Active transport: Active transport allows mineral ions to be absorbed into plant root hairs from very dilute solutions in the soil. Plants require ions for healthy growth.

The idea

Plant root hair cells absorb mineral ions such as nitrate, phosphate and potassium from soil solution. These ions are often present in the soil at a lower concentration than inside the root hair cell, so diffusion would not move them into the cell. Active transport uses energy from respiration and carrier proteins in the cell membrane to move the ions from the more dilute soil solution into the more concentrated cell contents, against the concentration gradient. The ions are then used to make compounds needed for healthy growth, including proteins, DNA and chlorophyll. A root hair cell has a long, thin extension that increases its surface area for absorption, and it has many mitochondria to release the energy needed for active transport.

What earns the marks

• Root hair cells absorb mineral ions such as nitrate, phosphate and potassium from soil solution.
• The soil solution can be more dilute than the cell contents, so the ions are moved against a concentration gradient by active transport.
• Active transport in root hair cells requires energy released by respiration, which is why root hair cells contain many mitochondria.
• The absorbed mineral ions are used by the plant to make substances needed for healthy growth, including proteins, DNA and chlorophyll.
• A root hair cell has a large surface area because of its long, thin extension, which increases the rate of absorption of ions and water.
• If active transport is prevented, for example by a lack of oxygen, the plant cannot absorb enough mineral ions and growth is affected.

Common mistakes and how to put them right

• Writing that mineral ions enter root hair cells by diffusion.

  Put it right: Because the soil solution is more dilute than the cell contents, the ions must be taken in by active transport.

• Saying that plants need mineral ions only for energy.

  Put it right: Mineral ions are used to make proteins, DNA and chlorophyll, while energy comes from respiration of glucose.

• Forgetting to link the structure of the root hair cell to its function.

  Put it right: The long extension increases surface area, and the many mitochondria supply energy for active transport.

Exam tips

• Name at least one mineral ion, such as nitrate or phosphate, and state that it is absorbed from a very dilute soil solution.
• Explain the advantage of active transport by referring to the concentration gradient between the soil solution and the cell contents.
• Link the structure of the root hair cell, including its large surface area and many mitochondria, to the process of absorbing ions.