Mass Transport in Plants — Xylem

Xylem

Structure of Xylem

Xylem is a transport tissue in plants responsible for transporting water and dissolved mineral ions from the roots to the leaves.

Key structural features of xylem vessels

• Dead cells with no cytoplasm, nucleus or organelles, creating a hollow lumen allowing water to flow through without obstruction
• Lignified walls — cell walls reinforced with lignin, a waterproof and tough substance that provides structural support and prevents the vessel from collapsing under tension
• No end walls — adjacent xylem cells form long continuous tubes so water can continuously move upwards
• Pits — gaps in the lignified walls allow lateral movement of water between vessels and surrounding cells; if a vessel is blocked or damaged, water can still move up the plant via adjacent vessels

How does water enter the xylem?

Water is absorbed by root hair cells from the soil by osmosis (soil water has a higher water potential than root hair cells).

Cohesion-Tension Theory

How does the cohesion-tension theory work?

1. Transpiration
   • Water evaporates from the surfaces of mesophyll cells in the leaf and diffuses out of the stomata in the leaf
   • This creates a low water potential in the mesophyll cells.
2. Tension
   • As water leaves the mesophyll cells, it creates a pulling force (tension/negative pressure) on the water in the xylem vessels.
   • Water is pulled up the xylem in a continuous column.
3. Cohesion
   • Water molecules are attracted to each other by hydrogen bonds (this is called cohesion), so when water is pulled from the top of the xylem, it pulls the whole column of water upwards
4. Adhesion
   • Water molecules can form hydrogen bonds with other molecules (e.g. xylem vessel walls).
   • This allows water to be pulled up the continuous column in the xylem, and prevents the water column from breaking.
5. Water uptake by roots
   • Water enters through root hair cells and moves into xylem tissue in the root

Transpiration

Transpiration is the loss of water vapour from the leaves of a plant, mainly through the stomata. It is a consequence of gas exchange.

Factors affecting transpiration rate:

• Light intensity
  • High light intensity increases rate of transpiration
  • Stomata open in light for photosynthesis (to let in carbon dioxide)
• Temperature
  • Higher temperature increases rate of transpiration
  • Increases the kinetic energy of water molecules, so they evaporate from the cells at a faster rate
• Humidity
  • Higher humidity decreases the rate of transpiration
  • Higher humidity means the water potential of the surrounding air is higher (less negative), so the water potential gradient between the leaf and the air is reduced, slowing the diffusion of water vapour out of the leaf
• Wind (air movement)
  • Higher air movement increases the rate of transpiration
  • High air movement removes water vapour from around the leaf surface, maintaining a steep water potential gradient

Potometer Experiment

A potometer is used to measure the rate of water uptake by a plant, which can be used as an estimate of the rate of transpiration.

How does a potometer work?

1. A leafy shoot is cut under water (to prevent air entering the xylem)
2. The shoot is sealed into the potometer apparatus with an airtight seal
3. An air bubble is introduced into the capillary tube
4. As the plant transpires, it takes up water, and the air bubble moves along the capillary tube
5. The distance the bubble moves over a set time is measured to calculate the rate of water uptake (e.g. ml per minute)

Using a potometer to investigate factors affecting transpiration

The potometer can be used to test the effect of different environmental conditions on transpiration rate:

• Wind — use a fan at different speeds or distances
• Temperature — place in water baths at different temperatures
• Light intensity — use a lamp at different distances
• Humidity — enclose in plastic bags with wet/dry cotton wool

Exam Questions