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

An electrically insulating peptidoglycan layer and a permanent inward sodium concentration that blocks any nutrient flux without hormonal signaling.

A hydrophobic lipid bilayer and the presence of concentration gradients across the membrane.

A rigid cellulose scaffold surrounding the membrane and an absence of extracellular fluid in the gut lumen preventing diffusion.

A crystalline protein shell encasing the cell and active phagocytosis of all luminal contents irrespective of size or polarity.