Final answer:
Substances enter the cell by receptor-mediated endocytosis through the binding of receptors in clathrin-coated pits. This selective process involves aggregating receptors at the plasma membrane, forming a vesicle, and transporting the substance into the cell. The correct answer 4 to the question is Clathrin-coated pits.
Step-by-step explanation:
Substances that enter the cell by receptor-mediated endocytosis bind to receptors that collect in specialized domains of the plasma membrane known as clathrin-coated pits. Receptor-mediated endocytosis is a specific type of endocytosis where cells take up targeted substances from the outside environment. This process begins when the targeted substance binds to its specific receptor on the cell surface.
The bound receptors then aggregate in areas of the plasma membrane that contain a specific protein called clathrin. These regions are known as clathrin-coated pits. Once a sufficient number of receptor-ligand complexes gather in these pits, the plasma membrane invaginates, or folds inward, forming a vesicle that is covered in clathrin. This vesicle is referred to as a clathrin-coated vesicle. The vesicle then pinches off from the plasma membrane and enters the cell, delivering its contents for further processing.
This mechanism is highly selective and efficient for the uptake of large molecules and particles that cannot cross the plasma membrane by simple diffusion. Important substances such as cholesterol, which is carried in the blood by low-density lipoproteins (LDL), are commonly taken up by cells through this process. Defects in receptor-mediated endocytosis, like those seen in familial hypercholesterolemia where LDL receptors are defective or absent, can lead to serious health conditions.
Given the options presented in the question, the correct answer is 4) Clathrin-coated pits.