7.10 Water: Leaves → atmosphere

Question 1

Describe the structure and function of xylem vessels in leaves. (5 marks)

Mark Scheme:

1. Location of Xylem Vessels:
   • Xylem vessels are located within the vascular bundles (veins) of the leaves. (1 mark)
2. Continuous Pathway:
   • Xylem vessels in leaves are continuous with those in the stem, providing an uninterrupted pathway for water transport from roots to leaves. (1 mark)
3. Cell Composition:
   • Xylem vessels are composed of dead, lignified cells that form hollow tubes. (1 mark)
4. Lignin in Cell Walls:
   • The cell walls contain lignin, which provides strength and makes them waterproof, preventing water loss from the vessels. (1 mark)
5. Facilitation of Water Transport:
   • The structure of xylem vessels allows for efficient upward movement of water and dissolved minerals through the plant. (1 mark)

Question 2

Explain how water exits xylem vessels into leaf tissues. (5 marks)

Mark Scheme:

1. Movement Through Pits:
   • Water moves out of the xylem vessels through pits, which are unlignified areas in the xylem vessel walls. (1 mark)
2. Entry into Bundle Sheath Cells:
   • Through pits, water enters the bundle sheath cells, which are intermediary cells surrounding the vascular tissue. (1 mark)
3. Facilitation by Bundle Sheath Cells:
   • Bundle sheath cells facilitate the transfer of water from the xylem vessels into the mesophyll cells. (1 mark)
4. Regulation of Water Flow:
   • Bundle sheath cells regulate the movement of water and minerals, ensuring selective uptake into the leaf tissues. (1 mark)
5. Connection to Mesophyll Cells:
   • From bundle sheath cells, water can move into mesophyll cells via symplastic or apoplastic pathways. (1 mark)

Question 3

Compare the symplastic and apoplastic pathways in water movement within leaf tissues. (6 marks)

Mark Scheme:

1. Definition of Symplastic Pathway:
   • Water moves from cell to cell through the cytoplasm via plasmodesmata. (1 mark)
2. Definition of Apoplastic Pathway:
   • Water travels through the cell walls and intercellular spaces without entering the cytoplasm. (1 mark)
3. Regulation of Water Movement:
   • Symplastic Pathway allows for selective uptake and regulated movement of water and solutes. (1 mark)
   • Apoplastic Pathway provides rapid movement of water but is less regulated. (1 mark)
4. Role of the Endodermis:
   • The Casparian strip blocks the apoplastic pathway at the endodermis, forcing water to enter the symplast. (1 mark)
5. Efficiency and Speed:
   • Apoplastic Pathway is faster, facilitating rapid water movement to areas needed for transpiration.
   • Symplastic Pathway is slower but allows for control and selectivity. (1 mark)

Question 4

Explain the role of the endodermis and Casparian strip in regulating water and mineral uptake in dicot roots. (5 marks)

Mark Scheme:

1. Location and Composition:
   • The endodermis is a single layer of tightly packed cells surrounding the vascular tissues in roots.
   • The Casparian strip is composed of suberin and lignin, making it waterproof. (1 mark)
2. Barrier to Apoplastic Flow:
   • The Casparian strip blocks the apoplastic pathway, forcing water and minerals to cross cell membranes into the symplast of endodermal cells. (1 mark)
3. Selective Uptake:
   • This selective barrier ensures that only necessary minerals and water are actively transported into the xylem, preventing the leakage of unwanted substances. (1 mark)
4. Regulation of Ion Absorption:
   • The Casparian strip allows the plant to regulate ion uptake by controlling which ions pass through the plasma membranes of endodermal cells. (1 mark)
5. Prevention of Backflow:
   • It prevents the backflow of solutes from the xylem into the surrounding cortex, maintaining the integrity of the water and mineral transport system. (1 mark)

Question 5

Explain how active transport of mineral ions by root hair cells affects water uptake. (5 marks)

Mark Scheme:

1. Active Transport Definition:
   • Active transport involves the movement of mineral ions against their concentration gradients using energy (ATP). (1 mark)
2. Ion Accumulation in Root Hair Cells:
   • Root hair cells actively transport ions (e.g., K⁺, Ca²⁺, NO₃⁻) into the cytoplasm, increasing the solute concentration inside the cells. (1 mark)
3. Lowering of Water Potential:
   • The accumulation of ions lowers the water potential (Ψ) inside the root hair cells, making it more negative compared to the surrounding soil water. (1 mark)
4. Enhanced Osmotic Gradient:
   • A steeper osmotic gradient is established, driving more water to enter the root hair cells via osmosis from the soil. (1 mark)
5. Support for Xylem Transport:
   • Increased water uptake due to active ion transport supports the continuous flow of water through the xylem, aiding overall plant hydration and nutrient transport. (1 mark)

Question 6

Describe the structure of xylem and how it is adapted for efficient water transport. (5 marks)

Mark Scheme:

1. Xylem Composition:
   • Xylem consists of xylem vessel elements and tracheids, which are dead, lignified cells that form continuous tubes for water transport. (1 mark)
2. Cell Walls and Lignin:
   • Xylem cell walls are thick and lignified, providing strength and waterproofing, preventing the collapse of vessels under negative pressure (tension). (1 mark)
3. Formation of Xylem Vessels:
   • Xylem vessel elements join end-to-end, with their end walls dissolving to form continuous tubes that allow uninterrupted water flow. (1 mark)
4. Pits in Xylem Cells:
   • Pits are unlignified areas in the xylem cell walls that allow lateral water movement between vessels and neighboring cells, maintaining water flow continuity. (1 mark)
5. Narrow Diameter of Xylem Vessels:
   • Xylem vessels often have a narrow diameter, which reduces the risk of air bubbles (air locks) disrupting the water column, ensuring efficient transport. (1 mark)

Question 7

Explain the cohesion-tension theory and its components in the context of water movement in plants. (6 marks)

Mark Scheme:

1. Cohesion-Tension Theory Overview:
   • The cohesion-tension theory explains how transpiration generates a tension (negative pressure) that pulls water upwards through the xylem. (1 mark)
2. Cohesion of Water Molecules:
   • Water molecules exhibit cohesion due to hydrogen bonding, creating a continuous column of water from the roots to the leaves. (1 mark)
3. Adhesion to Xylem Walls:
   • Adhesion of water molecules to the xylem cell walls helps stabilize the water column and prevent it from breaking under tension. (1 mark)
4. Transpiration Pull:
   • Transpiration at the leaf surfaces evaporates water, creating a negative pressure (tension) that pulls water upwards through the xylem. (1 mark)
5. Continuous Water Flow:
   • The cohesion and adhesion properties ensure a continuous flow of water, allowing for the uptake of water and dissolved minerals from the roots to the leaves. (1 mark)
6. Energy-Free Process:
   • The process relies on physical forces (cohesion and tension) rather than biochemical energy (ATP), making it an efficient, passive mechanism for water transport. (1 mark)

Question 8

Discuss the step-by-step process of water movement within leaves, from xylem to transpiration. (6 marks)

Mark Scheme:

1. Arrival at Leaf Xylem:
   • Water arrives at the leaves through xylem vessels located within the vascular bundles (veins). (1 mark)
2. Exit from Xylem to Bundle Sheath Cells:
   • Water moves out of the xylem vessels through pits into bundle sheath cells. (1 mark)
3. Transfer to Mesophyll Cells:
   • From bundle sheath cells, water moves into mesophyll cells via symplastic or apoplastic pathways. (1 mark)
4. Utilization in Photosynthesis:
   • Water is used in the light-dependent reactions of photosynthesis, being split into oxygen, protons, and electrons. (1 mark)
5. Evaporation from Mesophyll Cells:
   • Excess water moves from mesophyll cell walls into intercellular air spaces within the leaf. (1 mark)
6. Diffusion Through Stomata:
   • Water vapor diffuses out of the leaf through stomatal openings into the atmosphere, completing the transpiration process. (1 mark)

Question 9

Explain how environmental conditions such as temperature and wind influence the transpiration rate and water uptake in plants. (6 marks)

Mark Scheme:

1. High Temperature Effects:
   • Increased temperature raises the rate of water evaporation from mesophyll cells, increasing the transpiration rate. (1 mark)
2. Low Humidity Effects:
   • Low humidity creates a steeper water vapor gradient, enhancing the transpiration pull and increasing water uptake. (1 mark)
3. Wind Effects:
   • Wind removes the saturated air around the stomata, maintaining a high transpiration rate by preventing reabsorption of water vapor. (1 mark)
4. Transpiration Pull Enhancement:
   • Increased transpiration due to high temperature and wind strengthens the transpiration pull, promoting more efficient water transport. (1 mark)
5. Water Stress Risks:
   • Excessive transpiration can lead to water stress or dehydration if water uptake cannot keep pace with water loss. (1 mark)
6. Adaptive Responses:
   • Plants may adjust stomatal opening or alter leaf orientation in response to these factors to regulate transpiration and maintain water balance. (1 mark)

Question 10

Describe the importance of pits in xylem vessel walls for maintaining water transport in plants. (5 marks)

Mark Scheme:

• Definition of Pits:
  • Pits are unlignified areas in the xylem vessel walls that create pores. (1 mark)
• Facilitation of Lateral Water Movement:
  • Pits allow water to move laterally from one xylem vessel to another and into adjacent cells, maintaining continuous water flow. (1 mark)
• Prevention of Air Locks:
  • By enabling lateral bypassing around obstructions, pits help prevent the formation of air bubbles (embolisms) that could disrupt the water column. (1 mark)
• Maintenance of Water Column Integrity:
  • Pits support the cohesion-tension mechanism by maintaining the integrity of the water column under negative pressure. (1 mark)
• Adaptation to Tall Plants:
  • In tall plants, pits ensure efficient upward transport of water over long distances, reducing the risk of transport disruptions. (1 mark)

Question 11

Describe how xylem vessels in leaves are adapted to facilitate efficient water transport. (5 marks)

Mark Scheme:

1. Continuous Pathway:
   • Xylem vessels in leaves are continuous with those in the stem, providing an uninterrupted pathway for water transport from roots to leaves. (1 mark)
2. Lignified Cell Walls:
   • Xylem vessel elements have lignified cell walls, which provide strength and waterproofing, preventing collapse under negative pressure (tension). (1 mark)
3. Hollow Structure:
   • Xylem vessels are hollow tubes, allowing unobstructed flow of water and dissolved minerals. (1 mark)
4. Narrow Diameter:
   • The narrow diameter of xylem vessels reduces the risk of air bubbles (embolisms) disrupting the water column. (1 mark)
5. Pits in Vessel Walls:
   • Pits are unlignified areas in the vessel walls that allow lateral water movement, maintaining continuous water flow. (1 mark)

Question 12

Explain how pits in xylem vessel walls contribute to the maintenance of the water column in plants. (5 marks)

Mark Scheme:

1. Definition of Pits:
   • Pits are unlignified areas in the xylem vessel walls, creating pores. (1 mark)
2. Lateral Water Movement:
   • Pits allow water to move laterally between adjacent xylem vessels and into neighboring cells, ensuring continuous water flow. (1 mark)
3. Prevention of Air Locks:
   • By enabling lateral bypassing around obstructions, pits help prevent the formation of air bubbles (embolism) that could disrupt the water column. (1 mark)
4. Maintenance of Water Column Integrity:
   • Pits support the cohesion-tension mechanism by maintaining the integrity of the water column under negative pressure. (1 mark)
5. Adaptation to Tall Plants:
   • In tall plants, pits ensure efficient upward transport of water over long distances, reducing the risk of transport disruptions. (1 mark)

Question 13

Describe the role of bundle sheath cells in the movement of water from xylem vessels to mesophyll cells. (5 marks)

Mark Scheme:

1. Intermediary Role:
   • Bundle sheath cells act as intermediary cells facilitating the transfer of water from xylem vessels to mesophyll cells. (1 mark)
2. Water Movement Through Pits:
   • Water exits the xylem vessels through pits and enters the bundle sheath cells. (1 mark)
3. Symplastic and Apoplastic Pathways:
   • From bundle sheath cells, water moves into mesophyll cells via symplastic or apoplastic pathways. (1 mark)
4. Selective Regulation:
   • Bundle sheath cells regulate the movement of water and minerals, ensuring selective uptake into the leaf tissues. (1 mark)
5. Facilitation of Efficient Transport:
   • They help in coordinating water flow, maintaining efficient transport to areas where it is needed for transpiration and photosynthesis. (1 mark)

Question 14

Explain how symplastic and apoplastic pathways differ in water movement within leaf tissues and their significance. (6 marks)

Mark Scheme:

1. Definition of Symplastic Pathway:
   • Water moves cell-to-cell through the cytoplasm via plasmodesmata. (1 mark)
2. Definition of Apoplastic Pathway:
   • Water travels through the cell walls and intercellular spaces without entering the cytoplasm. (1 mark)
3. Regulation of Movement:
   • Symplastic Pathway allows for selective uptake and regulated movement of water and solutes. (1 mark)
   • Apoplastic Pathway provides rapid movement of water but is less regulated. (1 mark)
4. Role of the Endodermis:
   • The Casparian strip blocks the apoplastic pathway at the endodermis, forcing water to enter the symplast. (1 mark)
5. Efficiency and Speed Comparison:
   • Apoplastic Pathway is faster, facilitating rapid water movement to areas needed for transpiration.
   • Symplastic Pathway is slower but allows for more precise control over water and nutrient uptake. (1 mark)

Question 15

Discuss how the cohesion-tension theory explains the upward movement of water in plants. (6 marks)

Mark Scheme:

1. Cohesion-Tension Theory Overview:
   • The cohesion-tension theory explains how transpiration generates a tension (negative pressure) that pulls water upwards through the xylem. (1 mark)
2. Cohesion of Water Molecules:
   • Water molecules exhibit cohesion due to hydrogen bonding, creating a continuous column of water from the roots to the leaves. (1 mark)
3. Adhesion to Xylem Walls:
   • Adhesion of water molecules to the xylem cell walls helps stabilize the water column and prevent it from breaking under tension. (1 mark)
4. Transpiration Pull:
   • Transpiration at the leaf surfaces evaporates water, creating a negative pressure (tension) that pulls water upwards through the xylem. (1 mark)
5. Continuous Water Flow:
   • The cohesion and adhesion properties ensure a continuous flow of water, allowing for the uptake of water and dissolved minerals from the roots to the leaves. (1 mark)
6. Energy-Free Process:
   • The process relies on physical forces (cohesion and tension) rather than biochemical energy (ATP), making it an efficient, passive mechanism for water transport. (1 mark)

Question 16

Explain how transpiration creates a negative pressure in the xylem and its effect on water movement. (5 marks)

Mark Scheme:

1. Definition of Transpiration:
   • Transpiration is the loss of water vapor from stomata in leaves. (1 mark)
2. Creation of Negative Pressure:
   • As water vapor exits the leaf, it evaporates from the mesophyll cells into the air spaces, creating a negative pressure (tension) in the xylem. (1 mark)
3. Transpiration Pull:
   • This negative pressure pulls water upwards through the xylem vessels from the roots to the leaves. (1 mark)
4. Cohesion-Tension Mechanism:
   • The cohesion between water molecules maintains the continuous water column, ensuring that the pull affects the entire column from roots to leaves. (1 mark)
5. Effect on Water Movement:
   • The negative pressure generated by transpiration drives the mass flow of water and dissolved minerals against gravity, maintaining the plant’s hydration and nutrient supply. (1 mark)

Question 17

Describe how guard cells regulate transpiration and the impact on water movement within the plant. (5 marks)

Mark Scheme:

1. Function of Guard Cells:
   • Guard cells control the opening and closing of stomata. (1 mark)
2. Stomatal Opening:
   • When guard cells take up potassium ions (K⁺) and water, they inflate, causing the stomata to open. (1 mark)
3. Stomatal Closing:
   • Under water stress or high salt concentrations, guard cells lose K⁺ ions and water, causing them to deflate and close the stomata. (1 mark)
4. Regulation of Transpiration Rate:
   • Open stomata increase transpiration rates, enhancing the transpiration pull and water uptake.
   • Closed stomata reduce transpiration, conserving water within the plant. (1 mark)
5. Impact on Water Movement:
   • By regulating stomatal aperture, guard cells balance the need for CO₂ uptake for photosynthesis with the need to conserve water, thus controlling water movement within the plant. (1 mark)

Question 18

Explain how environmental factors such as temperature and wind influence the transpiration rate and consequently water uptake in plants. (6 marks)

Mark Scheme:

1. High Temperature Effects:
   • Increased temperature raises the rate of water evaporation from mesophyll cells, increasing the transpiration rate. (1 mark)
2. Low Humidity Effects:
   • Low humidity creates a steeper water vapor gradient, enhancing the transpiration pull and increasing water uptake. (1 mark)
3. Wind Effects:
   • Wind removes the saturated air around the stomata, maintaining a high transpiration rate by preventing reabsorption of water vapor. (1 mark)
4. Transpiration Pull Enhancement:
   • Increased transpiration due to high temperature and wind strengthens the transpiration pull, promoting more efficient water transport. (1 mark)
5. Water Stress Risks:
   • Excessive transpiration can lead to water stress or dehydration if water uptake cannot keep pace with water loss. (1 mark)
6. Adaptive Responses:
   • Plants may adjust stomatal opening or alter leaf orientation in response to these factors to regulate transpiration and maintain water balance. (1 mark)

Question 19

Describe the step-by-step process of water movement within leaves, from xylem to transpiration. (6 marks)

Mark Scheme:

1. Arrival at Leaf Xylem:
   • Water arrives at the leaves through xylem vessels located within the vascular bundles (veins). (1 mark)
2. Exit from Xylem to Bundle Sheath Cells:
   • Water moves out of the xylem vessels through pits into bundle sheath cells. (1 mark)
3. Transfer to Mesophyll Cells:
   • From bundle sheath cells, water moves into mesophyll cells via symplastic or apoplastic pathways. (1 mark)
4. Utilization in Photosynthesis:
   • Water is used in the light-dependent reactions of photosynthesis, being split into oxygen, protons, and electrons. (1 mark)
5. Evaporation from Mesophyll Cells:
   • Excess water moves from mesophyll cell walls into intercellular air spaces within the leaf. (1 mark)
6. Diffusion Through Stomata:
   • Water vapor diffuses out of the leaf through stomatal openings into the atmosphere, completing the transpiration process. (1 mark)

Question 20

Explain how the structure of bundle sheath cells facilitates the transfer of water from xylem vessels to mesophyll cells. (5 marks)

Mark Scheme:

• Location within Vascular Bundles:
  • Bundle sheath cells are located surrounding the vascular tissues (xylem and phloem) within the leaf’s vascular bundles. (1 mark)
• Intermediary Role:
  • They act as intermediary cells, facilitating the transfer of water from xylem vessels to mesophyll cells. (1 mark)
• Plasmodesmata Presence:
  • Bundle sheath cells contain plasmodesmata, allowing symplastic movement of water into mesophyll cells. (1 mark)
• Selective Regulation:
  • They regulate the movement of water and minerals, ensuring that only necessary substances are transferred to the mesophyll. (1 mark)
• Support of Photosynthetic Processes:
  • By controlling water flow, bundle sheath cells maintain optimal hydration of mesophyll cells, supporting efficient photosynthesis. (1 mark)