Question

phospholipids are the building blocks of cell membranes. in a cell membrane, phospholipids are arranged in two rows of molecules, called a lipid bilayer. both the extracellular environment (outside of the cell) and the intracellular environment (inside of the cell) contain water. which model best represents the lipid bilayer of a cell membrane?

Explanation:

Phospholipids have a hydrophilic (water - attracting) head (blue circles) and a hydrophobic (water - repelling) tail (orange chains). In a cell membrane, the extracellular and intracellular environments are aqueous (contain water). So, the hydrophilic heads should face the aqueous environments (extracellular and intracellular), and the hydrophobic tails should face each other (inside the bilayer).

Looking at the options:

• In the first option, the tails are facing the aqueous environments, which is incorrect.
• In the second option, there are two bilayers, which is not the structure of a cell membrane's lipid bilayer.
• In the third option, the tails are facing the aqueous environments, which is incorrect.
• In the fourth option, the hydrophilic heads (blue circles) face the extracellular (yellow) and intracellular (pink, aqueous) environments, and the hydrophobic tails (orange chains) face each other, which matches the structure of the lipid bilayer.

Answer:

The fourth model (the one in the bottom - right corner) best represents the lipid bilayer of a cell membrane.