B6. Transport in Plants - Xylem Flashcards
Structure of the xylem
What are xylem vessels?
How are they adapted and why?
Figure 1: A xylem vessel with internal detail showing.
Xylem vessels are the part of the xylem tissue that actually transports the water and ions. Xylem vessels are very long, tube-like structures formed from dead cells (vessel elements) joined end to end. There are no end walls on these cells, making an uninterrupted tube that allows water to pass up through the middle easily.
Water movement up a plant - Cohesion and tension (4 steps)
- Water evaporates from the leaves at the ‘top’ of the xylem. This is a process called transpiration.
- This creates tension (suction), which pulls more water into the leaf.
- Water molecules are cohesive (they stick together) so when some are pulled into the leaf others follow. This means the whole column of water in the xylem, from the leaves down to the roots, moves upwards.
- Water then enters the stem through the roots.
Factors affecting transpiration rate (4 factors, 3,3,2,3)
- Light intensity:
- the lighter it is the faster the transpiration rate (ie. there’s a positive correlation between light intensity and transpiration rate).
- This is because the stomata open when it gets light to let in CO2 for photosynthesis.
- When it’s dark the stomata are usually closed, so there’s little transpiration.
- Temperature:
- the higher the temperature the faster the transpiration rate.
- Warmer water molecules have more energy so they evaporate from the cells inside the leaf faster.
- This increases the water potential gradient between the inside and outside of the leaf, making water diffuse out of the leaf faster.
- Humidity:
- the lower the humidity, the faster the transpiration rate (i.e. there’s a negative correlation between humidity and transpiration rate).
- If the air around the plant is dry, the water potential gradient between the leaf and the air is increased, which increases transpiration rate.
- Wind:
- the windier it is, the faster the transpiration rate.
- Lots of air movement blows away water molecules from around the stomata.
- This increases the water potential gradient, which increases the rate of transpiration.
Tip: Transpiration’s really a side effect of ________________-the plant needs to open its stomata to let in CO2 so that it can produce glucose, but this also lets water out.
Tip: Transpiration’s really a side effect of photosynthesis-the plant needs to open its stomata to let in CO2 so that it can produce glucose, but this also lets water out.
Estimating transpiration rate - potometers A potometer is a special piece of apparatus used to estimate transpiration rates. It actually measures water uptake by a plant, but it’s assumed that water uptake by the plant is directly related to water loss by the leaves. You can use it to estimate how different factors affect the transpiration rate.
Here’s what you’d do: (8 STEPS)
- Cut a shoot underwater to prevent air from entering the xylem. Cut it at a slant to increase the surface area available for water uptake.
- Assemble the potometer under the water and insert the shoot with the apparatus still under the water, so no air can enter.
- Remove the apparatus from the water but keep the end of the capillary tube submerged in a beaker of water.
- Check that the apparatus is watertight and airtight.
- Dry the leaves, allow time for the shoot to acclimatise and then shut the tap.
- Remove the end of the capillary tube from the beaker of water until one air bubble has formed, then put the end of the tube back into the water.
- Record the starting position of the air bubble.
- Start a stopwatch and record the distance moved by the bubble per unit time, e.g. per hour. The rate of air bubble movement is an estimate of the transpiration rate.
Remember, only change one variable (e.g. temperature) at a time. All other conditions (e.g. light intensity, humidity) must be kept constant.
Figure 5: Diagram showing how to use a potometer.
