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Transport of materials and the theories behind it is taught in the note for Transport in Plants. Read it below and look for the section on this topic.

Transport in Plants

12 min read

Why plants need transport

Plants take in water and mineral salts through their roots and make sugars in the leaves. These must be carried to every cell. Xylem carries water and minerals upward from the roots; phloem carries sugars and amino acids to where they are used or stored.

Transport in a plantWater and mineral salts enter through the roots and travel up the stem in xylem vessels to the leaves. Sugars made by photosynthesis in the leaves travel down and around the plant in phloem tubes.soilxylem: water and mineral salts go upphloem: sugars go down and aroundleafleafroots
A root hair cellA root hair cell with a long thin extension that increases its surface area. Water from the soil enters it by osmosis and passes across the root cortex cells to the xylem, which carries it up the plant.root hair: big surface areaxylemcortex cellswater in the soil

Movement of water: from soil to air

Water enters the root hairs by osmosis, crosses the cortex, and enters the xylem. It rises through the stem and leaves the leaf as vapour through the stomata (transpiration). The theory that explains how water can rise to the top of a tall tree is the cohesion–tension theory: evaporation from the leaves creates a pull (tension); water molecules stick together (cohesion) and to the walls of the xylem (adhesion), so the whole column is pulled up. Root pressure and capillarity help only a little.

How water rises in a treeWater evaporates from the spongy cells of the leaves through the stomata, which is transpiration. This pulls water up the narrow xylem vessels in a continuous column, because water molecules stick to each other, cohesion, and to the walls, adhesion. This cohesion–tension theory explains how water reaches the tops of the tallest trees. Root pressure helps a little.water moves upwater vapour lost (transpiration)roots take up watercohesion: water moleculesstick together in a chain

Factors affecting transpiration

FactorEffect on transpirationWhy
LightIncreasesStomata open
TemperatureIncreasesFaster evaporation
HumidityDecreases when highSmaller gradient of water vapour
WindIncreasesRemoves water vapour near the leaf
Water supplyDecreases if shortStomata close

Movement of sugars: translocation

Sugars made in the leaves (the source) are moved through the phloem to the roots, fruits and growing points (the sink). The pressure-flow (mass-flow) hypothesis says that sugar is actively loaded into the phloem, water enters by osmosis and builds up pressure, and the sap flows to the sink where the sugar is removed.

Try itWater lost by a leaf

Water loss depends on the surface area, and how open the stomata are. Choose values for a leaf.

Check yourself Which tissue carries water up a plant?
Check yourself What is the main force pulling water up a tall tree?
Check yourself The movement of sugars in the phloem is called...

Key terms

Transpiration
Loss of water vapour from the leaves.
Cohesion
Attraction between water molecules.
Translocation
Movement of sugars in the phloem.
Sink
A part of the plant that uses or stores sugar.

Summary

  • Xylem carries water and minerals up; phloem carries sugars both ways.
  • Cohesion–tension: transpiration pulls a continuous column of water.
  • Transpiration rises with light, heat and wind and falls with humidity.
  • Phloem transport follows the pressure-flow hypothesis.