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 part) and a hydrophobic (water - repelling) tail (orange part). The extracellular and intracellular environments are aqueous (contain water). So the hydrophilic heads should face the aqueous environments (outside and inside the cell), and the hydrophobic tails should face each other (in the middle of the bilayer). Looking at the options, the first model (top - left) has the hydrophilic heads (blue) facing the extracellular (yellow) and intracellular (pink, aqueous) environments, and the hydrophobic tails (orange) facing each other.

Answer:

The top - left model (with extracellular environment labeled, blue heads facing out and in, orange tails in between) best represents the lipid bilayer.