Which molecules in the cell membrane help prevent the membrane from dissolving in water and maintain a stable barrier?

Introduction / Context:
This biology question focuses on cell membrane structure. Understanding why cell membranes do not simply dissolve in water is central to cell biology. The membrane must form a stable barrier that separates the interior of the cell from the external environment while remaining flexible and selectively permeable.

Given Data / Assumptions:

• The question is about molecules that stop the membrane from dissolving in water.
• The cell membrane is assumed to be the plasma membrane that surrounds cells.
• Options mention phospholipids, fatty acids, adenosine triphosphates, and none of the above.

Concept / Approach:
The fundamental structure of the plasma membrane is a phospholipid bilayer. Each phospholipid molecule has a hydrophilic head that is attracted to water and two hydrophobic tails that repel water. In an aqueous environment, phospholipids spontaneously arrange themselves into a bilayer with hydrophobic tails facing inward and hydrophilic heads facing outward toward water. This arrangement is energetically stable and prevents the membrane from dissolving, because the hydrophobic region is shielded from surrounding water. Other components such as proteins and cholesterol are embedded in the bilayer, but it is the amphipathic nature of phospholipids that is primarily responsible for forming a stable membrane barrier.

Step-by-Step Solution:

Verification / Alternative check:
You can check diagrams in cell biology textbooks that label the plasma membrane as a phospholipid bilayer with embedded proteins. These diagrams emphasise amphipathic phospholipid molecules forming the main barrier. Experimental studies of artificial membranes also show that phospholipids will spontaneously form bilayers in water, supporting this concept. The presence of this bilayer explains how the membrane is able to be fluid yet resistant to dissolving in the surrounding watery environment.

Why Other Options Are Wrong:

Fatty acids alone in solution are incorrect because, although they can assemble into structures like micelles, they do not typically form full cell membranes by themselves in modern cells. In addition, the question asks specifically about the molecules that maintain the membrane barrier.

Adenosine triphosphates are incorrect because they are energy carriers used in metabolism and active transport, not structural components that hold the membrane together.

None of the above is wrong because phospholipids clearly are the structural molecules that build and stabilise the cell membrane, so there is at least one correct choice among the options.

Common Pitfalls:
Learners sometimes overemphasise proteins when thinking about membranes, because many functions such as transport and signalling are protein driven. However, without the underlying phospholipid bilayer, those proteins would not have a stable platform. Another mistake is to assume that any lipid, like a generic fatty acid, can form a membrane identical to that of a cell, which is not accurate. Remember that the amphipathic nature of phospholipids, with both hydrophilic and hydrophobic parts, is the key property that prevents the membrane from dissolving in water.

Final Answer:
The molecules that maintain a stable barrier and prevent the cell membrane from dissolving in water are Phospholipids arranged in a bilayer.