Part 8 — Analytical & Higher-order NEET MCQs (Q176–200)

Q176.

If stomata remain closed for a prolonged period, which sequence of events occurs first?
A. Photosynthesis decreases due to CO₂ shortage ✅
B. Transpiration increases
C. Phloem transport accelerates
D. Root pressure rises

Explanation:

A. (Correct) Closed stomata block CO₂ entry → photosynthesis declines.
B. Transpiration decreases, not increases.
C. Phloem transport depends on source strength, which decreases later.
D. Root pressure unrelated.

Q177.

In the ringing experiment, if only the xylem is removed, the plant:
A. Dies immediately
B. Shows sugar accumulation above ring
C. Dies of water stress ✅
D. Shows guttation

Explanation:

A. Not immediate, but stress develops.
B. Sugar accumulation happens when phloem is removed.
C. (Correct) Removal of xylem cuts off water supply → wilting, death.
D. Guttation unrelated.

Q178.

The rate of translocation in phloem is approximately:
A. 0.1 mm/hr
B. 1 cm/hr
C. 1 m/hr ✅
D. 10 m/sec

Explanation:

A. Too slow.
B. Still too slow.
C. (Correct) Measured by radioactive tracers, ~1 m/hr.
D. Far too fast.

Q179.

A student applies vaseline on both surfaces of a dorsiventral leaf. Which observation is correct?
A. Transpiration continues as normal
B. Transpiration stops almost completely ✅
C. Transpiration increases
D. Photosynthesis accelerates

Explanation:

A. Wrong — stomata blocked.
B. (Correct) Vaseline blocks stomata and cuticle → minimal transpiration.
C/D. Impossible.

Q180.

The pressure-flow hypothesis in phloem is supported by:
A. Sap exudes under positive pressure when phloem is cut ✅
B. Negative pressure recorded in phloem
C. Mineral accumulation in xylem
D. Air embolism in vessels

Explanation:

A. (Correct) Positive hydrostatic pressure in phloem causes sap exudation.
B. Negative pressure is xylem.
C. Minerals = xylem.
D. Embolism affects xylem.

Q181.

Which statement best differentiates xylem from phloem transport?
A. Xylem flow is driven by negative pressure, phloem by positive pressure ✅
B. Both are always unidirectional
C. Both rely on active pumping
D. Both transport only water

Explanation:

A. (Correct) Xylem = tension (negative), phloem = pressure flow (positive).
B. Phloem is bidirectional.
C/D. Incorrect.

Q182.

If a plant experiences potassium deficiency, which will be most directly affected?
A. Xylem sap ascent
B. Stomatal regulation ✅
C. Chlorophyll synthesis
D. Casparian strip development

Explanation:

A. Not main role.
B. (Correct) K⁺ regulates guard cell osmotic balance.
C. Mg, N more critical.
D. Casparian strip = suberin, not K⁺.

Q183.

A plant shows necrosis at leaf margins. Which deficiency is suspected?
A. Nitrogen
B. Iron
C. Potassium ✅
D. Calcium

Explanation:

A. Causes chlorosis.
B. Interveinal chlorosis.
C. (Correct) K deficiency → leaf edge necrosis.
D. Root tip death.

Q184.

Which property of water explains cohesion-tension mechanism?
A. High specific heat
B. Strong hydrogen bonding ✅
C. High compressibility
D. Low boiling point

Explanation:

A. Helps thermoregulation.
B. (Correct) Hydrogen bonds → cohesion → continuous column.
C. Water is incompressible.
D. Boiling point unrelated.

Q185.

During drought, ABA-induced stomatal closure helps the plant by:
A. Increasing photosynthesis
B. Reducing water loss ✅
C. Increasing root pressure
D. Enhancing guttation

Explanation:

A. Photosynthesis decreases.
B. (Correct) ABA closes stomata, conserves water.
C/D. Not major effects.

Q186.

In a tall tree, maximum negative water potential occurs in:
A. Soil
B. Root hair
C. Leaf mesophyll ✅
D. Stem xylem

Explanation:

A. Soil has higher ψ.
B. Roots intermediate.
C. (Correct) Evaporation creates strongest tension at mesophyll.
D. Stem xylem less negative.

Q187.

Which mineral deficiency reduces RuBisCO activity directly?
A. Nitrogen
B. Magnesium ✅
C. Potassium
D. Calcium

Explanation:

A. N = proteins, but RuBisCO needs Mg²⁺.
B. (Correct) Mg²⁺ is cofactor of RuBisCO.
C. K regulates stomata.
D. Ca = structure.

Q188.

In an actively photosynthesizing leaf, the phloem sap has high concentration of:
A. Glucose
B. Sucrose ✅
C. Starch
D. Fructose

Explanation:

A/D. Too reactive for long-distance transport.
B. (Correct) Sucrose is the major transport sugar.
C. Starch is storage, not transport.

Q189.

Why does girdling at the base of a tree eventually kill roots?
A. Sugars cannot reach roots ✅
B. Water cannot move up
C. Oxygen supply stops
D. Mineral uptake ceases immediately

Explanation:

A. (Correct) Phloem removed → no downward sugar supply → roots die.
B. Xylem intact → water still moves.
C/D. Not main reason.

Q190.

Which type of movement is faster?
A. Diffusion
B. Facilitated diffusion
C. Active transport
D. Bulk flow ✅

Explanation:

A-C. Slower, short-distance.
D. (Correct) Bulk flow in xylem/phloem is rapid, over meters.

Q191.

Which experiment provided strong support for mass flow hypothesis?
A. Girdling experiment
B. Aphid stylet experiment ✅
C. Potometer
D. Pressure bomb

Explanation:

A. Shows phloem role.
B. (Correct) Aphids feed on phloem → sap exudes under pressure, confirming pressure-flow.
C/D. Measure transpiration/water potential, not phloem.

Q192.

Which is the main driving force for water movement into root hairs?
A. Root pressure
B. Lower water potential inside root cells ✅
C. Cohesion-tension
D. Guttation

Explanation:

A. Root pressure develops later.
B. (Correct) Active ion uptake → lowers ψ inside roots → water enters.
C. Applies to ascent of sap.
D. Not absorption mechanism.

Q193.

Why is phloem transport considered an active process?
A. Sieve tubes are dead
B. Companion cells use ATP for loading/unloading ✅
C. Sugars move passively down gradient
D. Root pressure pushes sucrose

Explanation:

A. Wrong; sieve tubes are living.
B. (Correct) Active loading/unloading needs ATP.
C. Not passive diffusion.
D. Root pressure = xylem.

Q194.

If a plant is placed in 0.3M sucrose solution, plasmolysis occurs. Which term describes this solution relative to cell sap?
A. Hypotonic
B. Hypertonic ✅
C. Isotonic
D. Pure water

Explanation:

A. Hypotonic → swelling.
B. (Correct) Hypertonic → water leaves → plasmolysis.
C. Equal potentials.
D. Opposite case.

Q195.

Which tissue is the primary pathway for upward mineral transport?
A. Phloem
B. Xylem ✅
C. Cortex
D. Epidermis

Explanation:

A. Phloem = sugars.
B. (Correct) Xylem moves minerals with water.
C/D. Not main pathway.

Q196.

Which condition will most reduce transpiration rate?
A. High wind
B. High humidity ✅
C. High light
D. High temperature

Explanation:

A/C/D. Increase transpiration.
B. (Correct) High humidity reduces vapor gradient.

Q197.

Why do guard cells open stomata on sunny mornings?
A. High CO₂ concentration inside
B. Active uptake of K⁺ reduces ψs, water enters ✅
C. ABA accumulation increases
D. Low humidity

Explanation:

A. CO₂ decrease, not increase, triggers.
B. (Correct) K⁺ entry → water uptake → turgidity → opening.
C. ABA closes stomata.
D. Low humidity causes closure.

Q198.

The “transpiration ratio” refers to:
A. CO₂ fixed per water molecule lost
B. Water lost per CO₂ fixed ✅
C. Water absorbed per ion absorbed
D. Starch synthesized per sucrose transported

Explanation:

A. Reverse definition.
B. (Correct) Transpiration ratio = water lost / CO₂ gained.
C/D. Not definitions.

Q199.

Which factor explains why phloem can transport in both upward and downward directions simultaneously?
A. Pressure gradients between multiple sources and sinks ✅
B. Capillary action
C. Root pressure
D. Cohesion of water

Explanation:

A. (Correct) Different sieve tubes maintain independent pressure gradients → bidirectional.
B-D. Not explanations.

Q200.

Which statement best integrates xylem and phloem function?
A. Xylem transports water upward, phloem distributes sugars bidirectionally ✅
B. Both xylem and phloem transport sugars
C. Both transport only minerals
D. Phloem supplies roots, xylem supplies leaves only

Explanation:

A. (Correct) Xylem → unidirectional water/mineral flow, phloem → bidirectional sugar/assimilate flow.
B. Only phloem transports sugars.
C. Minerals in xylem only.
D. Oversimplified.

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Part 8 — Analytical & Higher-order NEET MCQs (Q176–200)