Transport in Plants
Plants move water, minerals and food through two transport tissues. This chapter covers xylem and phloem, the transpiration stream that pulls water up, translocation of food, and how plants can clean pollutants (phytoremediation).
The factors that affect transpiration are a common Paper-3 investigation.
Content standard 19.1 goes beyond naming xylem and phloem to expect detail on how each tissue is built for its job. A xylem vessel is a tube of dead, lignified cells joined end to end with their cross-walls broken down, strong enough to resist collapsing under the tension created by transpiration.
Phloem, by contrast, is living: sieve tube elements connect through perforated sieve plates and are supported by companion cells, whose mitochondria supply the energy needed to actively load sugar into the tube at the source before it is carried to a sink.
Water first enters the plant through root hairs, thin extensions of root epidermal cells that greatly increase surface area for absorption; water moves in by osmosis while mineral ions, usually less concentrated in the soil than inside the root, are taken up by active transport. Root pressure adds a small push from below, seen as guttation on humid nights, but the much larger force pulling water to the top of a tall plant is the cohesion-tension created by transpiration, which is why factors changing the rate of transpiration are examined so often.
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Content standards in this chapter
- 19.1 Vascular Tissues (Xylem and Phloem)
- 19.2 Transport of Water and Mineral Salts in Plants
- 19.3 Translocation
- 19.4 Phytoremediation
Key concepts
- Vascular tissues
- Xylem is made of dead, hollow, lignified vessels; phloem is made of living sieve tubes and companion cells.
- Transport of water and minerals
- Water travels up the xylem in the transpiration stream, driven by evaporation from the leaves.
- Transpiration
- The loss of water vapour from the leaves through the stomata, which creates the pull that draws water up.
- Factors affecting transpiration
- Light, temperature, humidity and air movement change the rate of transpiration.
- Translocation
- The movement of sugars in the phloem from sources (leaves) to sinks (growing or storage tissues).
- Phytoremediation
- Using plants to absorb and remove pollutants such as heavy metals from soil or water.
- Root pressure
- Root cells actively pump mineral ions into the xylem, lowering the water potential inside so water moves in by osmosis and builds up a positive pressure that pushes the sap partway up the stem. Root pressure is strongest at night or in humid conditions when transpiration is low, and it can be seen as guttation, small droplets forced out at leaf tips.
- Cohesion-tension theory
- Evaporation of water from the leaf cells creates tension that pulls water upward through the xylem. Water molecules cling to each other by cohesion and to the lignified xylem walls by adhesion, so the pull at the top drags an unbroken column of water all the way from the roots without the column breaking.
- Xylem vessel structure
- A xylem vessel is formed from dead cells joined end to end with their end walls broken down, producing a continuous hollow tube. The walls are strengthened with lignin, which provides support and prevents the vessel from collapsing under the tension created during transpiration, while pits in the wall allow sideways movement of water.
- Phloem structure
- Phloem tissue is built from living sieve tube elements joined end to end through perforated sieve plates, allowing sap to flow between them. Each sieve tube element is supported by an adjacent companion cell, packed with mitochondria that supply the energy needed to actively load sugar into the sieve tube.
- Mechanism of translocation
- At the source, such as a leaf, companion cells actively load sugar into the sieve tubes, lowering the water potential there so water enters by osmosis and raises the pressure. This pressure difference pushes the sugary sap through the sieve tubes towards a sink, such as a growing shoot or storage root, where sugar is removed and used or stored.
- Water and mineral uptake at the root
- Root hairs greatly increase the surface area of the root for absorption. Water enters the root hair cells by osmosis, moving from the soil solution, which has a higher water potential, into the cell sap. Mineral ions are often at a lower concentration in the soil than in the root, so they are absorbed by active transport, which requires energy from respiration.
- Guard cells and stomata
- Each stoma is bordered by two guard cells that change shape as they become turgid or flaccid. When guard cells absorb water and become turgid, their uneven cell walls cause them to curve apart, opening the stoma; when they lose water and become flaccid, the stoma closes, directly controlling the rate of transpiration.
- Phytoremediation mechanisms
- In phytoextraction, plant roots absorb pollutants such as heavy metals from soil or water and store them in the shoots, which can later be harvested and safely removed. In phytostabilisation, plant roots instead bind and immobilise pollutants within the soil, reducing their movement into groundwater without removing them from the site.
- Hyperaccumulator plants
- A hyperaccumulator is a plant species able to absorb and tolerate unusually high concentrations of a specific pollutant, such as a heavy metal, without being poisoned. Examples include ferns and certain grasses used to clean up soil contaminated with metals like arsenic or lead, making them useful tools in phytoremediation projects.
- Investigating transpiration
- A potometer measures the rate of water uptake by a leafy shoot as a proxy for the rate of transpiration, tracked by timing the movement of an air bubble along a graduated capillary tube. Blue cobalt chloride paper, which turns pink as it absorbs moisture, can be pressed onto a leaf surface to compare the rate of water loss from the upper and lower epidermis.
How this chapter is examined
SPM Biology is assessed over three papers. Paper 1 has 40 objective questions (40 marks) in 1 hour 15 minutes; Paper 2 carries 100 marks across Sections A, B and C in 2 hours 30 minutes; Paper 3 is the practical, testing science process skills. Content from this chapter can appear in any of them, so lessons drill recall for Paper 1 and structured answers for Paper 2.
Common exam angles
- Comparing xylem and phloem in structure and function.
- Explaining how each factor changes the rate of transpiration.
- Describing a potometer experiment (Paper 3).
- Explaining root pressure and guttation as evidence for pressure from the root.
- Describing the pressure-flow (source-to-sink) mechanism of translocation.
- Comparing phytoextraction and phytostabilisation as methods of phytoremediation.
Common mistakes
What students write: Saying xylem carries food.
What earns the mark: Xylem carries water and minerals; phloem carries food (sugars) by translocation.
What students write: Writing that transpiration pushes water up.
What earns the mark: Transpiration creates a pull (tension) at the top; water is drawn up, not pushed.
What students write: Confusing high humidity with a high transpiration rate.
What earns the mark: High humidity lowers the rate of transpiration because the air is already moist.
What students write: Calling phloem cells dead.
What earns the mark: Phloem sieve tubes are living; it is xylem vessels that are dead and hollow.
What students write: Saying root pressure alone explains water rise in a tall tree.
What earns the mark: Root pressure only pushes water partway up; the cohesion-tension pull created by transpiration at the leaves is the main force lifting water to the top of a tall plant.
What students write: Describing xylem vessels as living cells.
What earns the mark: Mature xylem vessels are dead, and it is this dead, hollow, lignified structure that lets water pass through freely and resist collapse under tension.
What students write: Saying companion cells conduct the sugar sap themselves.
What earns the mark: Sugar sap flows through the sieve tube elements; the adjacent companion cells supply the energy for active loading but do not carry the main flow of sap.
What students write: Writing that translocation always moves sugar from the roots to the leaves.
What earns the mark: Translocation moves sugar from a source, wherever sugar is being made or released, to a sink, wherever it is being used or stored, so direction depends on the plant's needs, not a fixed root-to-leaf path.
What students write: Saying minerals enter the root by osmosis like water.
What earns the mark: Water enters root hair cells by osmosis, but mineral ions, often at lower concentration outside the root, are absorbed by active transport, which needs energy.
What students write: Saying stomata are always open during the day and closed at night with no exceptions.
What earns the mark: Stomatal opening mainly follows guard cell turgidity driven by light and other factors, so a stoma can close during the day under stress such as very low humidity or high temperature.
What students write: Confusing phytoextraction with phytostabilisation.
What earns the mark: Phytoextraction removes pollutants by storing them in the harvested shoot; phytostabilisation leaves pollutants in the soil but makes them less mobile, so the two achieve different outcomes.
What students write: Saying any plant can be used for phytoremediation equally well.
What earns the mark: Only certain hyperaccumulator species can absorb and tolerate high concentrations of a specific pollutant without being poisoned, so plant choice matters for a phytoremediation project.
What students write: Thinking guttation is caused by transpiration.
What earns the mark: Guttation is caused by root pressure pushing water out of leaf tips, and it happens when transpiration is low, such as at night, not because of transpiration itself.
What students write: Saying a potometer measures transpiration directly.
What earns the mark: A potometer measures the rate of water uptake by the shoot, which is used as an estimate of the transpiration rate, since a small amount of the water taken up is also used in the plant's cells.
Study this chapter
Processes in this chapter
Structures in this chapter
Experiments in this chapter
Frequently asked questions
What is the difference between xylem and phloem?
How does water move up a tall plant?
Which factors affect the rate of transpiration?
What is root pressure and how is it different from the transpiration pull?
How does the pressure-flow mechanism explain translocation?
How do guard cells control transpiration?
What is the difference between phytoextraction and phytostabilisation?
Source:SRC-DSKP-EN, SRC-FORMAT
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