# Water uptake and movement

> Biology · CIE A-Level
> Source: https://www.owlsprep.com/study/cie-9700-u7-water-uptake-and-movement/

This sub-topic covers how water enters plant roots, moves through root tissues to the xylem, and travels up the plant stem to the leaves. You will learn key movement pathways and core biological mechanisms driving water transport.

**Prerequisites:** [Osmosis and water potential](https://www.owlsprep.com/study/cie-9700-u2-osmosis-water-potential/); [Plant cell and tissue structure](https://www.owlsprep.com/study/cie-9700-u1-plant-cell-structure/)

## Learning objectives

- Explain how water is taken up by root hair cells from soil
- Describe the three pathways of water movement through plant roots
- Explain the cohesion-tension theory of water movement up the xylem
- Outline the movement of water from leaf xylem to the atmosphere

## Water Uptake by Root Hair Cells

**Root hair cell** — Specialised epidermal cell on plant roots adapted for maximum water and mineral ion uptake from soil

*Example:* A single mature root can have over 100 million root hairs, increasing root surface area by up to 100x

Root hair cells have several key adaptations: a long thin projection to maximise surface area for absorption, a thin cell wall for a short diffusion distance, and a lower (more negative) water potential than soil water due to accumulated solutes in the vacuole.

**Worked example:** Explain why water moves into a root hair cell from soil, where soil water potential is -20 kPa and the root hair vacuole water potential is -300 kPa

1. Water moves by osmosis down a water potential gradient, from higher (less negative) water potential to lower (more negative) water potential, across a selectively permeable membrane.
2. In this example, soil has a higher water potential (-20 kPa) than the root hair cell vacuole (-300 kPa), creating a steep downward gradient.
3. Water therefore moves into the root hair cell across its cell surface membrane by osmosis.

> **Exam tip:** Always reference water potential gradients when explaining osmosis in plant transport, do not just describe differences in concentration.

## Water Movement Pathways Through the Root

After entering the root hair cell, water moves across the root cortex to reach the endodermis and xylem, via three possible pathways:

**Apoplast Pathway** — Water moves through non-living components: the porous cell walls and the intercellular gaps between cells. Water never crosses a cell membrane in this pathway.

**Symplast Pathway** — Water moves through the living cytoplasm of cells, which is connected between adjacent cells via plasmodesmata (cytoplasmic bridges).

**Vacuolar Pathway** — Water moves through the vacuoles of each cell, in addition to the cytoplasm and plasmodesmata. This is the slowest pathway for water movement.

- At the endodermis (the innermost layer of the root cortex), the apoplast pathway is blocked by the Casparian strip, a waxy, waterproof layer in endodermal cell walls.
- This blockage forces all water to cross the endodermal cell membrane, giving the plant control over which mineral solutes enter the xylem.

**Worked example:** Explain why all water moving to the xylem must pass through the cytoplasm of endodermal cells

1. Endodermal cells have a waxy waterproof layer called the Casparian strip embedded in their cell walls.
2. The Casparian strip blocks the apoplast pathway, which is the movement of water through cell walls.
3. Water cannot bypass endodermal cells via the cell wall, so it must cross the cell membrane and enter the endodermal cytoplasm to reach the xylem.

> **Exam tip:** Make sure to explicitly state that the Casparian strip only blocks the apoplast pathway, not all water movement.

## Mechanisms of Water Movement Up the Xylem

Water must move from the roots, up the xylem in the stem, to the leaves even in very tall trees. The main accepted mechanism for this movement is the cohesion-tension theory, with root pressure contributing only a minor effect in small plants.

**Cohesion-tension Theory** — Model where transpiration (evaporation of water from leaves) creates negative tension that pulls continuous columns of water up the xylem, held together by cohesive hydrogen bonds between water molecules.

> **info**
>
> Adhesion, the attraction between water molecules and the hydrophilic cellulose walls of xylem, also helps prevent the water column from breaking under gravity.

**Worked example:** Explain two adaptations of xylem vessels that make them well-suited for water transport according to cohesion-tension theory

1. First, xylem vessels are dead, hollow cells with no cytoplasm or organelles, which means there is very little resistance to the flow of large volumes of water.
2. Second, xylem cell walls are thickened with lignin, which is strong and allows the vessels to resist collapse under the negative tension created by transpiration pull.
3. Third (a third adaptation), their narrow diameter helps maintain the continuous water column via adhesion between water and the cell wall.

## Water Movement From Leaf to Atmosphere

Once water reaches the xylem in the leaf, it moves out into the leaf mesophyll cells along a water potential gradient. Water evaporates from the surface of mesophyll cell walls into the air spaces inside the leaf, then diffuses out through open stomata to the atmosphere in a process called transpiration.

**Worked example:** Outline the sequence of water movement from leaf xylem to the atmosphere outside the leaf

1. Leaf mesophyll cells have a lower water potential than leaf xylem due to continuous evaporation, so water moves out of the xylem into mesophyll cells.
2. Water evaporates from the surface of mesophyll cell walls into the air spaces inside the leaf.
3. Water vapour potential is higher inside the leaf than outside, so water vapour diffuses out through open stomata into the atmosphere.

## Common pitfalls

- **Wrong:** Saying water moves into root hair cells by diffusion, not osmosis
  - Why it fails: Diffusion describes movement of solutes/gases down their own concentration gradient; water movement across a selectively permeable membrane is specifically osmosis
  - Correct: Always state water movement into plant cells occurs by osmosis down a water potential gradient
- **Wrong:** Claiming the Casparian strip blocks all water movement through the endodermis
  - Why it fails: It only blocks the apoplast (cell wall) pathway, it does not stop water movement entirely
  - Correct: State that the Casparian strip blocks the apoplast pathway, forcing water through the symplast pathway to allow control over solute entry
- **Wrong:** Confusing cohesion and adhesion when describing cohesion-tension theory
  - Why it fails: Many students swap the terms: cohesion is attraction between water molecules, adhesion is between water and xylem walls
  - Correct: Cohesion holds the water column together; adhesion prevents the column from breaking. Remember: *Co*hesion = *co*mpatible water molecules sticking together
- **Wrong:** Claiming root pressure is the main force driving water movement up tall trees
  - Why it fails: Root pressure is a weak positive pressure that only contributes to water movement in small, short plants
  - Correct: Always state that cohesion-tension (driven by transpiration pull) is the main mechanism for water movement up the xylem in all plants, especially tall trees

## Cheatsheet

| Component | Description | Key Exam Fact |
| --- | --- | --- |
| Apoplast pathway | Through cell walls / intercellular spaces | Blocked by Casparian strip at endodermis |
| Symplast pathway | Through cytoplasm / plasmodesmata | Always open, controlled by cell membrane |
| Vacuolar pathway | Through cell vacuoles | Slowest pathway for water movement |
| Cohesion-tension | Main mechanism for xylem transport | Driven by transpiration pull, cohesive water molecules |
| Root pressure | Weak positive pressure from ion uptake | Only minor role in small plants |
| Casparian strip | Waxy layer in endodermal cell walls | Forces water through symplast, controls solute entry |

## What's next

Mastering water uptake and movement provides the foundation for understanding transpiration, plant adaptations to different environments, and the transport of organic solutes in phloem. This sub-topic is frequently assessed alongside required practicals investigating transpiration rate, and links to questions about plant responses to water stress. Understanding the cohesion-tension theory and water pathways will help you answer both multiple choice and extended response questions on plant transport in the CIE exam.

- [Transpiration](https://www.owlsprep.com/study/cie-9700-u7-transpiration/)
- [Translocation](https://www.owlsprep.com/study/cie-9700-u7-translocation/)
- [Xerophyte adaptations](https://www.owlsprep.com/study/cie-9700-u7-xerophyte-adaptations/)

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