Which one among the following statements about facilitated transport in plants is correct?

Correct — D, Facilitated transport requires assistance of carrier proteins. Facilitated diffusion is the movement of a solute down its own concentration gradient through a membrane protein — either a channel or a carrier — and it spends no metabolic energy. What makes it 'facilitated' rather than plain diffusion is exactly the protein: polar and charged solutes such as sugars and ions cannot cross the hydrophobic core of the lipid bilayer unaided, so a membrane protein must provide the path. Because the protein is a fixed number of binding sites, facilitated transport is also saturable and selective, unlike simple diffusion.

• (a)All passive transports are types of facilitated transport. — The implication runs the other way. Facilitated transport is one kind of passive transport, but simple diffusion of oxygen, carbon dioxide or a small non-polar molecule straight through the lipid bilayer is passive and uses no protein at all.
• (b)Facilitated transport is an example of active transport. — Active transport spends ATP and can push a solute against its gradient. Facilitated transport does neither; it only speeds up movement that the concentration gradient was already driving.
• (c)Water moves across plasma membrane by active transport. — Water crosses by osmosis, which is passive, and its rate is raised by aquaporins — water channels. No pump moves water directly; where water follows a pump it does so because the pump moved a solute first.

Membrane transport divides first by whether energy is spent. Passive transport moves a substance down its electrochemical gradient and needs no ATP; it includes simple diffusion straight through the bilayer, facilitated diffusion through a channel or carrier protein, and osmosis for water. Active transport uses ATP, directly or indirectly, and can move a substance against its gradient. Facilitated diffusion therefore sits inside the passive family but is set apart by its dependence on a membrane protein, which makes it selective, saturable and sensitive to inhibitors.

Each wrong option here is a real statement about transport that has been shifted by one step: (a) inverts a subset relation, (b) moves facilitated transport into the wrong energy class, (c) puts water into the wrong mechanism. The safe method is to test each statement against two questions in turn — does it need ATP, and does it need a protein. Facilitated transport answers no to the first and yes to the second, and only option (d) says that.

• Facilitated diffusion is passive: it moves a solute down its gradient and spends no ATP.
• It requires a membrane protein — a channel or a carrier — which makes it selective and saturable.
• Simple diffusion of small non-polar molecules such as oxygen needs no protein, so not every passive transport is facilitated.
• Water crosses membranes by osmosis, helped by aquaporin channels, which is still passive.
• Active transport spends ATP and is the only route that can move a solute against its gradient.

• Treating 'facilitated' as a synonym for 'active' because both involve proteins.
• Assuming water needs a pump; water is always passive, and aquaporins are channels, not pumps.
• Reading option (a) too fast — facilitated transport is a subset of passive transport, not the whole of it.

A four-statement item where every distractor is a true idea attached to the wrong term, so it rewards clean definitions rather than broad familiarity.

• practice — not a real PYQ

  Which one of the following moves across the plasma membrane without any protein and without spending ATP?

  • (a)Glucose into a red blood cell
  • (b)Oxygen into a cell
  • (c)Sodium out of a nerve cell
  • (d)Potassium into a nerve cell

  Answer(b) Oxygen into a cell — it is small and non-polar, so it diffuses straight through the lipid bilayer.

• practice — not a real PYQ

  Aquaporins present in a cell membrane are best described as

  • (a)carrier proteins that pump water using ATP
  • (b)channel proteins that speed the passive movement of water
  • (c)enzymes that split water molecules
  • (d)receptors for antidiuretic hormone

  Answer(b) channel proteins that speed the passive movement of water — they raise the rate of osmosis but cannot move water against its potential gradient.