Lesson 3 of 10 / Nutrition and transport in plants
Root hairs: water and ion uptake
Why do water and mineral ions need different explanations?
In this lesson: Explain root-hair adaptations and distinguish water uptake from uptake of mineral ions.
About 5 min
The key ideaA root hair provides a large surface for uptake. Water enters by osmosis down a water-potential gradient; mineral ions can be actively transported against their concentration gradients.
Water enters by osmosis from higher soil water potential to lower cell water potential, across the partially permeable membrane.
Outer olive line: cell wall. Inner green line: cell membrane. Blue: vacuole. Purple: nucleus. Brown oval: mitochondrion. Arrows show net direction, except the paired equal-state arrows. Original schematic, not to scale.
Explanation
A root hair is a long, narrow extension of an epidermal cell. It increases the surface area in contact with water films around soil particles. A thin cell wall gives a short route for movement into the cell, while the cell membrane controls passage into the cytoplasm.
When soil water has a higher water potential than the root-hair cell contents, water enters by osmosis through the partially permeable cell membrane. Dissolved solutes in the cell help create the lower internal water potential. The direction depends on the actual gradient; water does not always enter simply because it is outside.
Mineral ions such as nitrate ions are needed for growth. If their concentration is lower in the soil solution than inside the cell, uptake against the gradient uses active transport and energy supplied by respiration. Root-hair cells contain mitochondria that support this energy supply.
Water can move across the root cortex towards the xylem through cell walls and through cells. Where water crosses cell membranes, osmosis follows water-potential gradients. Water entering the xylem joins the transpiration stream. Mineral ions are solutes, so their uptake should not be called osmosis.
Step by step
- 1
Name what moves
Separate water molecules from dissolved mineral ions.
- 2
Compare the correct gradient
Use water potential for water and concentration for a named ion.
- 3
Link the adaptation
Connect surface area, a short path or respiratory energy to the relevant uptake process.
Worked example
Uptake from dilute soil solution
Nitrate concentration is lower in soil water than in a root-hair cell, but nitrate still enters. Explain how.
One way to explain it
Nitrate ions are taken up by active transport against their concentration gradient, using energy supplied by respiration in the root-hair cell.
Why this answer works
- The movement is from lower to higher nitrate concentration.
- Name active transport rather than osmosis.
- State the source of energy.
Is this true? "Water and nitrate ions both enter by osmosis."
Osmosis describes water movement only. Nitrate uptake can use active transport when it is against a concentration gradient.