IGCSE Biology 0610: Active Transport of Nitrate Ions in Root Hairs

A hydroponics company grows lettuce plants with their roots suspended directly in a nutrient solution. This solution is kept much less concentrated in nitrate ions than the cell sap inside the root hair cells. Despite this, the root hair cells continue to accumulate nitrate ions from the solution, so the concentration difference between the inside of the cells and the solution keeps growing larger over time.

(a) Name the process by which the root hair cells take up nitrate ions against this concentration gradient, and state where the energy for this process comes from. [2]

(b) Explain how the structure of the root hair cell membrane allows nitrate ions to move against their concentration gradient, referring to protein carriers. [3]

(c) Explain why root hair cells contain unusually large numbers of mitochondria, linking your answer to the process described in (a). [3]

Part (a): Naming the process and its energy source

The root hair cells take up nitrate ions by active transport. Active transport moves particles through a cell membrane from a region of lower concentration to a region of higher concentration, against a concentration gradient, and this requires energy, which comes from respiration taking place inside the root hair cells.

Part (b): The role of protein carriers

The cell membrane of a root hair cell contains protein carriers. Each protein carrier can bind to a specific type of particle, in this case a nitrate ion, on the outside of the membrane. Using energy released by respiration, the protein carrier then changes shape, moving the bound nitrate ion across the membrane and releasing it on the inside of the cell. This happens even though the concentration of nitrate ions inside the cell is already higher than in the surrounding solution, because the protein carrier, rather than the ions moving on their own, actively carries the ions against the gradient. This is different from diffusion, where particles move on their own, down a concentration gradient, with no energy needed and no protein carrier required.

Part (c): Explaining the large number of mitochondria

Active transport is an energy-consuming process: every nitrate ion moved into the root hair cell against its concentration gradient requires energy from respiration. As the nitrate ions continue to accumulate inside the cell, the concentration gradient the cell is working against becomes larger and larger, so a continuous and reliable supply of energy is needed to keep the protein carriers working. Mitochondria are the site of respiration, where this energy is released. Root hair cells therefore contain unusually large numbers of mitochondria so that they can release enough energy, quickly enough, to sustain the active transport of nitrate ions into the cell.

Final answers

(a) Active transport; the energy comes from respiration inside the root hair cells.
(b) Protein carriers in the membrane bind nitrate ions and use energy from respiration to change shape and carry the ions across the membrane, against their concentration gradient.
(c) Because active transport needs a continuous supply of energy from respiration, root hair cells have unusually large numbers of mitochondria to release enough of this energy.