biology : class11 Xylem Sap Accumulation : Mechanism, Guttation, Importance : Root Pressure MCQ Questions and Answers
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Xylem Sap Accumulation
MCQ questions and answers.
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MCQ Questions
1. A well-watered herbaceous plant is cut near the base of its stem, and xylem sap slowly exudes from the cut surface even when transpiration is negligible. Which condition most directly accounts for this accumulation of xylem sap?
A.
Positive hydrostatic pressure generated in the root xylem by continued ion loading and water entry
B.
Negative pressure generated in the xylem by rapid evaporation from mesophyll cells
C.
Direct pumping of water upward by mature xylem vessels
D.
Expansion of air bubbles trapped inside the xylem lumen
Correct Answer:
Positive hydrostatic pressure generated in the root xylem by continued ion loading and water entry
Explanation:
When ions accumulate in the root xylem, the xylem water potential decreases, causing water to enter osmotically from surrounding root tissues. Continued entry produces positive hydrostatic pressure and can force xylem sap out of a cut stem.
2. Which sequence best explains the accumulation of xylem sap during root-pressure development in a hydrated plant?
A.
Ion release into xylem, increased xylem water potential, water loss from xylem, and sap accumulation
B.
Active ion loading into xylem, decreased xylem water potential, osmotic water entry, and positive pressure buildup
C.
Stomatal opening, increased evaporation, xylem dehydration, and positive pressure buildup
D.
Cavitation, air expansion, water displacement, and solute accumulation in xylem
Correct Answer:
Active ion loading into xylem, decreased xylem water potential, osmotic water entry, and positive pressure buildup
Explanation:
Solute accumulation in the xylem lowers its water potential. Water then enters from adjacent root tissues, and because the incoming water is confined within the conducting system, hydrostatic pressure and xylem sap volume increase.
3. A root system is supplied with water containing an inhibitor that greatly reduces active ion transport into the xylem. Under otherwise identical conditions, what is the most likely effect on xylem sap accumulation?
A.
It will increase because the xylem water potential becomes more positive
B.
It will remain unchanged because xylem sap accumulation depends only on transpiration
C.
It will decrease because the osmotic gradient driving water entry into the xylem is weakened
D.
It will reverse because water will move from xylem into the surrounding soil immediately
Correct Answer:
It will decrease because the osmotic gradient driving water entry into the xylem is weakened
Explanation:
Active ion loading helps lower the water potential of xylem sap. If ion transport is inhibited, less water enters osmotically into the xylem, so xylem sap accumulation and the associated positive pressure are reduced.
4. Why does xylem sap accumulation become especially detectable as exudation from a cut stem when transpiration is low?
A.
Low transpiration prevents the xylem from containing water
B.
Low transpiration removes the negative pull that would otherwise oppose or mask positive root-generated pressure
C.
Low transpiration causes the phloem to pump water directly into the xylem
D.
Low transpiration makes xylem walls actively secrete liquid sap
Correct Answer:
Low transpiration removes the negative pull that would otherwise oppose or mask positive root-generated pressure
Explanation:
When transpiration is low, the tension generated by evaporation is weak. Positive pressure arising from water accumulation in the root xylem can therefore become evident as sap exuding from a cut surface.
5. Two identical plants are kept in moist soil. In plant P, xylem solute concentration rises while water entry continues; in plant Q, xylem solute concentration remains constant. Which comparison is most consistent with xylem sap accumulation?
A.
Plant P develops a greater tendency for water entry and positive xylem pressure than plant Q
B.
Plant Q develops greater pressure because constant solute concentration always causes water loss
C.
Both plants develop identical pressure because xylem solute concentration does not affect water movement
D.
Plant P loses xylem water because increasing solute concentration raises xylem water potential
Correct Answer:
Plant P develops a greater tendency for water entry and positive xylem pressure than plant Q
Explanation:
A rise in xylem solute concentration lowers xylem water potential, strengthening the osmotic gradient for water entry. Continued water influx increases the amount of xylem sap and promotes positive pressure.
6. A pressure probe records increasing pressure in the root xylem while the surrounding root cells remain hydrated. Which interpretation is most appropriate?
A.
Xylem sap is accumulating because water entry exceeds the rate at which sap leaves the xylem
B.
Xylem sap is disappearing because positive pressure drives water into the root cortex
C.
The xylem is under tension because positive pressure is equivalent to negative pressure
D.
Pressure is increasing solely because xylem vessels are becoming narrower without any change in sap volume
Correct Answer:
Xylem sap is accumulating because water entry exceeds the rate at which sap leaves the xylem
Explanation:
A sustained rise in xylem pressure indicates net accumulation of water within the xylem. In root-pressure conditions, osmotic water entry can exceed outward movement, producing positive hydrostatic pressure.
7. If the plasma membranes of root cells suddenly become highly permeable to ions, which change would most directly reduce the accumulation of xylem sap?
A.
The ion gradient between root tissues and xylem would be maintained more effectively
B.
Solute accumulation in the xylem would be reduced, weakening osmotic water entry
C.
Water would become unable to cross into the xylem because aquaporins would close permanently
D.
The xylem would accumulate sap faster because ion leakage increases xylem water potential
Correct Answer:
Solute accumulation in the xylem would be reduced, weakening osmotic water entry
Explanation:
Maintaining a solute gradient is important for lowering xylem water potential. Excessive ion leakage across root-cell membranes would diminish effective solute accumulation in the xylem and thereby reduce osmotic water influx.
8. A root is placed in a solution with a water potential lower than that of the xylem sap. Assuming xylem ion accumulation continues, what is the most likely consequence for xylem sap accumulation?
A.
Water entry into the root and xylem will be opposed, so sap accumulation and positive pressure will decline
B.
Water will enter the xylem faster because the external solution has lower water potential
C.
Xylem solutes will disappear immediately, but water accumulation will remain unchanged
D.
The xylem will generate pressure independently of water movement
Correct Answer:
Water entry into the root and xylem will be opposed, so sap accumulation and positive pressure will decline
Explanation:
For net water entry into the root, the root system must have a lower water potential than its surroundings. A surrounding solution with lower water potential opposes water uptake, reducing the water available for xylem sap accumulation.
9. Which observation provides the strongest direct evidence that xylem sap has accumulated under positive pressure rather than merely being present in the vessels?
A.
A cut stem releases xylem liquid that continues to flow briefly without external pressure
B.
Xylem vessels contain water when examined microscopically
C.
Leaves contain chlorophyll during daylight
D.
Roots are in contact with moist soil
Correct Answer:
A cut stem releases xylem liquid that continues to flow briefly without external pressure
Explanation:
Continued exudation from a cut stem indicates that the xylem liquid is under positive hydrostatic pressure. Mere presence of water in xylem vessels does not demonstrate pressure-driven accumulation.
10. In a root system, xylem solute concentration rises initially, but xylem sap accumulation soon stops even though the roots remain hydrated. Which explanation best accounts for this outcome?
A.
The osmotic gradient has disappeared or water entry has become balanced by sap loss and leakage
B.
The xylem must have converted positive pressure into atmospheric pressure without any change in water movement
C.
Solute accumulation always prevents water from entering the xylem after a short delay
D.
Hydrated roots cannot maintain any concentration difference between xylem sap and surrounding tissues
Correct Answer:
The osmotic gradient has disappeared or water entry has become balanced by sap loss and leakage
Explanation:
Xylem sap accumulation requires net water entry. It stops when the driving gradient is lost or when outward flow, leakage, or other water losses equal the rate of water entry, so no further increase in xylem sap occurs.
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