Which two membrane features are most essential for explaining why some nutrients require transport proteins while others do not?

• A. An electrically insulating peptidoglycan layer and a permanent inward sodium concentration that blocks any nutrient flux without hormonal signaling.
• B. A hydrophobic lipid bilayer and the presence of concentration gradients across the membrane.
• C. A rigid cellulose scaffold surrounding the membrane and an absence of extracellular fluid in the gut lumen preventing diffusion.
• D. A crystalline protein shell encasing the cell and active phagocytosis of all luminal contents irrespective of size or polarity.

Answer: (B)