Identify Structure D. The cell membrane. Each letter marks a definite structure of the membrane. Letter A marks the structure represented by a branched sequence on the integral protein. B marks a branched structure on the cell membrane. C marks a large elongated structure outside the cell. D marks part of the membrane, which consists of two rows of molecules. Each molecule has a head and a tail. E marks a molecule, which is one of the membrane components. Such molecules are randomly integrated in the membrane. F marks a structure that penetrates two layers of the membrane.
a. protein
b. phospholipid bilayer of membrane
c. cholesterol
d. extracellular matrix
e. glycoprotein

Each molecule in the membrane phospholipid bilayer has two heads and a tail, both heads being hydrophilic (having affinity for water) and the tail being hydrophobic (having no affinity for water, and therefore repelling it).

This structure, with the heads, which are hydrophilic facing the intracellular and extracellular environments of cells, thus prevents the passage of polar compounds.