9.1 Essential ideas

9.1.1 Transport in the xylem

You probably recall from Chapter 5, that all plant phyla (except Bryophyta) are vasculated, meaning they have vessel elements that conduct substances through the body of the plant. Water and inorganic solutes are moved through vessels called xylem. Xylem is made of the remains of dead cells that run the length of the roots and stem and branch into each individual leaf as part of a vascular bundle.

vascular bundels

Figure 9.1.1a – Xylem forms a continuous tube through the plant’s superstructure

Xylem vessels are long, continuous tubes composed of non-living cellulose cell walls. They are reinforced by annular rings of lignin, a very tough organic polymer. As a result, xylem tubes also play an important role in supporting plant structure.

lignin

Figure 9.1.1b – Lignin

Lignin reinforces the structure of the xylem vessel, like the wire mesh in a garden hose. Xylem vessels play an important role in maintaining the structure of the plant.

The transpiration stream

Xylem sap moves in an uninterrupted stream through the plant, at an average rate of about 15m/hour or faster. At the end of a xylem vessel, water evaporates into the spongy mesophyll layer of the leaf, where it might become involved in photosynthesis. The majority of the water, however, is not used in photosynthesis and instead is released into the atmosphere through small pores on the underside of the leaf called stomata (singular: stoma).

cross-section leaf

Figure 9.1.1c – Cross-section of a leaf

The evaporation of water through stomata is called transpiration. Transpiration is a more or less continuous process, and is an inevitable consequence of gas exchange in leaves. The opening of the stoma is regulated by two bean-shaped guard cells. As long as there is sufficient water present, the guard cells keep the stoma open to allow carbon dioxide in and oxygen out. With stomata open, the moist mesophyll layer is exposed to drier atmospheric air, and water evaporates. Tension is created by the adhesion of water molecules to cell walls as well as by evaporation. This tension pulls the column of water along the xylem vessels. Water has strong cohesive properties, due to the hydrogen bonds between individual molecules, so the column remains an unbroken chain in the xylem vessel. This known as the cohesion-tension mechanism.

stoma open - stoma closed

Figure 9.1.1b – Swollen vacuoles cause the guard cells to open the stoma pore

Transpiration ensures a steady supply of water for photosynthetic plants. This water must be replaced by absorption through the roots. Water is absorbed through the roots by osmosis, and essentially travels upwards through capillary action. At the root, the osmotic gradient is created by active uptake of mineral ions. As minerals are pumped into the root from the soil, water travels down the gradient to maintain root pressure. In other words, the transpiration stream is pushed up by osmosis in the roots and pulled up by evaporation from the leaves. The combination of these two physical forces accounts for the upward movement of water through the xylem vessel. 

Key questions

  • What is transpiration?
  • Which properties of water allow it to move through xylem?
  • What are the consequences of the active uptake of water at the roots?

Extended essay

  • Some of the best EEs are related to plant science. Plants are easy to manipulate and experimental data is repeatable, yielding reliable results. Why not try to link transpiration rates to climate change (Topic 4) or photosynthesis (Topic 2)?

In the lab

  • Have a closer look at stomata and learn to use an important statistical tool. See: Page 9.2.1 
  • Design an experiment to investigate factors that affect transpiration. See: Page 9.2.2