Final answer:
Cholesterol molecules within the cell membrane provide necessary flexibility, contributing to membrane fluidity. The correct answer is option b).
Step-by-step explanation:
The part of cell membranes that gives flexibility to the structure is the cholesterol molecules. Cell membranes are dynamic and fluid structures primarily composed of a phospholipid bilayer that provides the basic membrane structure, with embedded proteins serving various functions.
However, it is the presence of cholesterol within the membrane that significantly contributes to its flexibility. Cholesterol molecules are interspersed among the phospholipids, and they interact with the fatty acid tails, preventing them from packing too closely, which maintains membrane fluidity at varying temperatures.
In summary, while phospholipids form the bilayer and integral and peripheral proteins have their respective roles, the incorporation of cholesterol into the cell membrane is crucial for providing the necessary flexibility. This fluidity is vital for membrane functionality, allowing the movement and function of embedded proteins and facilitating the dynamic nature of cellular membranes.