Which property of phospholipids drives the spontaneous formation of the bilayer shown?

Answer: Amphipathic structure: hydrophilic heads face water; hydrophobic tails face inward Each phospholipid has a polar head and nonpolar tails; in water, tails self-associate away from water while heads contact it, generating a bilayer without energy input.

The fluid mosaic model describes the cell membrane as

Answer: A fluid phospholipid bilayer with proteins embedded and free to drift laterally Phospholipids and most proteins move laterally within the bilayer — the membrane is a 2D fluid with embedded "mosaic" of proteins.

The fluid mosaic model emphasizes that membrane proteins:

Answer: Move laterally within the lipid bilayer like floating mosaic tiles The model views the membrane as a 2D fluid in which proteins drift laterally; this lateral mobility is essential for signaling, transport, and membrane fusion.

AP Biology · Topic 2.4

Plasma Membranes Practice

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

Practice questions

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Sample questions

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Sample 1difficulty 2/5
$A student adds purified phospholipids to water and observes that they spontaneously self-assemble into bilayer sheets and vesicles without energy input. The teacher asks why this organization happens spontaneously when water is the solvent on both sides.$
$Which property of phospholipids drives the spontaneous formation of the bilayer shown?$
- A
  $ATP hydrolysis builds the bilayer molecule by molecule$
- B
  $Amphipathic structure: hydrophilic heads face water; hydrophobic tails face inward$
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- C
  $Phospholipids are entirely hydrophobic and avoid water$
- D
  $Phospholipids are entirely hydrophilic and bind water tightly$
Why
$Each phospholipid has a polar head and nonpolar tails; in water, tails self-associate away from water while heads contact it, generating a bilayer without energy input.$
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Sample 2difficulty 2/5
$The fluid mosaic model describes the cell membrane as$
- A
  $A solid crystalline lattice of cholesterol with surface glycoproteins$
- B
  $A pure protein network with embedded carbohydrates and no lipid layer$
- C
  $A rigid lipid sheet with proteins anchored permanently in fixed positions$
- D
  $A fluid phospholipid bilayer with proteins embedded and free to drift laterally$
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Why
$Phospholipids and most proteins move laterally within the bilayer — the membrane is a 2D fluid with embedded "mosaic" of proteins.$
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Sample 3difficulty 3/5
$The fluid mosaic model emphasizes that membrane proteins:$
- A
  $Spontaneously flip from one leaflet to the other quickly$
- B
  $Are covalently fixed in position by the cytoskeleton$
- C
  $Span only one leaflet of the bilayer$
- D
  $Move laterally within the lipid bilayer like floating mosaic tiles$
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Why
$The model views the membrane as a 2D fluid in which proteins drift laterally; this lateral mobility is essential for signaling, transport, and membrane fusion.$
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Sample 4difficulty 3/5
$Which change would <strong> decrease </strong> membrane fluidity at a given temperature?$
- A
  $More unsaturated phospholipids$
- B
  $Higher temperature$
- C
  $More cholesterol at low temp$
- D
  $More saturated phospholipids$
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Why
$Saturated tails pack tightly, reducing fluidity. Unsaturated tails have kinks; cholesterol at low temp prevents tight packing.$
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Sample 5difficulty 3/5
$The two leaflets of a cell membrane differ in lipid and protein composition. This asymmetry is$
- A
  $Functionally important — e.g., sugars on outer leaflet for cell-cell recognition$
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- B
  $Random — leaflet composition is determined by passive diffusion alone$
- C
  $Quickly eliminated by lateral and flip-flop diffusion within the bilayer$
- D
  $Found only in prokaryotic plasma membranes — eukaryotes lack asymmetry$
Why
$Lipid flip-flop is rare, allowing inner/outer leaflets to maintain distinct compositions tied to function.$
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