Question

bricks make good walls for a house or an apartment building. however, the walls must have gaps for doors and windows. the doors and windows allow things to pass in and out of the building.
some materials pass through easily. for example, on a cold winter day, cold air goes inside easily through an open window. other materials do not pass through as easily. for example, the trash will not go out the door unless someone or something carries it out.
like the walls of a building, the cell membrane also has openings and channels that allow materials to pass through. the cell uses these openings to take in materials that it needs, such as water and nutrients, and to remove wastes.
the cell membrane also acts to protect the cell. for example, most large molecules cannot pass through the membrane. large molecules in the cell’s environment could damage the cell if they gained entry.

Explanation:

Since no specific question is provided, it's hard to give a targeted answer. If you want to analyze the text (e.g., identify the scientific concept, summarize, etc.), here's a brief breakdown:

The text compares building walls (with doors/windows for material passage) to cell membranes (with openings/channels for material transport and protection). It explains selective permeability of cell membrane (allowing needed materials in, wastes out, blocking harmful large molecules).

Answer:

The text explains the cell membrane's function (selective permeability, protection) by analogy to building walls with doors/windows.