7.8 - Water transport in Plants

Question 1

Steps of using a potometer

Answer 1

1. Cut the shoot underwater at slant to increase the surface area for water uptake.
2. Assemble the potometer with the shoot submerged in water.
3. Keep the capillary tube end of the potometer submerged throughout the experiment.
4. Check that the apparatus is airtight.
5. Dry the leaves, and give the shoot time to acclimatise.
6. Shut the tap, form an air bubble and record its position.
7. Measure the distance the air bubble moves and the time taken.
8. Change one variable at a time and keep everything else constant.

Question 2

Factors affecting the transpiration rate (4):

Answer 2

• Light intensity- At high light intensities, stomata open for maximum CO2absorption for photosynthesis, increasing the transpiration rate.
• Temperature- At high temperatures, evaporation of water molecules is faster due to higher kinetic energy, increasing the transpiration rate.
• Humidity- Low humidity increases the water vapour gradient between the leaf and atmosphere, increasing the transpiration rate.
• Wind speed - High wind speeds increase the water vapour gradient between the leaf and atmosphere, increasing the transpiration rate.

Question 3

Why does transpiration occur?

Answer 3

1. Water evaporates (changes from liquid water into gaseous water vapour) from the moist surfaces of mesophyll cells.
2. Stomata open so they can absorb carbon dioxide for photosynthesis.
3. This provides a pathway for water vapour loss through the open stomata.
4. Water vapour moves down a water potential gradient from the air spaces in the leaf into the atmosphere

Question 4

Cohesion tension theory (3 factors):

Answer 4

• Cohesion- Hydrogen bonding causes water molecules to stick together and move as one continuous column.
• Adhesion- Hydrogen bonding between polar water molecules and non-polar cellulose in xylem vessel walls pulls water upwards through the xylem.
• Transpiration pull- Evaporation of water at leaves creates the transpiration pull, and this tension is transmitted down the whole water column due to cohesion.

Question 5

Calculating the rate of transpiration

Answer 5

Question 6

How does water move through a plant

Answer 6

1. Water enters a plant’sroot hair cells via osmosis.
2. It moves through the cell cytoplasm or cell walls towards the xylem.
3. The xylem transports water from the roots up to the leaves.
4. Water is used for photosynthesis.
5. Some water evaporates from leaf cells by transpiration and diffuses out of the plant.