Question

what is the main responsibility of protein channels attached to the cell membrane? keeping cell turgidity at normal levels for homeostasis extra support for the cell exterior and mechanical stress allowing specific molecules across the cell membrane

Explanation:

Protein channels in the cell membrane are integral membrane proteins that form pores. Their main function is to facilitate the passage of specific molecules (like ions, small polar molecules) across the selectively permeable cell membrane. The first option relates to osmotic pressure/turgor (more about water movement and cell wall in plants), the second is about structural support (e.g., cytoskeleton or cell wall components, not protein channels). So the correct function is allowing specific molecules across.

Answer:

allowing specific molecules across the cell membrane