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Fundamentals Of Life Science > Membrane Transport > Flashcards

Membrane Transport Flashcards

1
Q

What is the transport of inorganic ions and small molecules across the membrane required for? (3)

A
  • Regulation of intracellular ion concentrations
  • Uptake of nutrients
  • Excretion of metabolic waste products
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2
Q

Does passive transport require energy?

A

no (driven by electrochemical gradients/conc)

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3
Q

What are the types of passive transport?

A

Simple diffusion

Facilitated diffusion

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4
Q

What is the difference between simple and facilitated diffusion?

A

In facilitated diffusion membrane proteins are involved

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5
Q

What is the electrochemical gradient?

A

The combined force of concentration gradient and membrane potential (membrane potential only applies to charged molecules)

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6
Q

What is the membrane potential?

A

The difference in charge between the inside and the outside of a cell

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7
Q

What affects the ability of a solute to cross the membrane by simple diffusion? (4)

A
  • concentration gradient
  • electrochem gradient (ions don’t cross)
  • hydrophobicity (polar molecules don’t tend to cross bilayer)
  • size
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8
Q

What types of membrane proteins are involved in facilitated diffusion?

A
  • channels

- transporters

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9
Q

What are the channels and transporters required for in facilitated diffusion?

A

to transport inorganic ions and small molecules across the membrane (passively along conc/electrochem grad)

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10
Q

What are channels required for?

A

To transport ions (membranes impermeable to them)

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11
Q

What are 4 features of ion channels?

A
  1. exhibit ion selectivity
  2. driven by conc/electrochem grad
  3. V fast (10^7 molecules per sec)
  4. Regulated (open + close in response to stimulus)
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12
Q

What is one of the most common ion channel?

A

K+ channel

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13
Q

What are K+ leak channels

A

K+ ion channels that are constantly open (not gated)

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14
Q

How many times more effective are K+ channels than Na+ channels?

A

10,000 fold more effective

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15
Q

Describe how a K+ ion moves through a K+ channel

A
  1. Due to being in a high conc a hydrated K+ ion enters the channel where there are negatively charged amino acids at the pore entrance
  2. As it passes through it gets dehydrated
  3. The K+ ion passes through the selectivity filter which is lined with carbonyl oxygens of amino acids
  4. As the K+ leaves the channel it gets rehydrated and moves out into the are of lower electrochem gradient
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16
Q

How is the K+ ion channel selective?

A
  1. Anions prevented by entering due to negative amino acids at entrance
  2. Hydrated cations including K+ are too big to pass through selectivity filter + must be dehydrated (requires energy). In K+ the energy lost by dehydration is regained by dehydrated K+ ions interacting with carbonyl oxygens of amino acids lining selectivity filters (act as surrogate water molecules) (Na+ would only be able to interact with 2 O molecules in contrast to K+ 4 (energetically unfavourable))
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17
Q

What are the features of Transporter proteins?

A
  • Exhibit selectivity to certain solutes
  • Slower transporters
  • oscilate between 2 conformations regardless of if a solute is bound
  • Transports solutes down electrochem/conc gradients
18
Q

What are 5 features of glucose transporters (Gluts)

A
  • uniporters (only transport glucose)
  • expressed by most cells
  • many isoforms
  • 12 pass membrane spanning proteins
  • alternate between 2 conformations
19
Q

What happens when someone has Glut1 deficiency syndrome?

A
  • seizures
  • microcephaly
  • retarded development
20
Q

Explain how Glut 1 maintains unidirectional transport.

A

When glucose is transported into the erythrocyte, it is phosphorlyated to Glucose-6-phosphate which is not recognised by Glut 1 and prevents the transporter working in the opposite direction.

21
Q

What 2 transporters are involved in active transport?

A
  • ATP-driven pumps

- coupled receptors

22
Q

What is active transport?

A

Inorganic ions and small molecules are transported actively (‘uphill’) against their concentration/electrochemical gradients

23
Q

What is the role of ATP driven pumps?

A

To move solutes against their conc/electrochem gradient by expending energy (primary active transport)

24
Q

What are 7 features of the (Na+K+ pump)

A
  • expressed by all cells
  • up to 30 million of them on some cells
  • Hydrolyses ATP (primary active transport
  • Utilises 30% of energy available in most cells (more in nerve cells)
  • Continually expels Na+ from the cell in exchange for K+)
  • Maintains k+ and Na+ electrochem gradients
  • Stabilises osmotic balance so that secondary active transport can’t occur
25
Q

How does the Na+K+ pump work?

A
  1. 3Na+ ions bind, the pump hydrolyses ATP and is phosphorylated
  2. Na+ dependent phosphorylation causes the pump to change conformationally and Na+ is transferred across the membrane and released
  3. 2K+ ions bind and the pump is dephosphorylated
  4. K+ dependent dephospho rylation causes the pump to return to normal conformation and K+ is transferred across the membrane and released
26
Q

What is the role of coupled receptors?

A

To move solutes against their conc/electrochem gradient by coupling their transport to the Na+ gradient created by Na+K+ATPase (does not depend directly on the hydrolysis of ATP)

27
Q

What are 2 features of the Na+/glucose symporter?

A

oscillates between two conformations

-binding is cooperative (Na+ binding increases affinity for glucose)

28
Q

Where is glucose most likely to bind to the Na/glucose sympoter and why?

A

extracellular side where Na+ conc is higher (glucose more likely to enter cells then leave them)

29
Q

Why are glucose symporters required for absorption of glucose in the GI tract?

A

There are tight junctions so glucose can’t pass through paracellular pathway

30
Q

What do tight junctions prevent?

A

Paracellular movement

31
Q

What are tight junctions?

A

Junctions that define the boundary between apical and basal domaints

32
Q

What are the functions of tight junctions?

A

Barrier function - Prevents large macromolecular movement paracellularly

Fence Function - Prevents proteins diffusing between the 2 domains

33
Q

How is glucose absorbed in the ileum?

A
  1. Na+ grad created by Na+/K+ATPase
  2. Na+/ glucose symporter transports glucose into the epithelial cells (against conc grad) using Na+ electrochem gradient
  3. Glut2 transports glucose out of cells (along conc grad) and into blood supply (fac diffusion)
34
Q

What does glucose absorption depend on?

A

The asymmetric distribution of transporters in epithelial cells
Na+/glucose symporter must be in apical membrane
Glut2 must be in basolateral membrane

35
Q

Where is the Na+/Ca+ anti porter expressed by?

A

Cardiomyocytes in heart muscles

36
Q

Is the conc Ca+ conc greater in the interstitial fluid or in the cardiomyocytes?

A

Interstitial fluid

37
Q

What is the role of the antiporter in cardiomyoctes?

A

To use Na+ electrochem gradient to transport Ca2+ out of the cell against its gradient (causes muscle relaxation)

38
Q

How is muscle contraction caused?

A

By a rise in intracellular Ca2+

39
Q

How would you treat heart failure?

A

By preventing Ca2+ being transported out of cell (increase strength of heartbeat)

  • ouabain and digoxin inhibit Na+K+ATPase
  • This inhibition increases intracellular Na+ and a reduction in Na+ gradient
  • So effectiveness of anti porter reduced so fewer Ca2+ transported out of the cell
40
Q

What is the structure of an aquaporin?

A
  • tetrameric complex
  • made up of four monomers (each acting as a water channel)
  • Each monomer has a six pass membrane spanning protein
41
Q

What is the role of the aquaporin?

A

to allow passage of water whist preventing the passage of ions

Fundamentals Of Life Science (26 decks)

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