Read Membrane Transport by Barrier and Energy

The bilayer interior is hydrophobic. Charged ions cross it very poorly without proteins; large polar molecules also cross slowly. Smaller uncharged polar molecules pass more readily, and lipid-soluble molecules pass most readily. For the NSEB choices, increasing direct permeability is Ca²⁺ < glucose < glycerol < vitamin A. This is permeability through the lipid bilayer itself, not the rate after transport proteins are added.

The hydrophilic solute in the NSEB diagram is surrounded by water outside the membrane. Crossing the hydrophobic core unaided has a high activation-energy barrier. Binding to the transporter replaces some solute–water interactions and lowers that barrier, allowing the molecule to pass without irreversible chemical conversion. The question's graph compares the route with and without a transporter; it does not show that transport increases the barrier.

In the two-panel PYQ, process I bends the membrane around cargo and pinches off a vesicle: endocytosis is energy-dependent. Process II moves individual solute molecules through a membrane protein down their concentration gradient: facilitated diffusion is passive and needs no direct ATP input. Protein involvement alone does not make transport active; direction and energy source decide.

Receptor-mediated endocytosis can internalise cholesterol-containing particles; exocytosis releases mucus. ATP synthase and glucose transporters are membrane proteins. In the nephron, the ascending limb is effectively water-impermeable while salts move out, helping establish the medullary gradient.

For direct diffusion through a lipid bilayer, a polar solute must shed its hydration shell before entering the hydrophobic core and regain water after crossing. More hydroxyl groups generally mean stronger hydration and a larger dehydration barrier. Across glycol, glycerol and erythritol, glycol is least hydrated and diffuses fastest; greater hydration raises, rather than lowers, the activation barrier.

Follow each membrane separately. At the apical surface, SGLT brings glucose in together with Na⁺: it is a symporter driven by the Na⁺ gradient. At the basolateral surface, GLUT lets glucose leave through a uniporter. The Na⁺/K⁺ ATPase spends ATP to pump Na⁺ out and K⁺ in, maintaining the gradient that powers SGLT. Thus choose B and C.

Simple diffusion, channel transport and facilitated diffusion can all move solute downhill without direct ATP. The route explicitly coupled to energy moves solute against its concentration gradient, so active transport is the most energy-demanding. Choose D.