7.05 Transport of Water

Questions

Why Most Stomata are on Lower Epidermis:

• Answer: Reduces water loss as lower epidermis is less exposed to sunlight, helping prevent excessive transpiration.

Factors Affecting Transpiration Rate:

• Increased Wind Speed:
  • Effect: Removes water vapor around leaves faster, maintaining a steep water potential gradient, increasing transpiration.
• Increased Temperature:
  • Effect: Warmer temperatures increase the rate of water evaporation from mesophyll, speeding up transpiration.

Question 1

Describe the pathway of water transport in plants from the soil to the atmosphere. (5 marks)

Mark Scheme:

1. Soil to Roots:
   • Water is absorbed by the roots from the soil through root hairs. (1 mark)
2. Roots to Xylem:
   • Water moves from the root cortex into the xylem vessels. (1 mark)
3. Xylem Transport:
   • Water is transported upwards through the xylem from the roots to the stem and leaves. (1 mark)
4. Leaves to Atmosphere:
   • Water travels within the leaves to mesophyll cells or remains in cell walls. (1 mark)
5. Evaporation:
   • Water evaporates from the mesophyll cell walls into air spaces and exits through stomata into the atmosphere. (1 mark)

Question 2

Explain the role of transpiration in the water transport mechanism of plants. (5 marks)

Mark Scheme:

1. Definition of Transpiration:
   • Transpiration is the loss of water vapor from the plant to the atmosphere, primarily through stomata. (1 mark)
2. Primary Driver:
   • Transpiration creates a water potential gradient by lowering water potential in the leaves. (1 mark)
3. Continuous Flow:
   • This gradient drives the continuous flow of water from soil → roots → xylem → leaves → atmosphere. (1 mark)
4. Cooling Effect:
   • Transpiration helps in cooling the plant by evaporative cooling. (1 mark)
5. Nutrient Transport Support:
   • The flow of water aids in the transport of minerals from the soil to various parts of the plant. (1 mark)

Question 3

What factors affect the rate of transpiration in plants? Provide two examples and explain their effects. (6 marks)

Mark Scheme:

1. Wind Speed:
   • Increased Wind Speed:
     • Removes water vapor from around the leaves more quickly, maintaining a steep water potential gradient, thereby increasing transpiration rate. (2 marks)
2. Temperature:
   • Increased Temperature:
     • Raises the rate of water evaporation from the mesophyll cells, thereby speeding up transpiration. (2 marks)
3. Humidity:
   • Low Humidity:
     • Creates a greater gradient for water vapor to diffuse out, increasing transpiration.
     • High Humidity:
       • Reduces the gradient, thereby decreasing transpiration. (2 marks)

Question 4

Explain the concept of water potential gradients and how they drive water movement in plants. (5 marks)

Mark Scheme:

1. Definition of Water Potential:
   • Water potential is the potential energy of water in a system, measured in pascals (Pa), determining the direction of water movement. (1 mark)
2. High to Low Gradient:
   • Water moves from areas of higher water potential to areas of lower water potential. (1 mark)
3. Water Pathway Driven by Gradient:
   • In plants, water moves from the soil (high water potential) → roots → xylem → leaves → atmosphere (low water potential). (1 mark)
4. Factors Influencing Water Potential:
   • Solute Concentration: Higher solute concentration lowers water potential.
   • Pressure: Positive pressure increases water potential, while negative pressure (tension) decreases it. (1 mark)
5. Role in Transport Mechanism:
   • The water potential gradient ensures the continuous flow of water throughout the plant, supporting nutrient transport and cellular functions. (1 mark)

Question 5

Describe the process of water uptake by roots and its entry into the xylem. (5 marks)

Mark Scheme:

1. Water Absorption by Root Hairs:
   • Water is absorbed from the soil into the root hairs through osmosis due to a higher solute concentration inside root cells. (1 mark)
2. Movement through Root Cortex:
   • Water travels from root hairs through the cortex via the apoplast and symplast pathways. (1 mark)
3. Entry into the Endodermis:
   • Water reaches the endodermis, where the Casparian strip forces it to move through the symplast of endodermal cells. (1 mark)
4. Loading into Xylem Vessels:
   • Water enters the xylem vessels from the endodermis and pericycle through cell membranes. (1 mark)
5. Upward Transport via Xylem:
   • Once in the xylem, water is transported upwards through the plant driven by transpiration pull and root pressure. (1 mark)

Question 6

Explain the role of mesophyll cells in the transpiration process. (5 marks)

Mark Scheme:

1. Location and Structure:
   • Mesophyll cells are located within the leaf tissue, consisting of palisade mesophyll (photosynthesis) and spongy mesophyll (gas exchange). (1 mark)
2. Water Transport to Cell Walls:
   • Water travels from the xylem to the mesophyll cell walls. (1 mark)
3. Evaporation from Cell Walls:
   • Water evaporates from the cell walls into the air spaces within the leaf. (1 mark)
4. Diffusion into Air Spaces:
   • The evaporated water diffuses through the air spaces towards the stomata. (1 mark)
5. Exit Through Stomata:
   • Water vapor exits the leaf through the open stomata, completing the transpiration process. (1 mark)

Question 7

Why are most stomata located on the lower epidermis of leaves? (5 marks)

Mark Scheme:

1. Reduced Exposure to Sunlight:
   • Lower epidermis is less exposed to direct sunlight, minimizing water loss through transpiration. (1 mark)
2. Protection from Wind:
   • Located on the underside, stomata are protected from strong winds, which can increase transpiration rates. (1 mark)
3. Humidity Levels:
   • The lower epidermis often experiences higher humidity, reducing the gradient for water vapor loss. (1 mark)
4. Temperature Regulation:
   • The underside tends to be cooler, decreasing the rate of water evaporation. (1 mark)
5. Energy Conservation:
   • By positioning stomata on the lower epidermis, plants conserve water while still allowing necessary gas exchange for photosynthesis. (1 mark)

Question 8

Describe the role of guard cells in the regulation of stomata. (5 marks)

Mark Scheme:

1. Structure of Guard Cells:
   • Guard cells are specialized parenchyma cells surrounding each stoma. (1 mark)
2. Control of Stomatal Opening:
   • Guard cells change shape by turgor pressure changes, opening the stomata when they swell with water and closing when they lose water. (1 mark)
3. Response to Environmental Signals:
   • Guard cells respond to light, carbon dioxide levels, and humidity, adjusting stomatal opening accordingly. (1 mark)
4. Facilitation of Transpiration and Gas Exchange:
   • By regulating stomatal opening and closing, guard cells control the rate of transpiration and gas exchange (CO₂ intake for photosynthesis and O₂ release). (1 mark)
5. Water Conservation:
   • Guard cells help conserve water by closing stomata during drought conditions or at night when photosynthesis is not occurring. (1 mark)

Question 9

Explain how transpiration contributes to the upward movement of water in plants. (5 marks)

Mark Scheme:

1. Transpiration Pull:
   • Water vapor loss from stomata creates a negative pressure (tension) in the xylem, pulling water upwards from the roots. (1 mark)
2. Continuous Column of Water:
   • Maintains a continuous column of water in the xylem from roots to leaves. (1 mark)
3. Cohesion and Adhesion:
   • Cohesion between water molecules and adhesion to the xylem walls help maintain the integrity of the water column during upward transport. (1 mark)
4. Water Potential Gradient:
   • Transpiration maintains a water potential gradient from the soil (high water potential) to the atmosphere (low water potential), driving water movement. (1 mark)
5. Energy-Free Process:
   • The mechanism of transpiration pull does not require energy (ATP), making it an efficient, passive process for water transport. (1 mark)

Question 10

Discuss how increased wind speed and temperature affect the transpiration rate in plants. (6 marks)

Mark Scheme:

• Increased Wind Speed:
  • Effect: Removes water vapor from around the leaves more quickly, maintaining a steep water potential gradient.
  • Result: Increases transpiration rate by continuously pulling water through the plant. (2 marks)
• Increased Temperature:
  • Effect: Raises the rate of water evaporation from the mesophyll cells and cell walls.
  • Result: Speeds up transpiration by enhancing the evaporation process. (2 marks)
• Combined Impact:
  • Both factors strengthen the transpiration pull, leading to greater water uptake and increased water flow through the plant. (1 mark)
• Potential Consequences:
  • Excessive transpiration can lead to water stress or dehydration if water uptake cannot keep pace with loss. (1 mark)