7.2 Transport Mechanisms

2026 Syllabus Objectives

By the end of this topic, you should be able to:

  1. State that some mineral ions and organic compounds can be transported within plants dissolved in water
  2. Describe the transport of water from the soil to the xylem through the apoplast pathway (including lignin and cellulose) and the symplast pathway (including the endodermis, Casparian strip, and suberin)
  3. Explain that transpiration involves evaporation of water from the internal surfaces of leaves, followed by diffusion of water vapour to the atmosphere
  4. Explain how hydrogen bonding is involved in moving water in the xylem by cohesion-tension and by adhesion to cellulose in cell walls
  5. Make annotated drawings of transverse sections of xerophytic plant leaves to explain how they reduce water loss
  6. State that assimilates dissolved in water (such as sucrose and amino acids) move from sources to sinks in phloem sieve tubes
  7. Explain how companion cells transfer assimilates to phloem sieve tubes, with reference to proton pumps and cotransporter proteins
  8. Explain mass flow in phloem sieve tubes down a hydrostatic pressure gradient from source to sink

1. What Travels Inside Plants, and How?

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Plants need to move many different substances around their bodies — from the roots all the way up to the leaves and back again.

Some substances travel dissolved (mixed) in water. These include:

  • Mineral ions — for example, nitrate ions (used to make proteins) and magnesium ions (used to make chlorophyll). These are absorbed from the soil and carried upward dissolved in water inside the xylem (the water-carrying tubes of the plant).
  • Organic compounds — substances made by the plant, such as sucrose (a type of sugar) and amino acids (the building blocks of protein). These travel dissolved in water inside the phloem (the food-carrying tubes of the plant).

Key idea: Water is the "vehicle" — mineral ions and organic compounds hitch a ride by dissolving in it.

2. How Water Travels from the Soil to the Xylem

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Water enters the plant through the root hair cells (tiny extensions of root cells that hugely increase the surface area for absorption). From there, it must cross the root to reach the xylem vessels, which carry it upward through the plant.

There are two pathways water can take across the root:

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