Aquaporins are membrane proteins that

Answer: Allow rapid passive water flow across the membrane Aquaporins are channel proteins specifically permeable to water, enabling osmosis to occur much faster than diffusion alone.

Facilitated diffusion differs from simple diffusion in that it

Answer: Uses transport proteins (channels or carriers) but moves solutes down their gradient without ATP Still passive (down gradient, no ATP), but uses protein channels or carriers for solutes that can't cross the bilayer alone.

Channel proteins differ from carrier proteins in that channels

Answer: Form continuous pores allowing rapid passive flow Channels are open (or gated) pores; carriers bind solute and flip between conformations to release it on the other side.

AP Biology · Topic 2.7

Facilitated Diffusion Practice

Part of Cell Structure and Function.(ENE-1.G)

Practice questions

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Sample questions

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Sample 1difficulty 2/5
$Aquaporins are membrane proteins that$
- A
  $Bind glucose for facilitated diffusion uptake$
- B
  $Allow rapid passive water flow across the membrane$
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- C
  $Catalyze ATP synthesis from a proton gradient$
- D
  $Allow active sodium transport across the membrane$
Why
$Aquaporins are channel proteins specifically permeable to water, enabling osmosis to occur much faster than diffusion alone.$
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Sample 2difficulty 2/5
$Facilitated diffusion differs from simple diffusion in that it$
- A
  $Uses transport proteins and ATP hydrolysis to move solutes against their concentration gradient$
- B
  $Uses transport proteins to move solutes up their gradient without consuming any ATP$
- C
  $Uses transport proteins (channels or carriers) but moves solutes down their gradient without ATP$
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- D
  $Moves solutes directly through the lipid bilayer along the gradient without any membrane proteins$
Why
$Still passive (down gradient, no ATP), but uses protein channels or carriers for solutes that can't cross the bilayer alone.$
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Sample 3difficulty 2/5
$Channel proteins differ from carrier proteins in that channels$
- A
  $Bind specific solutes and undergo conformational changes$
- B
  $Always require ATP$
- C
  $Form continuous pores allowing rapid passive flow$
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- D
  $Only handle large molecules$
Why
$Channels are open (or gated) pores; carriers bind solute and flip between conformations to release it on the other side.$
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Sample 4difficulty 3/5
$Why does glucose require a channel/carrier protein to cross the bilayer despite moving down its gradient?$
- A
  $Glucose is positively charged and repelled by phospholipid heads$
- B
  $Glucose is too small to be detected$
- C
  $Glucose binds water tightly and only crosses with ATP$
- D
  $Glucose is large and polar; the hydrophobic core blocks unaided diffusion$
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Why
$Polar/large solutes like glucose cannot pass through the hydrophobic membrane interior; protein carriers (e.g., GLUT) provide a hydrophilic path down the gradient.$
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Sample 5difficulty 4/5
$Kidney collecting duct cells have aquaporin channels in their membranes. When the hormone vasopressin signals dehydration, more aquaporins are inserted, and water reabsorption rises sharply. Sodium/potassium permeability is unchanged.$
$Aquaporins are an example of which transport process?$
- A
  $Facilitated diffusion of water through a selective channel (no ATP)$
  check_circle
- B
  $Active transport of water that requires ATP$
- C
  $Endocytosis of water droplets$
- D
  $Ion pumping that secondarily moves water$
Why
$Aquaporins are channel proteins that allow water to follow its osmotic gradient passively, no ATP required - the hallmark of facilitated diffusion.$
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