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Yr 1 - Membranes, Lipids, Signalling > Passive Transport > Flashcards

Passive Transport Flashcards

1
Q

What is a pure phospholipid bilayer only permeable to?

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

What are the 2 forms of passive transport?

A
  1. Simple diffusion

2. Facilitated diffusion

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3
Q

What characterises passive transport?

A
  1. No metabolic energy required

2. Net movement is awn concentration gradient

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4
Q

What molecules can cross the bilayer via simple diffusion?

A

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

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5
Q

What does a large difference in concentration cause?

A

A faster rate of diffusion

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6
Q

What does facilitation diffusion involve?

A
  • Integral membrane proteins that help molecules cross bilayer
  • Proteins are specific to molecules being transported
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7
Q

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

A

Higher affinity of transporter for molecule

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8
Q

What are the 4 examples of facilitated diffusion?

A
  1. Ionophore
  2. Ion channels
  3. Glucose transport
  4. Aquaporins
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9
Q

What is job of ionophores?

A

Tranport ions across membranes

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10
Q

What are ionophores produced by?

A

Bacteria as antibiotics to discharge ion gradients of target cells

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11
Q

What are the 2 types of ionophores?

A
  1. Carriers

2. Channel-forming

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12
Q

What are carrier ionophores?

A
  • Hydrophobic molecules that carry ion in their core

- Shield it from hydrophobic membrane environment

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13
Q

What are channel-forming ionophores?

A
  • Membrane-spanning hydrophobic proteins that form a hydrophilic channel
  • Allows ions to pass freely through cell membrane
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14
Q

What is Valinomycin an example of?

A

Carrier ionophore

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15
Q

What is Gramicidin A an example of?

A

Channel-forming ionophore

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16
Q

Describe Valinomycin

A
  • Macrocyclic molecule
  • Specific to K+ ions
  • Effective as antibiotic
17
Q

How does Valinomycin transport K+?

A
  1. K+ binds to interior

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

18
Q

How is Valinomycin effective as antibiotic?

A
  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
19
Q

Describe Gramicidin A

A
  • 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)
20
Q

How is Gramicidin A effective as antibiotic?

A

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

1st antibiotic used clinically (1939)

21
Q

What is job of ion channels?

A

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

22
Q

Describe meaning of ‘gated’ channels

A

Ion channels normally closed by opened by specific stimulus

  1. Voltage-gated channe;s
  2. Ca2+ activated channels
  3. Non-selective cation channels (TRP)
23
Q

Describe voltage-gated channels

A

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

24
Q

Describe Ca2+ activated channels

A

Ca2+ activated K+ channels

25
Q

Describe non-selective cation channels

A

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

26
Q

What are ion channels important for?

A
  1. Maintaining osmotic balance
  2. Signal transduction
  3. Nerve impulses
27
Q

How is glucose transported into and out of cell?

A

Glucose transporter (GLUT1) moves glucose across membrane

Can move glucose either way (depends on concentrations)

28
Q

Describe GLUT1

A
  • Integral membrane protein
  • 12 transmembrane alpha-helices
  • Central pore through which glucose can move
29
Q

Describe transport of glucose into cell

A
  1. Glucose binds to transporter
  2. Induces conformational change in transporter
  3. Glucose molecule moved into cytosol by diffusion
  4. Transporter resets
30
Q

What happens to glucose inside cytosol?

A

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

31
Q

How is glucose stopped from moving back out of cell?

A

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

32
Q

What is glucose transport specific for?

A

For D-isomer of glucose

Has higher affinity (lower Km) for glucose than other sugars

33
Q

What are aquaporins?

A

Water channel proteins required for bulk flow of H20 across cell membranes

Rapid transfer of water down concentration gradient

34
Q

Describe aquaporins

A
  • Tetramer with 4 pores through which H20 can pass

- Identical 28 kDa subunits (each subunit containing 6 transmembrane alpha-helices)

35
Q

Where are aquaporins abundant?

A

Abundant in erythrocytes and kidney cells

Yr 1 - Membranes, Lipids, Signalling (24 decks)

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