Final answer:
The import and export of small molecules through the cell membrane are governed by transport proteins, which include integral proteins forming channels and carrier proteins for active transport. These proteins enable the selective permeability of the cell, facilitating the movement of ions, water, and specific molecules like hormones and neurotransmitters.
Step-by-step explanation:
The import and export of small molecules in cells are facilitated by specific protein components known as transport proteins. These proteins are essential for maintaining cellular homeostasis and allow for selective permeability of the cell membrane.
Integral proteins and carrier proteins are two fundamental components that enable this process. Integral proteins form channels that allow small ions and molecules soluble in water to pass through, while carrier proteins interact specifically with certain molecules, such as hormones and neurotransmitters, to facilitate their passage.
Moreover, carrier proteins are involved in active transport and can function as either a uniporter, symporter, or antiporter, enabling the movement of ions or molecules across the cell membrane. During mRNA export from the nucleus to the cytoplasm, proteins including nuclear transport receptor, poly-A-binding protein, and CAP-binding proteins, as well as others, are critical for the process, indicating their role in the export of larger molecules such as mRNA.