Water transport in plants and investigating transpiration practical

Describe the function of the xylem

Transports water and mineral ions through the stem, up the plant to the leaves

Describe how xylem tissue is adapted for its function

- Cells joined with no end walls form a continuous long tube, so water flows as a continuous column

- Cells contain no cytoplasm which makes water flow easier

- Thick cell walls with lignin to withstand tension and prevent water loss

- Pits in side walls allow lateral water movements

Explain the cohesion tension theory of water transport in the xylem

Leaf- Water is lost from the leaf by transpiration from the mesophyll cells into open air spaces through the stomata. This reduces the water potential in the mesophyll cells, so water is drawn out of the xylem down the water potential gradient.

Xylem- This creates tension in the xylem. Hydrogen bonds in water result in cohesion between water molecules so water is pulled up as a continuous column. The water also sticks to the walls of the xylem

Root- Water enters roots via osmosis

Evidence to support Cohesion tension

- If a trunk or stem is damaged and a xylem cell is broken water does not leak out, which would happen if the vessels were under pressure. Once air enters, the tree can no longer draw up water because the continuous column of water has been broken

- The trunks of trees reduce in diameter during the daytime when transpiration is at its greatest. This is because adhesion of water molecules to walls of xylem results in a tension which pulls the xylem walls in. A t night when transpiration is at its lowest there is less tension so the diameter increases

Describe the factors that affect transpiration rate

1. Light intensity- Stomata open when it is light to let in co2 for photosynthesis. When its dark, there is not photosynthesis so the stomata close. Little transpiration happens in darkness.

2. Temperature- Water particles have more kinetic energy in warmer conditions so they evaporate from the leaf faster. This increases water potential gradient between the inside and outside of the leaf so the rate of transpiration is increased.

3. Humidity- If the air around the pants is dry, the water potential gradient is increased so transpiration occurs faster. In humid environments the water potential gradient is reduced as there is a lot more water in the atmosphere so the rate of transpiration decreases.

4. Air movement- Lots of wind blows water molecules away from the stomata increasing the water potential gradient and therefore increasing the transpiration rate.

Describe how to set up a potometer

1. Cut a shoot underwater at a slant → prevent air entering xylem and cut at an angle to increases the surface area available for water uptake.

2. Assemble potometer with capillary tube end submerged in a beaker of water

3. Insert shoot underwater

4. Ensure apparatus is watertight / airtight

5. Dry leaves and allow time for plant to

acclimatise

6. Shut tap to reservoir

7. Form an air bubble - quickly remove end

of capillary tube from water

Describe how a potometer can be used to measure the rate of transpiration

1. Record position of air bubble

2. Record distance moved in a certain amount of time (eg. 1 minute)

3. Calculate volume of water uptake in a given time: ○ Use radius of capillary tube to calculate cross-sectional area of water (πr 2 ) ○ Multiply this by distance moved by bubble

4. Calculate rate of water uptake - divide volume by time taken