biology : class11 Casparian Strip Function : Mechanism, Guttation, Importance : Root Pressure MCQ Questions and Answers
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Casparian Strip Function
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MCQ Questions
1. A dye is supplied to the soil surrounding a young root. It moves freely through the cortex but is unable to enter the central stele through the cell-wall spaces at a particular layer. Which structure most directly causes this blockage?
A.
The lignified thickening of xylem vessels
B.
The suberin-containing Casparian strip of the endodermis
C.
The cellulose wall of epidermal cells
D.
The starch sheath surrounding the vascular bundle
Correct Answer:
The suberin-containing Casparian strip of the endodermis
Explanation:
The Casparian strip is a band of suberin in the radial and transverse walls of endodermal cells. It blocks passive movement through the apoplast and forces water and dissolved substances to cross endodermal plasma membranes before entering the stele.
2. If the Casparian strip in a root were experimentally dissolved while the endodermal plasma membranes remained intact, which change would most directly occur?
A.
Apoplastic flow could bypass membrane screening at the endodermis
B.
Water could no longer move through the cortex
C.
Xylem vessels would lose their lignified walls
D.
Root hairs would become unable to absorb mineral ions
Correct Answer:
Apoplastic flow could bypass membrane screening at the endodermis
Explanation:
The Casparian strip normally interrupts the continuous cell-wall pathway. If it is removed, water and solutes can move apoplastically into the stele without first crossing an endodermal membrane, reducing the endodermis's control over entry.
3. Which sequence best describes the route of a soil ion that enters the root through the apoplast but encounters an intact Casparian strip?
A.
Soil solution → xylem lumen directly through cell walls
B.
Soil solution → cortex cell walls → endodermal membrane → symplast or stele
Explanation:
The Casparian strip blocks the apoplastic pathway at the endodermis. Consequently, an ion moving through cell walls must cross an endodermal plasma membrane and then may continue through the symplast toward the stele.
4. Why is the Casparian strip essential for selective mineral uptake by roots?
A.
It creates root hairs that increase the absorbing surface
B.
It prevents all solutes from entering the vascular cylinder
C.
It forces solutes in the apoplast to encounter selective membrane transporters
D.
It converts mineral ions into organic compounds before absorption
Correct Answer:
It forces solutes in the apoplast to encounter selective membrane transporters
Explanation:
By blocking an unrestricted apoplastic route into the stele, the Casparian strip requires ions to cross an endodermal membrane. Membrane proteins can then regulate which ions enter and in what amounts.
5. A root has an intact Casparian strip but its endodermal plasma membrane transport proteins are selectively inhibited. Which consequence is most likely?
A.
Apoplastic entry into the stele will remain completely unrestricted
B.
Selective entry of affected ions into the stele will decrease
C.
The Casparian strip will dissolve immediately
D.
Water will be unable to move through any root tissue
Correct Answer:
Selective entry of affected ions into the stele will decrease
Explanation:
The Casparian strip forces substances to cross endodermal membranes. If the relevant transport proteins are inhibited, the affected ions cannot efficiently cross this controlled barrier, even though they may be present in the cortical apoplast.
6. Which structural feature most directly gives the Casparian strip its ability to block apoplastic transport?
A.
A suberin-rich, hydrophobic band in endodermal cell walls
B.
A thin, highly permeable cellulose layer in epidermal walls
C.
A region of open xylem perforation plates
D.
A layer of chloroplasts surrounding the stele
Correct Answer:
A suberin-rich, hydrophobic band in endodermal cell walls
Explanation:
Suberin is hydrophobic and forms the sealing material of the Casparian strip. Its position in the radial and transverse endodermal walls interrupts the water-filled continuum of cell-wall spaces.
7. A mutation prevents Casparian strips from forming in the endodermis but does not alter root hairs or xylem structure. Which result is most directly predicted?
A.
Greater uncontrolled movement of soil solutes into the stele through cell walls
B.
Complete loss of water absorption by the epidermis
C.
Permanent closure of all xylem vessels
D.
Inability of cortical cells to contain cytoplasm
Correct Answer:
Greater uncontrolled movement of soil solutes into the stele through cell walls
Explanation:
Without Casparian strips, the endodermis no longer effectively interrupts the apoplastic pathway. Consequently, solutes can reach the stele through cell walls with less obligatory membrane-based selection.
8. Which statement best distinguishes the Casparian strip from a general thickening of a root cell wall?
A.
The Casparian strip is located specifically in endodermal radial and transverse walls and seals the apoplast
B.
The Casparian strip occurs only in xylem vessels and conducts water upward
C.
The Casparian strip is a cytoplasmic layer that synthesizes mineral ions
D.
The Casparian strip is an epidermal extension that absorbs water from soil
Correct Answer:
The Casparian strip is located specifically in endodermal radial and transverse walls and seals the apoplast
Explanation:
Its distinctive function depends on both composition and position. The Casparian strip occurs in the endodermis and seals the junctions between adjacent cells, thereby interrupting apoplastic movement into the vascular region.
9. A researcher places a water-soluble tracer in the cortical cell walls of a root and observes that it accumulates outside the endodermis but later appears in the stele. Which explanation is most consistent with an intact Casparian strip?
A.
The tracer crossed an endodermal plasma membrane before entering the stele
B.
The tracer passed directly through the suberin band by the apoplastic route
C.
The tracer entered the stele through xylem vessel walls from the outside
D.
The tracer was transported through the Casparian strip by root hairs
Correct Answer:
The tracer crossed an endodermal plasma membrane before entering the stele
Explanation:
An intact Casparian strip blocks the tracer's continued movement through endodermal cell walls. Its appearance in the stele therefore indicates that it crossed an endodermal membrane and continued through a membrane-controlled pathway.
10. How does the Casparian strip contribute to the maintenance of a controlled internal ionic environment in the root stele?
A.
By preventing any exchange between the root and soil
B.
By ensuring that entry through the endodermis is subject to membrane selectivity rather than unrestricted apoplastic flow
C.
By replacing the need for plasma membranes in endodermal cells
D.
By pumping every soil ion actively into the xylem lumen
Correct Answer:
By ensuring that entry through the endodermis is subject to membrane selectivity rather than unrestricted apoplastic flow
Explanation:
The Casparian strip acts as an apoplastic checkpoint. Because substances must cross endodermal membranes, selective permeability and transport regulation help control the ionic composition of materials entering the stele.
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