Transport in Plants, revision notes

Overview

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.

19.1 Vascular tissues: xylem and phloem

A flowering plant moves materials through two specialised transport tissues that run together in vascular bundles. Xylem carries water and dissolved mineral salts upward from the roots, while phloem carries dissolved food, mainly sucrose, from where it is made or stored to wherever it is needed.

Knowing the structure of each tissue is the foundation for every other idea in this chapter, because structure explains function.

A mature xylem vessel is a column of dead cells joined end to end, with the cross-walls between them broken down to leave a continuous hollow tube. The walls are thickened and waterproofed with lignin, which supports the plant and stops the vessel collapsing inward under the tension created during transpiration.

Pits in the lignified wall let water pass sideways from one vessel to the next. Because the cells are dead and empty, water meets little resistance as it is drawn up.

Phloem is living tissue. Sieve tube elements are joined end to end through perforated sieve plates, so sap can flow from one element to the next.

Each sieve tube element has lost most of its organelles, so it depends on an adjacent companion cell, which is packed with mitochondria, to supply the energy needed to load sugar into the tube by active transport. This partnership is why phloem transport, unlike the passive rise of water in xylem, needs a living, respiring tissue.

19.2 Uptake of water and mineral salts at the root

Water and mineral ions enter a plant through the root hairs, which are long, thin extensions of root epidermal cells. Each root hair greatly increases the surface area in contact with the soil solution, so absorption is faster.

This is a clear example of structure suiting function at the level of a single cell.

Water enters the root hair by osmosis. The soil solution has a higher water potential than the cell sap inside the root hair, so water moves down the water potential gradient across the partially permeable cell membrane, with no energy needed.

From cell to cell, water then crosses the root cortex towards the xylem at the centre.

Mineral ions such as nitrate, potassium and magnesium are usually at a lower concentration in the soil than inside the root. They therefore cannot diffuse in; instead they are absorbed by active transport, which moves ions against the concentration gradient using energy released by respiration.

This is why a waterlogged soil, low in oxygen, slows mineral uptake: less respiration means less energy for active transport.

19.2 The transpiration stream and the forces that move water

Transpiration is the loss of water vapour from the aerial parts of a plant, mostly through the stomata on the lower surface of the leaf. As water evaporates from the wet cell walls inside the leaf and diffuses out, it pulls on the water behind it.

This creates a tension, or negative pressure, at the top of the xylem.

Water is drawn up as a continuous column because its molecules are attracted to one another by cohesion and to the lignified xylem walls by adhesion. The column does not break, so the pull at the leaf is transmitted all the way down to the roots.

This is the cohesion-tension theory, and the moving stream of water it describes is the transpiration stream.

Root pressure adds a small upward push from below when root cells actively pump ions into the xylem and water follows by osmosis; it is strongest at night and can force droplets out of leaf tips as guttation. Root pressure alone only lifts water part of the way, so in a tall plant the transpiration pull is the main force.

19.3 Translocation and the pressure-flow mechanism

Translocation is the transport of dissolved food in the phloem from a source to a sink. A source is any part that releases sugar, such as a photosynthesising leaf or a storage organ being emptied; a sink is any part that uses or stores sugar, such as a growing shoot, a developing fruit or a storage root.

Because sources and sinks change with the season, phloem can carry sugar in different directions at different times.

The pressure-flow mechanism explains how the sap moves. At the source, companion cells use energy to load sugar into the sieve tubes by active transport.

This lowers the water potential inside the tube, so water enters from the nearby xylem by osmosis and raises the hydrostatic pressure. At the sink, sugar is removed, water leaves, and the pressure falls.

Sap therefore flows from the high-pressure source to the low-pressure sink by mass flow.

19.4 Phytoremediation

Phytoremediation is the use of living plants to remove, immobilise or break down pollutants such as heavy metals in contaminated soil or water. It is a low-cost, low-disturbance method that uses the plant's own transport and storage processes rather than digging out and treating the soil.

In phytoextraction, plant roots absorb pollutants and transport them to the shoots, where they accumulate. The shoots are then harvested and disposed of safely, so the pollutant is removed from the site.

In phytostabilisation, roots instead bind and hold the pollutant in the soil, reducing its movement into groundwater without taking it out of the ground. The two methods have different goals: one removes the pollutant, the other only limits its spread.

Only certain plants, called hyperaccumulators, can take up and tolerate unusually high concentrations of a specific pollutant without being poisoned. Examples include some ferns that accumulate arsenic and certain grasses used on metal-contaminated land.

Because a species usually accumulates one type of pollutant well, the plant must be matched to the contaminant at a site.

Key concepts to master

• 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.

Quick recall checklist

1. Can you define and explain Vascular tissues?
2. Can you define and explain Transport of water and minerals?
3. Can you define and explain Transpiration?
4. Can you define and explain Factors affecting transpiration?
5. Can you define and explain Translocation?
6. Can you define and explain Phytoremediation?
7. Can you define and explain Root pressure?
8. Can you define and explain Cohesion-tension theory?
9. Can you define and explain Xylem vessel structure?
10. Can you define and explain Phloem structure?
11. Can you define and explain Mechanism of translocation?
12. Can you define and explain Water and mineral uptake at the root?
13. Can you define and explain Guard cells and stomata?
14. Can you define and explain Phytoremediation mechanisms?
15. Can you define and explain Hyperaccumulator plants?
16. Can you define and explain Investigating transpiration?

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Source:SRC-DSKP-EN