Which structural feature of the cell membrane allows molecules such as oxygen and carbon dioxide to diffuse into and out of the cell

The correct answer to the question: Which structural feature of the cell membrane allows molecules such as oxygen and carbon dioxide to diffuse into and out of the cell, would be, C: The cell membrane is made up of a phospholipid bilayer, which happens to allow nonpolar molecules, such as oxygen and CO2, to diffuse through it, without having to have the mediation of transport proteins, or to have any kind of mediator, for that matter.

This process is known as selective permeability and it is the reason why oxygen, CO2, certain lipids and alcohol, are able to pass the membrane bilayer without having much restriction, and without needing mediators in the process. Also, the cell does not need to expend any energy in the exchange of any of these molecules.

It must be remembered that protein molecules have a different job, and that is to become channels through which ions, and bigger molecules, such as glucose, must pass. Also, cholesterol is simply a structural substance in the membrane that gives it fluidity and also reinforces the structure of it. This is why the answer is C, not A, or B, or D.