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Biological Membranes Quiz
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Which feature of phospholipids best explains why they spontaneously form a bilayer when placed in an aqueous environment?
- A They contain glycerol linked to two identical saturated fatty acids You picked ✕
- B They have a hydrophilic core that attracts water and ions You picked ✕
- C They contain glycerol, a phosphate group, and two fatty acid tails You picked ✕
- D They are amphipathic, with hydrophilic heads and hydrophobic tails You picked ✓
- E They are rigid molecules that pack tightly to form a solid sheet You picked ✕
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The answer is D: They are amphipathic, with hydrophilic heads and hydrophobic tails
Explanation
Why D is correct
See Membrane Structure. Option D is correct because phospholipids are amphipathic, which means each molecule has a water-loving (hydrophilic) end and a water-hating (hydrophobic) end. The hydrophilic end is the phosphate head, which interacts happily with water. The hydrophobic end is the pair of fatty acid tails, which are long chains of carbon and hydrogen that avoid water. When phospholipids are placed in water, they arrange themselves so that the tails point inwards, shielded from the water, and the heads point outwards towards it. The result is a bilayer: two layers of phospholipids with their tails meeting in the middle. This happens spontaneously, with no energy needed from the cell, because it is the most stable arrangement in water and keeps the hydrophobic tails away from it. It is also what lets a membrane work as a barrier, because the oily core stops charged and large polar substances from passing freely. Memory aid: the heads face the water and the tails hide from it.
- Amphipathic: each phospholipid has a hydrophilic phosphate head and hydrophobic fatty acid tails.
- In water: the tails point inwards and the heads point outwards, forming a bilayer.
- Energy: bilayer formation is spontaneous and needs no energy from the cell.
Why the other options are incorrect
- A. They contain glycerol linked to two identical saturated fatty acids: Phospholipids do contain glycerol and fatty acids, but the two tails are not always identical or saturated. This is a description of the structure and does not explain why a bilayer forms in water. Your answer
- B. They have a hydrophilic core that attracts water and ions: The core of a phospholipid bilayer is hydrophobic because it is formed by the fatty acid tails; a hydrophilic core would attract water and ions, which would disrupt the bilayer rather than explain its formation. Your answer
- C. They contain glycerol, a phosphate group, and two fatty acid tails: Phospholipids do contain glycerol, a phosphate group and two fatty acid tails, so the statement is true, but it simply lists the building blocks and does not explain why a bilayer spontaneously forms in water. Your answer
- E. They are rigid molecules that pack tightly to form a solid sheet: Membrane lipids are fluid and can move within the bilayer, so they are not rigid. The bilayer forms because of the way water treats the two ends of the molecule, not because the lipids lock into a solid sheet. Your answer
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1
During synthesis of an integral membrane protein, which factor is responsible for recognising the signal sequence on the nascent polypeptide and temporarily halting translation?
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2
Which erythrocyte membrane protein primarily links integral membrane proteins to the underlying spectrin-actin cytoskeleton, thereby helping maintain the biconcave shape of red blood cells?
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