Lesson 67 - Mass transport in plants - Xylem

Question 1

What are the 2 parts of a plant involved in mass transport?

Answer 1

Xylem - transport of water and mineral ions from the roots up to the leaves
Phloem - transport of sugars (sucrose) in solution from where they are produced in the leaves by photosynthesis to parts of the plant that need it for resp (all cells)

Question 2

What is the structure of the xylem?

Answer 2

• Long, tube-like structures made of dead cells (vessel elements) joined end to end.
• No end walls, making an uninterrupted tube that allows water to pass up through the middle easily.

Question 3

How is water moved up a plant?

Answer 3

1. Water evaporates from the leaves at the ‘top’ of the xylem, process called transpiration.
2. This creates tension (suction) up the leaf.
3. Water molecules are cohesive (stick together), so when some are pulled into the leaf, others follow.
4. There is adhesion between the water molecules and the side of the xylem.
5. This supports a continuous column of water to move up the xylem.
6. Water enters the roots by osmosis through the root hair cells.

Question 4

What is transpiration?

Answer 4

The evaporation of water from a plant’s surface, e.g. the leaves.
Water evaporates from the moist cell walls and accumulates in the spaces between cells in the leaf. When the stomata open, it moves out of the leaf down the water potential gradient.

Question 5

What are the 4 main factors that affect transpiration rate?

Answer 5

Light intensity
Temperature
Humidity
Wind

Question 6

How does light intensity affect transpiration rate?

Answer 6

• The lighter it is, the faster the transpiration rate.
• The stomata open when it gets light to let in CO2 for photosynthesis.

Question 7

How does temperature affect transpiration rate?

Answer 7

• The higher the temp. the faster the transpiration rate.
• Warmer water molecules have more energy, so they evaporate from the cells in the leaf faster. Which increases water potential gradient, so water diffuses out faster.

Question 8

How does humidity affect transpiration rate?

Answer 8

• The lower the humidity, the faster the transpiration rate.
• If the air round the plant is dry, the water potential gradient between the leaf and the air is increased, increasing transpiration rate.

Question 9

How does wind affect transpiration rate?

Answer 9

• 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, increasing rate of transpiration.

Question 10

What is a potometer?

Answer 10

A special piece of apparatus used to estimate transpiration rates. It actually measures water uptake by a plant, assuming that it’s directly related to water loss by the leaves.

Question 11

What is the method for using a potometer?

Answer 11

1. Cut a shoot underwater (to prevent air from entering the xylem). Cut it at a slant to increase the SA available for water uptake.
2. Assemble the potometer under the water and insert the shoot with the apparatus still under the water, so no air can enter.
3. Remove the apparatus from the water but keep the end of the capillary tube submerged in a beaker of water.
4. Check the apparatus is watertight/airtight.
5. Dry the leaves, allow time for the shoot to
   acclimatise and then shut the tap.
6. Remove the end of the capillary tube from the beaker of water until 1 air bubble has formed, then put the end of the tube back into the water.
7. Record the starting position of air bubble.
8. Start a stopwatch and record the distance moved by the bubble per unit time. The rate of air bubble movement is an estimate of the transpiration rate.
9. Remember, only change 1 variable (e.g. temp) at a time. All other conditions (e.g. light intensity, humidity) must be kept constant.

Question 12

How is water taken into the roots, and into the xylem?

Answer 12

• Minerals actively transported into xylem.
• This lowers the water potential in the xylem and water follows by osmosis.