Passive Transport

Question 1

What is a pure phospholipid bilayer only permeable to?

Answer 1

1. H20
2. Small hydrophobic molecules
3. Small uncharged molecules

Question 2

What are the 2 forms of passive transport?

Answer 2

1. Simple diffusion

2. Facilitated diffusion

Question 3

What characterises passive transport?

Answer 3

1. No metabolic energy required

2. Net movement is awn concentration gradient

Question 4

What molecules can cross the bilayer via simple diffusion?

Answer 4

Small molecules (gases, hydrophobic molecules such as O2, CO2, NO, urea)

Question 5

What does a large difference in concentration cause?

Answer 5

A faster rate of diffusion

Question 6

What does facilitation diffusion involve?

Answer 6

• Integral membrane proteins that help molecules cross bilayer
• Proteins are specific to molecules being transported

Question 7

What does a lower Km value result in for facilitated diffusion?

Answer 7

Higher affinity of transporter for molecule

Question 8

What are the 4 examples of facilitated diffusion?

Answer 8

1. Ionophore
2. Ion channels
3. Glucose transport
4. Aquaporins

Question 9

What is job of ionophores?

Answer 9

Tranport ions across membranes

Question 10

What are ionophores produced by?

Answer 10

Bacteria as antibiotics to discharge ion gradients of target cells

Question 11

What are the 2 types of ionophores?

Answer 11

1. Carriers

2. Channel-forming

Question 12

What are carrier ionophores?

Answer 12

• Hydrophobic molecules that carry ion in their core

- Shield it from hydrophobic membrane environment

Question 13

What are channel-forming ionophores?

Answer 13

• Membrane-spanning hydrophobic proteins that form a hydrophilic channel
• Allows ions to pass freely through cell membrane

Question 14

What is Valinomycin an example of?

Answer 14

Carrier ionophore

Question 15

What is Gramicidin A an example of?

Answer 15

Channel-forming ionophore

Question 16

Describe Valinomycin

Answer 16

• Macrocyclic molecule
• Specific to K+ ions
• Effective as antibiotic

Question 17

How does Valinomycin transport K+?

Answer 17

1. K+ binds to interior

2. It takes K+ from one side of membrane to other down concentration gradient

Question 18

How is Valinomycin effective as antibiotic?

Answer 18

1. Differential K and Na gradient on either side of membrane
2. Destroying electrochemical gradient by allowing free movement of K+ ions across membrane –> KILLS CELL

Question 19

Describe Gramicidin A

Answer 19

• Hydrophilic exterior and hydrophilic pore down middle which spans membrane
• Made up of alternating L- and D- amino acids (highly unusual as most proteins made exclusively of L- amino acids)

Question 20

How is Gramicidin A effective as antibiotic?

Answer 20

Allows Na+ and K+ to move readily through membrane and destroy electrochemical gradient ]

1st antibiotic used clinically (1939)

Question 21

What is job of ion channels?

Answer 21

Allows rapid and gated passage of anions (-) and cations (+) across membrane

Question 22

Describe meaning of ‘gated’ channels

Answer 22

Ion channels normally closed by opened by specific stimulus

1. Voltage-gated channe;s
2. Ca2+ activated channels
3. Non-selective cation channels (TRP)

Question 23

Describe voltage-gated channels

Answer 23

Change in voltage leads to passage membrane (Ca2+, K+, Na+ channels)

Question 24

Describe Ca2+ activated channels

Answer 24

Ca2+ activated K+ channels

Question 25

Describe non-selective cation channels

Answer 25

Activated by diverse stimuli (e.g. mechanical) and important for detecting taste (marmite) and touch sensations

Question 26

What are ion channels important for?

Answer 26

1. Maintaining osmotic balance 2. Signal transduction 3. Nerve impulses

Question 27

How is glucose transported into and out of cell?

Answer 27

Glucose transporter (GLUT1) moves glucose across membrane Can move glucose either way (depends on concentrations)

Question 28

Describe GLUT1

Answer 28

- Integral membrane protein - 12 transmembrane alpha-helices - Central pore through which glucose can move

Question 29

Describe transport of glucose into cell

Answer 29

1. Glucose binds to transporter 2. Induces conformational change in transporter 3. Glucose molecule moved into cytosol by diffusion 4. Transporter resets

Question 30

What happens to glucose inside cytosol?

Answer 30

It is phosphorylated by hexokinase --> converted into glucose-6-phosphate (G-6-P)

Question 31

How is glucose stopped from moving back out of cell?

Answer 31

G-6-P is not a substrate for glucose transporter

Question 32

What is glucose transport specific for?

Answer 32

For D-isomer of glucose Has higher affinity (lower Km) for glucose than other sugars

Question 33

What are aquaporins?

Answer 33

Water channel proteins required for bulk flow of H20 across cell membranes Rapid transfer of water down concentration gradient

Question 34

Describe aquaporins

Answer 34

- Tetramer with 4 pores through which H20 can pass | - Identical 28 kDa subunits (each subunit containing 6 transmembrane alpha-helices)

Question 35

Where are aquaporins abundant?

Answer 35

Abundant in erythrocytes and kidney cells