Questions: Cell Membrane Structure

Question 1

Why do phospholipids spontaneously form a bilayer in an aqueous environment?

• A. Covalent bonds form between the phospholipid tails
• B. The hydrophobic tails are repelled by water and cluster together while the hydrophilic heads face the water
• C. Enzymes actively assemble the phospholipids into a bilayer
• D. The polar heads repel each other and push the tails inward

Answer: (B)

Phospholipids are amphipathic — they have a hydrophilic (water-loving) head and hydrophobic (water-fearing) tails. In water, the tails are excluded from the aqueous environment and cluster together, while the heads interact favorably with water. This self-assembly is driven by the hydrophobic effect (a consequence of intermolecular forces), not by covalent bonding or enzymatic action.

Question 2

Cholesterol makes the cell membrane more rigid at most temperatures.

• T. True
• F. False

Answer: False

This is a common misconception. Cholesterol acts as a fluidity buffer: at high temperatures it restrains phospholipid movement (reducing fluidity), but at low temperatures it prevents tight packing of tails (increasing fluidity). The net effect is that cholesterol stabilizes membrane fluidity across a range of temperatures.

Question 3

What is the primary difference between integral (transmembrane) proteins and peripheral proteins in the cell membrane?

Model answer: Integral proteins are embedded in or span the entire phospholipid bilayer, while peripheral proteins are loosely attached to the membrane surface and do not penetrate the hydrophobic interior.

Integral proteins have hydrophobic regions that interact with the lipid tails, anchoring them within the bilayer. Many span the entire membrane (transmembrane proteins) and function as channels or receptors. Peripheral proteins associate with the membrane surface through interactions with integral proteins or polar head groups and can be removed without disrupting the bilayer.