Which term refers to the process of taking in substances by the cell membrane?

The term that refers to the process of taking in substances by the cell membrane is "endocytosis." This process involves the cell membrane folding inward to engulf external materials, allowing the cell to take in large molecules or particles that cannot pass through the membrane by diffusion. Endocytosis is critical for many cellular functions, including nutrient uptake, immune responses, and the internalization of signaling molecules.

Other processes such as phagocytosis can be considered a specific type of endocytosis focused on the uptake of large particles or microorganisms. However, phagocytosis is just one form of endocytosis, which also includes other types like pinocytosis, responsible for taking in liquids and small solutes.

In contrast, exocytosis is the process by which cells expel materials, while filtration is not a cellular process but rather a physical separation method relevant in different biological contexts, such as in kidneys for filtering blood. Thus, the term endocytosis encompasses the general mechanism of substance intake through the cell membrane, making it the correct answer in this context.