Final answer:
Proteins must pass through the cell membrane to enter or exit a cell, with transport proteins assisting the passage of larger molecules that cannot diffuse through the lipid bilayer on their own.
Step-by-step explanation:
For proteins to come in and out of the cell, they have to pass through the cell membrane. The cell membrane's selectively permeable nature allows it to control the passage of substances, ensuring that essential materials can enter the cell while preventing harmful materials from getting inside. Many proteins require special mechanisms to facilitate their transport because they are larger or otherwise unable to pass through the hydrophobic lipid bilayer of the membrane without assistance. This assistance comes in the form of transport proteins, including integral proteins that can form channels or pores, and carrier proteins that bind to substances and help carry them across the membrane. Additionally, cells expend energy to actively transport certain vital substances against their concentration gradient, highlighting the membrane's critical role in cellular function.