By the end of this lesson
- Explain water uptake by osmosis.
- Relate root-hair structure to absorption.
- Connect mineral-ion uptake to water potential.
BIG QUESTION
How does water cross from soil into a root against the pull of gravity?
Root hairs create a large exchange area; water follows a water-potential gradient into cells.
Root hairs extend into soil spaces
A root hair cellA root epidermis cell with a long extension that gives a large surface area for absorbing water by osmosis and mineral ions by active transport. has a long projection that greatly increases contact area with films of water around soil particles.
A thin wall and membrane give a short path into the cell.
Water follows a water-potential gradient
Soil water commonly has a higher water potential than root-hair cytoplasm. Water therefore enters through the partially permeable membrane by osmosisThe net movement of water molecules, through a partially permeable membrane, from a region of higher water potential to a region of lower water potential; a dilute solution has a higher water potential than a concentrated one..
Water then moves across cortex cells and into xylem, following water-potential gradients created by transpiration.
Mineral ions alter the gradient
Root cells absorb ions such as nitrate and magnesium, often by active transport when uptake is against a concentration gradient.
Solutes lower cell water potential, helping maintain water entry. Respiration supplies ATP for active transport, not for osmosis itself.
Water and mineral uptake
Both enter roots but by different mechanisms.
| Feature | Water | Mineral ions |
|---|---|---|
| Main process | Osmosis. | Diffusion or active transport. |
| Membrane requirement | Partially permeable membrane. | Specific carrier/channel proteins. |
| Energy | No direct ATP use. | Active uptake requires ATP. |
| Gradient | Down water-potential gradient. | Can move against concentration gradient. |
Language that earns precision
- root hair cell
- Epidermal root cell with a projection that increases absorption area.
- water potential
- A measure of water’s tendency to move; water moves from higher to lower potential.
- active transport
- Energy-requiring movement against a concentration gradient.
Common misconceptions
Claim: Roots suck water from soil.
Correction: Water crosses root-hair membranes by osmosis down a water-potential gradient.
Claim: Osmosis uses energy from respiration.
Correction: It is passive; respiration powers active ion transport.
Claim: Root hairs are tiny roots.
Correction: Each is a projection from one epidermal cell.
Match the answer to the command word
State higher soil water potential, partially permeable membrane and lower cell water potential.
Do not describe mineral-ion active transport as osmosis.
Recall, apply, then justify
Explain how a root hair cell absorbs water.
Hint
Potential, membrane, direction, process.
Suggested marking points
- Soil water has a higher water potential than the cell sap.
- The cell membrane is partially permeable.
- Water moves down the water-potential gradient.
- The process is osmosis.
Explain two root-hair adaptations for absorption.
Hint
Area and distance.
Suggested marking points
- The long projection provides a large surface area.
- This increases contact with soil water/mineral ions.
- The wall/membrane provides a short diffusion path.
- Many mitochondria can supply ATP for active mineral-ion uptake.
Predict the effect of waterlogged, oxygen-poor soil on mineral and water uptake.
Planning hint
Connect oxygen to ATP and solute gradients.
What a strong design includes
- Less oxygen reduces aerobic respiration.
- Less ATP is available for active ion transport.
- Fewer mineral ions accumulate in root cells.
- Cell water potential may be less negative.
- The water-potential gradient from soil may become smaller.
- Water uptake and growth can decline.