biology : class11 Hydrostatic Pressure Development : Mechanism, Guttation, Importance : Root Pressure MCQ Questions and Answers
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Hydrostatic Pressure Development
MCQ questions and answers.
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MCQ Questions
1. Which sequence most accurately describes the development of positive hydrostatic pressure in a root?
A.
Active ion accumulation in the stele → decrease in xylem water potential → osmotic entry of water → development of positive xylem pressure
B.
Evaporation from mesophyll → increase in xylem water potential → passive water entry into roots → development of positive xylem pressure
C.
Closure of stomata → active removal of ions from the stele → increase in xylem water potential → development of positive pressure
D.
Lignification of root hairs → decrease in soil water potential → outward movement of water from xylem → development of positive pressure
Correct Answer:
Active ion accumulation in the stele → decrease in xylem water potential → osmotic entry of water → development of positive xylem pressure
Explanation:
Active accumulation of mineral ions in the xylem lowers its water potential. Water then enters osmotically from surrounding root tissues, and the resulting accumulation of water generates positive hydrostatic pressure in the xylem.
2. A root is supplied with oxygen-free conditions while adequate water and mineral ions remain available outside it. Which immediate change most directly reduces root pressure?
A.
Failure of ATP-dependent ion accumulation into the xylem
B.
Complete cessation of osmosis through the root cortex
C.
Increase in xylem water potential caused by evaporation
D.
Conversion of positive xylem pressure into transpiration pull
Correct Answer:
Failure of ATP-dependent ion accumulation into the xylem
Explanation:
Root pressure depends on active ion transport, which requires metabolic energy. Oxygen deprivation limits ATP production, reducing ion accumulation in the xylem and consequently weakening the osmotic water entry that produces hydrostatic pressure.
3. Why does the endodermis contribute to hydrostatic pressure development in the root?
A.
Its Casparian strips help restrict uncontrolled apoplastic leakage, allowing solute accumulation within the stele
B.
Its chloroplasts manufacture sugars that directly push water into the xylem
C.
Its lignified walls actively pump water from the soil into root hairs
D.
Its cells eliminate the need for selective ion transport into the xylem
Correct Answer:
Its Casparian strips help restrict uncontrolled apoplastic leakage, allowing solute accumulation within the stele
Explanation:
The Casparian strips block the apoplastic pathway at the endodermis and force water and ions through selective membranes. This helps retain solutes in the stele, sustaining the water-potential gradient required for positive xylem pressure.
4. If active ion loading into a root xylem increases while the hydraulic conductivity of the root remains unchanged, what is the most likely initial effect?
A.
The xylem water potential becomes more negative and water enters more rapidly, increasing hydrostatic pressure
B.
The xylem water potential becomes less negative and water leaves the xylem into the soil
C.
The root immediately loses all pressure because solutes prevent water movement
D.
The pressure becomes negative because ion loading directly causes water evaporation
Correct Answer:
The xylem water potential becomes more negative and water enters more rapidly, increasing hydrostatic pressure
Explanation:
Greater ion loading lowers xylem water potential. Provided the root can conduct water, the resulting gradient enhances osmotic water entry and raises positive hydrostatic pressure within the xylem.
5. Under which condition would positive root pressure be most likely to produce guttation at leaf margins?
A.
High soil moisture, active root metabolism, and low transpiration during humid nighttime conditions
B.
Severe soil dryness, inactive root metabolism, and intense daytime transpiration
C.
Low root solute accumulation, high evaporative demand, and closed xylem vessels
D.
Frozen soil, oxygen deprivation, and rapid water loss from leaf surfaces
Correct Answer:
High soil moisture, active root metabolism, and low transpiration during humid nighttime conditions
Explanation:
Abundant soil water and active ion transport favor root-pressure development, while low transpiration reduces the outward loss of xylem water. The resulting positive pressure can force xylem sap through hydathodes, producing guttation.
6. A root is placed in a solution whose water potential is lower than that of the root xylem, even though ion transport remains active. What is the likely effect on hydrostatic pressure development?
A.
Water entry into the xylem is opposed, so positive root pressure is reduced or may not develop
B.
Water enters more rapidly because a lower external water potential always increases root pressure
C.
Ion transport reverses automatically and creates pressure without water entry
D.
The xylem pressure becomes positive because water potential is unrelated to water movement
Correct Answer:
Water entry into the xylem is opposed, so positive root pressure is reduced or may not develop
Explanation:
Water moves down a water-potential gradient. If the external solution has a lower water potential than the xylem, osmotic water entry into the xylem is hindered, limiting the accumulation of water needed to generate positive hydrostatic pressure.
7. Which observation best supports the conclusion that root pressure results from active solute accumulation rather than from simple physical imbibition?
A.
Root pressure decreases sharply after metabolic inhibitors are applied, despite continued contact with water
B.
Root pressure increases when the root is killed but remains immersed in water
C.
Root pressure is unaffected by temperature because water movement is purely passive
D.
Root pressure develops only when leaf surfaces are exposed to dry air
Correct Answer:
Root pressure decreases sharply after metabolic inhibitors are applied, despite continued contact with water
Explanation:
Metabolic inhibitors reduce active transport and therefore limit ion accumulation in the xylem. The resulting decline in root pressure indicates that active solute loading is essential for hydrostatic pressure development.
8. Why does root pressure generally fail to account for the rapid ascent of water to the tops of very tall trees?
A.
Its positive pressure is usually too small and develops mainly when transpiration is low
B.
It is generated only in leaves and therefore cannot affect xylem water
C.
It requires evaporation from leaves, which prevents upward water movement
D.
It makes xylem water potential equal to that of the surrounding soil
Correct Answer:
Its positive pressure is usually too small and develops mainly when transpiration is low
Explanation:
Root pressure can push xylem sap upward over limited distances, especially during periods of low transpiration. Its magnitude is generally insufficient to explain the long-distance ascent of water in tall plants.
9. A root system accumulates ions in the stele, but its endodermal membranes become highly permeable to those ions. What is the most direct consequence?
A.
Solute leakage reduces the water-potential gradient and weakens positive xylem pressure
B.
Solute leakage makes xylem water potential more negative and strengthens root pressure
C.
Ion leakage prevents any water from entering the root but increases xylem pressure
D.
The root develops pressure independently because water potential gradients are unnecessary
Correct Answer:
Solute leakage reduces the water-potential gradient and weakens positive xylem pressure
Explanation:
Retention of solutes in the stele is necessary to maintain a lower xylem water potential than that of surrounding tissues. Increased leakage dissipates this gradient, reducing osmotic water entry and hydrostatic pressure.
10. When a transpiring shoot is removed from a well-watered, actively respiring root system, sap exudes from the cut xylem surface. Which explanation is most appropriate?
A.
Continued root ion loading draws water into the xylem, and accumulated water is expelled by positive hydrostatic pressure
B.
Leaf evaporation continues inside the root and forces sap out of the cut surface
C.
The cut surface actively transports sugars into the xylem, creating pressure without water movement
D.
Removal of the shoot reverses the xylem water-potential gradient and causes water to leave the soil
Correct Answer:
Continued root ion loading draws water into the xylem, and accumulated water is expelled by positive hydrostatic pressure
Explanation:
An actively functioning root can continue loading ions into the xylem after the shoot is removed. Water enters osmotically, accumulates in the xylem, and is pushed out through the cut surface by the resulting positive hydrostatic pressure.
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