Plant organ system: Root hair cells are adapted for the efficient uptake of water by osmosis, and mineral ions by active transport.

The idea

Root hair cells are epidermal cells on young roots. Each has a long, thin extension that pushes between soil particles, greatly increasing the surface area in contact with soil water. This large surface area speeds up diffusion and osmosis, so water moves efficiently from the soil solution into the cell. Water enters by osmosis because the soil solution is usually more dilute than the cell contents, so water moves down a water potential gradient across the partially permeable membrane. Mineral ions are often at a lower concentration in the soil than inside the cell, so they cannot enter by diffusion alone. The cell uses energy from respiration to move them in by active transport, against the concentration gradient, using carrier proteins in the membrane. Root hair cells therefore contain many mitochondria to release the energy needed.

What earns the marks

• Root hair cells have a long, thin extension that increases the surface area of the cell membrane in contact with the soil solution.
• A large surface area increases the rate of osmosis and diffusion of water into the cell.
• Water enters by osmosis down a water potential gradient through a partially permeable membrane.
• Mineral ions are taken up by active transport, which requires energy from respiration.
• Active transport moves ions against a concentration gradient, often from a dilute soil solution into the cell.
• Root hair cells contain many mitochondria to supply the energy for active transport.
• The adaptations allow efficient uptake because both a short diffusion pathway and a large surface area are provided.

Common mistakes and how to put them right

• Saying water enters by active transport.

  Put it right by stating water moves by osmosis, a passive process, while mineral ions are moved by active transport.

• Saying mineral ions enter by diffusion because they are small.

  Put it right by explaining that ions are often more concentrated inside the cell, so active transport against the gradient is needed.

• Confusing the direction of the water potential gradient.

  Put it right by stating water moves from the more dilute soil solution into the more concentrated cell contents.

• Forgetting that active transport needs energy.

  Put it right by linking the many mitochondria in root hair cells to respiration supplying ATP for carrier proteins.

Exam tips

• Name the process for each substance: osmosis for water and active transport for mineral ions.
• Link each adaptation to its function, for example large surface area to faster uptake.
• Use the phrase against a concentration gradient when explaining active transport.
• Refer to the partially permeable membrane when describing osmosis.
• State that energy comes from respiration, not directly from sunlight.