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Cell Membranes and Transport: Question 9

Syllabus 4.2

Structured AS 6 marks

The soil solution surrounding a root hair cell contains nitrate ions at a much lower concentration than the concentration of nitrate ions already inside the root hair cell's cytoplasm. Despite this, the root hair cell continues to take up nitrate ions from the soil solution, using a specific carrier protein in its cell-surface membrane.

(a) Explain why the root hair cell cannot take up these nitrate ions by facilitated diffusion, and state the type of transport it must use instead. [2]

(b) Describe, with reference to changes in the shape of the carrier protein, how this type of transport moves a nitrate ion across the membrane. [3]

(c) State one structural difference between the carrier protein described above and a channel protein used in facilitated diffusion. [1]

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Worked solution

Part (a): Why facilitated diffusion cannot be used here

Facilitated diffusion, like simple diffusion, is a passive process: it always moves substances down their concentration gradient, from a region of higher concentration to a region of lower concentration, and never requires metabolic energy. Here, the nitrate ion concentration is lower in the soil solution than inside the root hair cell, so moving nitrate ions from the soil into the cell means moving them against their concentration gradient, from low to high concentration. Facilitated diffusion cannot achieve this. Instead, the cell must use active transport, which can move substances against their concentration gradient because it is powered by energy released from the hydrolysis of ATP.

Part (b): How the carrier protein moves the nitrate ion

The carrier protein involved in active transport works through a cycle of shape changes:

  1. A nitrate ion in the soil solution binds to a specific binding site on the carrier protein, on the outer surface of the cell-surface membrane.
  2. A molecule of ATP is hydrolysed to ADP and inorganic phosphate, and the phosphate group binds to the carrier protein.
  3. This causes the carrier protein to change shape, moving the binding site (with the nitrate ion still attached) through the membrane to the inner (cytoplasmic) surface.
  4. The nitrate ion is released into the cytoplasm, and the inorganic phosphate is also released from the protein.
  5. Losing the phosphate group allows the carrier protein to revert to its original shape, ready to bind another nitrate ion on the outer surface and repeat the cycle.

Part (c): Carrier protein versus channel protein

A carrier protein transports its specific substance by changing shape, cycling between two conformations to move the substance from one side of the membrane to the other. A channel protein, by contrast, forms a fixed (or gated) pore that spans the membrane; it does not change shape to move a substance across, instead, the substance simply passes through the open channel down its concentration gradient.

Final answers

  • (a) Facilitated diffusion only moves substances down a concentration gradient; since nitrate is at a lower concentration outside than inside, the root hair cell must use active transport, which uses ATP to move ions against their concentration gradient.
  • (b) The nitrate ion binds to the carrier protein on the outer surface; ATP hydrolysis (and phosphate binding) causes the protein to change shape, carrying the ion to the inner surface, where it is released; the protein then reverts to its original shape.
  • (c) A carrier protein changes shape to transport its substance; a channel protein forms a pore and does not change shape to transport a substance.