A eucalyptus tree can pull water more than 50 metres upward without a pump, while at the same time moving sugars from leaves to roots, fruits and growing shoots. Plant transport looks passive from the outside, but it depends on tightly coordinated membrane processes and whole-plant pressure systems.
A plant has no heart, no blood and no muscles to squeeze tubes. Yet water moves from roots to leaves and sugars move from leaves to roots.
Before reading on: how do you think a tall plant moves substances around its body? Write your best explanation, even if you are unsure.
Come back to this at the end of the lesson.
Core Content
Plants face the same fundamental challenge as animals: every cell needs water, mineral ions, sugars and gases, but most cells are far from the external environment. Unlike animals, plants cannot solve this by circulating fluid with a muscular pump. Instead, they use a combination of membrane transport at the cell level and specialised vascular tissue at the whole-organism level.
The result is a two-system design:
| Transport system | What it moves | Main direction | Driving mechanism |
|---|---|---|---|
| Xylem | Water and dissolved mineral ions | Mostly roots → stems → leaves | Transpiration pull, cohesion, tension, root uptake |
| Phloem | Sucrose and other organic solutes | Source → sink (can be upward or downward) | Pressure-flow generated by active loading and osmosis |
Transport begins in the roots. Root hair cells are specialised epidermal cells with a long, thin projection that increases surface area. This maximises contact with water films around soil particles.
Water enters root hair cells by osmosis. The cell sap inside the root hair cell has a lower water potential than the soil solution, so water moves across the partially permeable membrane into the cell.
Mineral ions such as nitrate, phosphate and potassium are often at lower concentration in soil than in the root hair cell, so plants use active transport to take them in against the concentration gradient using ATP.
Their long, narrow shape gives a high surface area to volume ratio, shortens diffusion distance and lets the plant explore more soil without building more root tissue.
Once water enters a root hair cell, it moves across the cortex toward the xylem through a combination of cell wall pathways and cytoplasmic pathways. The endodermis, with its Casparian strip, forces water and ions through cell membranes before they enter the vascular tissue, helping the plant regulate what gets in.
Xylem is made of dead, hollow cells joined end to end, forming continuous tubes. Their walls are thickened with lignin, which prevents collapse under tension and also provides structural support.
Xylem transport is mostly unidirectional: from roots to leaves. It is driven mainly by transpiration at the leaf surface rather than by pushing from below.
Water evaporates from moist cell walls inside the leaf and diffuses out through the stomata. This loss of water vapour is called transpiration. When water leaves the leaf, it creates tension in the xylem. Because water molecules cohere to each other and adhere to xylem walls, the whole water column is pulled upward.
Stomata are pores in the leaf epidermis, usually concentrated on the lower surface. Each stoma is surrounded by two guard cells that can open or close the pore.
Environmental conditions that increase transpiration include higher temperature, stronger wind and lower humidity. Conditions that reduce it include high humidity, low temperature and stomatal closure in drought conditions.
Phloem transports sucrose and other dissolved organic substances from sources to sinks.
Unlike xylem, phloem is made of living cells: sieve tube elements supported by companion cells. Companion cells provide ATP and metabolic support for loading sugars into the sieve tubes.
At the source, sucrose is actively loaded into the phloem. This lowers the water potential in the sieve tube, so water moves in from nearby xylem by osmosis. The increased water entry creates high hydrostatic pressure. At the sink, sucrose is unloaded and used or stored, so water potential rises and water leaves the phloem. Sap therefore moves from high pressure at the source to lower pressure at the sink.
Misconception: Plants move water upward by pumping it from the roots.
Root pressure exists in some situations, but the main mechanism in tall plants is transpiration pull working through cohesion and tension in the xylem.
Misconception: Xylem and phloem both just move “sap” around the plant.
Xylem mainly transports water and mineral ions upward. Phloem transports sugars and other organic solutes between sources and sinks, potentially in either direction depending on the organ involved.
Misconception: Stomata only exist to let water out.
Stomata are essential for gas exchange as well. They let CO₂ enter for photosynthesis, but water vapour escapes as a trade-off.
Activities
Construct a comparison of xylem and phloem under the headings: cells alive or dead, contents transported, direction of movement, driving force, and one structural adaptation. Then answer this question:
Why would a xylem vessel fail if it were made of living cells packed with cytoplasm?
Write your explanation here or in your book.
A tomato plant is growing in hot, dry, windy conditions. By midday its leaves wilt. Answer the following:
Write your responses here or in your book.
Assessment
1. Water enters a root hair cell mainly by:
2. Which structural feature of xylem most directly helps prevent collapse under tension?
3. Translocation in phloem moves sugars from:
4. Which environmental change would most likely increase transpiration rate?
5. A plant is placed in very salty soil. Which explanation best predicts the immediate effect on water uptake?
1. Explain how water moves from the soil into the root hair cell, then into the xylem, and finally up to the leaves. (3 marks)
1 mark root uptake; 1 mark movement into xylem; 1 mark cohesion-tension/transpiration pull
2. Compare xylem and phloem in terms of structure, what they transport, and the direction of movement. (3 marks)
1 mark structural comparison; 1 mark contents; 1 mark direction/driving mechanism
3. Explain why closing stomata helps a plant survive drought, but also reduces growth. (3 marks)
1 mark reduced water loss; 1 mark reduced CO2 entry/photosynthesis; 1 mark link to reduced sugar production/growth
Answers
SA1: Water enters root hair cells by osmosis because the root hair cell has a lower water potential than the surrounding soil solution. Water then moves across the cortex toward the xylem, while mineral ions may be actively transported into the vascular tissue. From there, transpiration at the leaf surface creates tension in the xylem, and cohesion between water molecules pulls the continuous water column upward to the leaves.
SA2: Xylem is made of dead, hollow, lignified vessels, while phloem is made of living sieve tube elements and companion cells. Xylem transports water and dissolved mineral ions, whereas phloem transports sucrose and other organic solutes. Xylem movement is mostly upward from roots to leaves, driven by transpiration pull, while phloem transport is source to sink and may move upward or downward depending on where sugars are being produced and used.
SA3: Closing stomata reduces water loss because less water vapour can diffuse out of the leaf. However, it also reduces the entry of carbon dioxide, which is needed for photosynthesis. With less photosynthesis, the plant produces less glucose and therefore less sucrose for respiration, storage and growth, so growth slows.
The key insight is that plants do not need a heart to move substances. Water transport in xylem is driven mostly by transpiration pull and cohesion, while sugar transport in phloem depends on source-sink pressure differences created by active loading and osmosis.
If your initial answer treated plant transport as simple diffusion through the whole organism, update it now with the roles of root hairs, xylem, stomata and phloem.
Sprint through questions on transport mechanisms in plants. Pool: lessons 1–7.
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