The model below shows the structure of a portion of a plasma membrane in an animal cell.

The model shows the structure of a portion of a plasma membrane in an animal cell. None of the parts are labeled, but the model includes a phospholipid bilayer that contains some embedded proteins. The phospholipids are arranged with their heads oriented toward the external and internal membrane surfaces and their fatty acid tails oriented toward the center of the membrane.

Which statement best explains the orientation of the phospholipid molecules in this model?

Question

The model below shows the structure of a portion of a plasma membrane in an animal cell.

The model shows the structure of a portion of a plasma membrane in an animal cell. None of the parts are labeled, but the model includes a phospholipid bilayer that contains some embedded proteins. The phospholipids are arranged with their heads oriented toward the external and internal membrane surfaces and their fatty acid tails oriented toward the center of the membrane.

Which statement best explains the orientation of the phospholipid molecules in this model?

BlackTom AI · Accepted Answer

The model describes a phospholipid bilayer where phospholipids have hydrophilic heads facing the aqueous environments on both sides of the membrane, and hydrophobic tails tucked toward the interior of the bilayer.

Reasoning for the correct option:
- Phospholipids are amphipathic molecules with a polar, hydrophilic head and nonpolar, hydrophobic tails. In an aqueous environment, they arrange themselves so that the heads interact with water, while the tails avoid water by clustering away from it. This drives the formation of a bilayer with the heads pointing outward toward the cytosol and extracellular space and the tails facing inward toward the interior of the membrane.

Why the other potential explanations (which would be incorrect in this context) would fail if considered:
- If one suggested that tails interact with water or that heads point inward, that would contradict the fundamental behavior of phospholipids in water and the observed structure of the bilayer. Tails are hydrophobic and minimize contact with water, so they are sequestered inside.
- Proposals that the orientation is random or that both leaflets expose hydrophobic regions to water would not produce a stable, continuous bilayer in an aqueous environment, and would not align with the image showing a consistent layer of heads at both surfaces.
- Any statement claiming only one side of the membrane interfaces with water, or asserting that the entire bilayer is hydrophobic on both surfaces, would overlook the known placement of polar heads facing the aqueous compartments on either side.

In summary, the arrangement where the hydrophilic phosphate-containing heads face the watery interiors on both sides of the membrane, while the hydrophobic tails point inward away from water, best explains the described phospholipid orientation in this model.

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