Final answer:
Cholesterol and phospholipids are the molecules that add stiffness and strength to the plasma membrane.
Cholesterol modulates the fluidity and provides mechanical stiffness, while phospholipids create the membrane's basic structure.
Therefore the correct answer is option are A) Cholesterol and B) Phospholipids.
Step-by-step explanation:
The molecules that add stiffness and strength to the plasma membrane are cholesterol and phospholipids. Phospholipids form the bilayer of the plasma membrane, providing a basic structure as well as a barrier for the cell. Cholesterol, on the other hand, is embedded within the tail bilayer of the plasma membrane and acts to provide flexibility, but at the same time, it modulates the fluidity and mechanical stiffness of the membrane, allowing it to withstand changes in temperature and other stresses.
Proteins, such as integral proteins, form transporters and are embedded in the plasma membrane, potentially spanning all or part of the membrane. While they are crucial for transport and cellular communication, they do not primarily contribute to the stiffness and strength of the membrane. Glycoproteins, which are carbohydrates attached to proteins, extend from the membrane's surface and are involved in cell recognition but do not specifically add rigidity to the membrane.