7.08 Water: Roots → leaves

Question 1

Describe the structure and function of root hairs in plant roots. (5 marks)

Mark Scheme:

1. Structure – Extensions of Epidermal Cells:
   • Root hairs are extensions of epidermal cells, making them thin and elongated. (1 mark)
2. Increased Surface Area:
   • Their length and density significantly increase the root’s surface area, enhancing water and mineral absorption. (1 mark)
3. Presence of Plasmodesmata:
   • Root hairs contain plasmodesmata, which allow symplastic transport of water and solutes between cells. (1 mark)
4. Rich in Aquaporins:
   • They are rich in aquaporins (water channel proteins) that facilitate rapid water movement into the root hair cells. (1 mark)
5. Function – Water and Mineral Absorption:
   • Root hairs absorb water and dissolved mineral ions from the soil, playing a crucial role in the plant’s hydration and nutrient uptake. (1 mark)

Question 2

Explain the concept of water potential and how it influences the movement of water in plants. (5 marks)

Mark Scheme:

1. Definition of Water Potential (Ψ):
   • Water potential is the potential energy of water in a system, measured in megapascals (MPa), indicating the direction of water movement. (1 mark)
2. Components of Water Potential:
   • It is influenced by solute concentration (solute potential, Ψs) and pressure (pressure potential, Ψp). (1 mark)
3. Gradient from Soil to Air:
   • Water moves from areas of higher water potential (soil, less negative) to areas of lower water potential (air, more negative). (1 mark)
4. Driving Force for Osmosis:
   • The water potential gradient creates a driving force for osmosis, facilitating the passive movement of water into root hairs. (1 mark)
5. Maintaining Continuous Flow:
   • By maintaining a steep water potential gradient, plants ensure a continuous flow of water from the soil through the roots and up the xylem to the leaves. (1 mark)

Question 3

Compare the apoplastic and symplastic pathways in the movement of water through root tissues. (6 marks)

Mark Scheme:

1. Definition of Apoplastic Pathway:
   • Apoplastic Pathway involves water moving through cell walls and intercellular spaces without crossing any cell membranes. (1 mark)
2. Definition of Symplastic Pathway:
   • Symplastic Pathway involves water moving cell-to-cell through the cytoplasm and plasmodesmata, crossing cell membranes. (1 mark)
3. Movement Through Root Cap and Epidermis:
   • In the apoplastic pathway, water moves freely through the root cap and epidermis without crossing membranes until it reaches the endodermis. (1 mark)
4. Role of Endodermis and Casparian Strip:
   • The Casparian strip in the endodermis blocks the apoplastic pathway, forcing water to enter the symplast for selective uptake. (1 mark)
5. Selective Absorption in Symplastic Pathway:
   • The symplastic pathway allows the plant to control and regulate the movement of water and minerals by selectively absorbing necessary ions through active transport in root hair cells. (1 mark)
6. Efficiency and Regulation Comparison:
   • The apoplastic pathway is faster but less regulated, while the symplastic pathway is slower but allows for more precise control over water and nutrient uptake. (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 in the xylem according to the cohesion-tension theory. (6 marks)

Mark Scheme:

1. Transpiration Occurs:
   • Water vapor exits the leaf via stomata, creating a water potential gradient. (1 mark)
2. Tension in the Xylem:
   • Evaporation generates a pulling force (negative pressure) on the water in the xylem, creating tension. (1 mark)
3. Cohesion of Water Molecules:
   • Hydrogen bonds between water molecules maintain the continuous water column within the xylem vessels. (1 mark)
4. Adhesion to Xylem Walls:
   • Water molecules adhere to the xylem vessel walls, supporting the water column and preventing breakage under tension. (1 mark)
5. Water Uptake from Roots:
   • Negative pressure in the xylem pulls water from the root hairs into the xylem vessels. (1 mark)
6. Mass Flow:
   • All water molecules move together efficiently, transporting water and minerals from roots to leaves in a continuous flow. (1 mark)

Question 9

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

Mark Scheme:

1. Increased Temperature:
   • Higher temperatures raise the rate of water evaporation from the mesophyll cells, increasing transpiration rate. (1 mark)
2. Lower Humidity:
   • Low humidity creates a steeper water potential gradient, enhancing the transpiration pull and increasing water uptake. (1 mark)
3. Increased Wind Speed:
   • Wind removes the saturated air surrounding the stomata, maintaining a steep gradient and thereby increasing transpiration. (1 mark)
4. Effect on Cohesion-Tension Mechanism:
   • Enhanced transpiration strengthens the transpiration pull, promoting more efficient water transport from roots to leaves. (1 mark)
5. Water Stress Risks:
   • Excessive transpiration due to high temperature and wind can lead to water stress or dehydration if water uptake cannot keep pace with 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. (5 marks)

Mark Scheme:

• Definition of Pits:
  • Pits are unlignified areas in the xylem vessel walls that allow for lateral water movement between adjacent xylem vessels and cells. (1 mark)
• Facilitation of Continuous Water Flow:
  • Pits enable the bypass of air bubbles (air locks) by allowing water to move laterally around obstructions, maintaining continuous water transport. (1 mark)
• Maintenance of Water Column Integrity:
  • By facilitating lateral water movement, pits help preserve the cohesion and adhesion of water molecules, ensuring the water column remains unbroken under tension. (1 mark)
• Efficiency in Tall Plants:
  • In tall plants, pits support the efficient upward movement of water by connecting multiple xylem vessels, reducing the likelihood of transport disruptions. (1 mark)
• Adaptation to Environmental Stresses:
  • Pits help plants adapt to varying environmental conditions by providing pathways for water movement even when parts of the xylem are blocked or damaged. (1 mark)