Facilitated diffusion uses carrier proteins but isn't active transport. Why? And why does it show "saturation"?
Diya Menon11th Sample
12 Sept 2026 · 1 min read
Ahh I thought I'd finished this chapter and then a practice question made me doubt everything. 🥲
NCERT lists three means of transport in plants: diffusion, facilitated diffusion and active transport. I know the standard line, diffusion is passive, from higher to lower concentration, no energy. Active transport uses ATP and moves things against the gradient. Fine.
But facilitated diffusion has "carrier proteins" or "channels" and I picture a protein as a machine that must use energy. So why does it count as passive? And NCERT says facilitated diffusion shows saturation, like active transport does. Saturation of what?
The sentence from the book that confuses me: "Facilitated diffusion cannot cause net transport of molecules from a low to a high concentration, which would require energy." Okay, but then both use proteins, so when I see "proteins + saturation + specificity" in an MCQ, how do I tell them apart?
Also: aquaporins. Are they carriers or channels? And is osmosis a type of diffusion or a different thing?
I made a Venn diagram but it has too much overlap and I just can't fill the middle. Can someone help me divide the circles?
8 answers & replies
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[+][–]Meera Patel11thSample· 1mo ago
The key question is the direction relative to the gradient and the energy. Facilitated diffusion: the protein gives a path (a channel or a carrier) to substances that can't cross the lipid bilayer easily, such as hydrophilic molecules and ions. It moves them down the gradient, so no ATP. Active transport: moves them up the gradient and needs ATP, and the proteins are called pumps.
The protein doesn't need energy because it's not pushing, it's only opening the door. The gradient does the pushing.
[+][–]Nisha Verma12thSample· 1mo ago
Saturation: the number of carrier proteins in the membrane is limited. At low concentration of the substance, rate rises with concentration. When all carriers are busy, even more substance doesn't speed it up, and the rate plateaus. That's saturation. Simple diffusion doesn't saturate; its rate keeps rising with the gradient.
Quick sorting table:
- Diffusion: no protein, no ATP, no saturation
- Facilitated: protein, no ATP, saturation, down gradient
- Active: protein (pump), ATP, saturation, up gradient
[+][–]Diya Menon11thSample· 1mo agoAuthor
That table is the Venn middle that I couldn't fill. Thank you!! ❤️
[+][–]Yash Tiwari11thSample· 1mo ago
Bhai facilitated diffusion is faster than active transport and slower than simple diffusion, na? Isiliye "facilitated"?
[+][–]Kavya NaikDropperSample· 1mo ago
Yash, "facilitated" just means it's helped by a protein, it isn't about the speed ordering. There's no fixed rule that it is faster than active transport. The word describes the process (diffusion with a helper), not the rate. Don't write the speed thing in an exam.
[+][–]Yash Tiwari11thSample· 1mo ago
ok ok samajh gaya bhai, sorry 😅
[+][–]Siddharth RaoDropperSample· 1mo ago
Aquaporins: they are water channels (channel proteins), so aquaporins are part of how water moves through membranes quickly, it's still osmosis/diffusion of water, no ATP. Porins are different: NCERT describes them as proteins forming large pores in the outer membrane of plastids, mitochondria and some bacteria, letting even small proteins through. So keep porins and aquaporins in separate boxes.
Osmosis = a special case of diffusion, for water across a selectively permeable membrane, down water potential gradient. So yes, osmosis is diffusion, just of water.
[+][–]Ishita GoyalDropperSample· 1mo ago
Also one more NCERT feature: facilitated diffusion is very specific, it allows cell to select substances for uptake, and it is sensitive to inhibitors that react with protein side chains. This is in the same paragraph as the saturation line. If an MCQ lists "highly selective" and "sensitive to inhibitors", it's talking about carrier-mediated transport (facilitated or active).
