question 7

What are some examples of carrier ionophores?

Answer 7

Glucose transporters

Valinomycin - transports K+ ions by passive diffusion
Very efficient (104 K+ per second)
Formed by D- and L-amino acid residues, participating in peptide bonds and ester linkages

Question 8

What are porins?

Answer 8

Beta-barrel structures with a central aqueous channel

The size of the channel and the residues lining it decide what types of substances pass through (e.g. cation or anion)

Question 9

Give an example of a porin?

Answer 9

Maltoporin - in a bacterial membrane
Facilitates the diffusion of maltodextrins (degradation products of starch)
18 stranded antiparallel b-barrels
Diameter - 5 Å

Maltodextrins hydrophobic faces interact with the aromatic side chains, to create a ‘greasy slide’ effect to cross the membrane

Question 10

What are ion channels?

Answer 10

Integral proteins form channels in membranes carrying salts/ions
They span the membrane and generate water filled pores that allow water molecules to stabilize the ions as they pass through the membrane

Question 11

How do ion channels work? example K+ channel

Answer 11

They contain: outer vestibule, selectivity filter and inner vestibule

Outer vestibule
Entrance is lined with charged side chains, which attract and repelling ions
KcsA K+ channel: anionic side chains (repel anions, e.g. Cl-)
They use charge to allow selectivity

Selectivity filter
The pore between 6-10 Å and the selectivity filter is 3 Å = very constricted
The selectivity filter strips the hydration shell
It is the correct distance for K+ but not smaller Na+

Question 12

What is significant about ion channels?

Answer 12

Ion channels are gated

Mechanosensitive channels - open in response to deformations in the lipid bilayer
Ligand-gated channels- open in response to an extracellular chemical stimulus (e.g. a neurotransmitter)
Signal-gated channels- open as response to a signalling molecule (e.g. Ca2+)
Voltage-gated channels- open in response to a change in membrane potential (e.g. nerve impulses)

Question 13

Describe an action potential that opens a voltage-gated channel?

Answer 13

Depolarisation
Repolarisation
Hyperpolarisation
Resting Potential

Question 14

What else do ion channels have?

Answer 14

A second gate
Ion channels spontaneously close a few milliseconds after opening and do not reopen until after the membrane has regained its resting membrane potential

Question 15

What are aquaporin?

Answer 15

They mediate the transmembrane movement of water
They do NOT allow the passage of ions/protons
There are as many as 50
Used in kidneys and some glands

Question 16

What is an example of an aquaporin?

Answer 16

Aquaporin 1 (AQP1)
The a-helices form a central pore with a narrow point of 2.8 Å wide = Van der Waals diameter of a water molecule
The pore is lined with hydrophobic groups to expedite passage of water through the pore
This re-orientates the water molecules
Separation of water molecules from each other is facilitated by Arg and His residues
To avoid passage of protons (via proton jumping) two conserved asparagine’s reorient the water molecule

Question 17

What are the transport proteins between cells?

Answer 17

Gap Junctions - direct flow of ions from one neighbouring cell to another
6 connexin subunits = 1 connexon (hemichannel)
They are intercellular channels needed for communication

Question 18

What are another group of transport proteins?

Answer 18

Asymmetrically situated transmembrane proteins, that alternate between two conformational states in which the ligand binding sites are exposed, in turn, to opposite sides of the membrane
e.g. GLUT1 (glucose transporter)

Question 19

What terminology describes the operation of transport proteins carrying out processes?

Answer 19

Uniport - the movement of a single molecule at a time e.g. GLUT1

Symport - simultaneously transports two different molecules in the same direction

Antiport - simultaneously transports two different molecules in opposite directions

Question 20

What is active transport?

Answer 20

An endergonic process coupled to ATP hydrolysis, where a substance in low concentration is moved to an area of higher concentration on the other side of the membrane

Question 21

What are the types of ion pumps used in active transport?

Answer 21

P-type ATPases - undergo phosphorylation as they transport cations
F-type ATPases - proton transporting complexes in mitochondria and bacterial membranes
V-type ATPases - proton transporting complexes in plant vacuoles and acidic vesicles (lysosomes)
A-type ATPases - transports anions
ABC transporters -transport a variety of substances, including drug molecules

Question 22

Describe the Na2+/K+ ATPase pump?

Answer 22

It has 2 subunits a & b
It moves 3 Na+ out and K+ in (Antiport)
Coupled with ATP hydrolysis

It has two conformational states E1 and E2
E1 - the binding site has a higher affinity for Na+ and lower affinity for K+
E2 - the binding site has a lower affinity for Na+ and a higher affinity for K+

Question 23

Describe how the Na2+/K+ pump works?

Answer 23

Na2+ binds to E1-ATP
Formation of 'high energy' aspartyl phosphate intermediate (ADP release) = E1-Pi-3Na2+
Na2+ transported out
K+ binds to E2-Pi
Phosphate hydrolysis = E2-2K+
K+ transported in and ATP binds = E1-ATP

Question 24

Describe the Ca2+ ATPase pump?

Answer 24

It pumps 2Ca2+ out of the cytosol

Coupled with ATP hydrolysis

Question 25

What are ABC transporters?

Answer 25

They pump - ions, sugars, amino acids and other polar and non-polar substances
Built from 4 molecules: 2 two highly conserved cytoplasmic nucleotide-binding domains, and two transmembrane domains

They are responsible for drug resistance

Question 26

Give an example of an ABC transporter pump?

Answer 26

P-glycoprotein
It pumps a variety of amphiphilic substances (including drugs) out of the cell
Therefore also a multidrug resistance (MDR) transporter

When overexpressed it leads to the inability of anticancer drugs to kill cancer cells

Question 27

Give an example of an ABS transporter pump linked to a disease?

Answer 27

CFTR - cystic fibrosis transmembrane conductance regulator
In 70% of cases Phe 508 is deleted (homozygotes have many problems)
CFTR is defective in individuals with cystic fibrosis that should allow Cl- ions to flow out of the cell
Reduced Cl- export results in thickened mucus that the lungs cannot easily clear
It is therefore diffucult to clear foreign particles sucha as bacteria

Question 28

What is secondary active transport?

Answer 28

An ion gradient maintained by an ATPase or other free energy-capturing cellular process drives the transport of another substance
Therefore active transport can be driven by ion gradients

Question 29

Give an example of secondary active transport?

Answer 29

Lactose permease:
It transports lactose into a cell
This is driven by the cotransport of H+, whose gradient is maintained by oxidative metabolism in gram-negative bacteria