Part 4 — Toxicity of Micronutrients, Absorption, and Translocation (Q76–100)

Q76.

Toxicity of micronutrients usually occurs when their concentration in tissues exceeds:
A. 1 µmol g⁻¹ dry weight
B. 10 µmol g⁻¹ dry weight ✅
C. 1 mol g⁻¹ dry weight
D. 100 µmol g⁻¹ dry weight

Explanation:

A. Too low for toxicity.
B. (Correct) Above 10 µmol g⁻¹ dry weight of tissue, micronutrients may become toxic.
C/D. Unrealistically high concentrations.

Q77.

Which micronutrient in excess causes bronzing of leaves and crinkling in cotton?
A. Manganese ✅
B. Boron
C. Copper
D. Zinc

Explanation:

A. (Correct) Excess Mn causes brown spots, crinkled leaves, and inhibits other nutrient uptake (Fe, Mg).
B-D. Not specific to bronzing/crinkling.

Q78.

Which toxicity interferes with the absorption of iron and magnesium?
A. Boron
B. Manganese ✅
C. Zinc
D. Copper

Explanation:

A. Boron toxicity causes tip necrosis.
B. (Correct) Mn toxicity inhibits Fe and Mg uptake, leading to deficiency symptoms of both.
C/D. Affect other metabolic pathways.

Q79.

Copper toxicity in plants primarily results in:
A. Enhanced root elongation
B. Reduced root growth and chlorosis ✅
C. Increased nodulation
D. Overproduction of auxin

Explanation:

A. Opposite effect.
B. (Correct) Excess Cu inhibits root elongation and causes leaf chlorosis due to oxidative stress.
C/D. Not outcomes of Cu toxicity.

Q80.

Excessive boron in soil causes:
A. Purple pigmentation in leaves
B. Necrosis at leaf margins and tips ✅
C. White chlorotic patches
D. Excess root branching

Explanation:

A. Phosphorus deficiency symptom.
B. (Correct) Boron toxicity → necrosis at leaf tips and margins.
C. Iron deficiency symptom.
D. Not related.

Q81.

Excessive zinc toxicity primarily causes:
A. Iron deficiency symptoms ✅
B. Phosphorus accumulation
C. Increased root pressure
D. Excess nitrogen fixation

Explanation:

A. (Correct) Zn toxicity interferes with Fe uptake → chlorosis.
B-D. Not observed effects.

Q82.

Which statement about passive absorption is correct?
A. Requires ATP
B. Depends on concentration gradient ✅
C. Requires metabolic inhibitors
D. Occurs only in symplast

Explanation:

A/C. ATP and inhibitors relate to active absorption.
B. (Correct) Passive absorption follows diffusion/electrochemical gradient.
D. Can occur via apoplast too.

Q83.

Active absorption of ions is supported by evidence that:
A. Uptake continues after root death
B. Uptake is inhibited by cyanide or lack of O₂ ✅
C. Uptake increases at low respiration
D. Uptake does not require carrier proteins

Explanation:

A. Dead roots show passive, not active uptake.
B. (Correct) Active uptake requires respiration and ATP → inhibited by CN⁻ and low O₂.
C. Opposite effect.
D. Carriers are essential.

Q84.

Which of the following transport proteins is directly involved in ion absorption?
A. Aquaporins
B. Proton pumps (H⁺-ATPases) ✅
C. Plastocyanin
D. Cytochrome oxidase

Explanation:

A. Aquaporins transport water, not ions.
B. (Correct) Proton pumps create electrochemical gradients that drive ion transport (active absorption).
C/D. Electron transport proteins, not ion transporters.

Q85.

Which type of transport is saturable due to limited carriers?
A. Active transport ✅
B. Passive diffusion
C. Osmosis
D. Bulk flow

Explanation:

A. (Correct) Carrier-mediated active transport saturates at high substrate concentration.
B/C/D. Not carrier-mediated, so non-saturable.

Q86.

Which pathway of absorption involves movement only through cell walls and intercellular spaces?
A. Apoplast ✅
B. Symplast
C. Transmembrane
D. Xylem

Explanation:

A. (Correct) Apoplastic = through cell walls, intercellular spaces, blocked at Casparian strip.
B. Symplast = cytoplasm via plasmodesmata.
C. Transmembrane = repeated membrane crossing.
D. Long-distance pathway.

Q87.

At the endodermis, apoplastic movement is blocked by:
A. Pectin
B. Suberin in Casparian strip ✅
C. Cutin
D. Lignin

Explanation:

A. Middle lamella component.
B. (Correct) Casparian strip with suberin blocks apoplast → ensures selective absorption.
C/D. Found elsewhere but not key here.

Q88.

Symplastic absorption is advantageous because:
A. It avoids plasmodesmata
B. It allows direct control by cytoplasm ✅
C. It bypasses living cells
D. It requires no carrier proteins

Explanation:

A. Wrong, symplast depends on plasmodesmata.
B. (Correct) Cytoplasm controls ion movement → regulation.
C. That’s apoplast.
D. Carriers may still be needed.

Q89.

The final entry of ions into xylem vessels occurs through:
A. Casparian strip directly
B. Pericycle cells ✅
C. Root cap cells
D. Pith

Explanation:

A. Casparian strip blocks apoplast, not direct entry.
B. (Correct) Pericycle is last living layer before xylem → ions cross here to enter vessels.
C/D. Not involved.

Q90.

In long-distance transport, minerals move primarily with:
A. Phloem sap
B. Xylem transpiration stream ✅
C. Diffusion across cortex
D. Starch grains

Explanation:

A. Phloem = sugars.
B. (Correct) Minerals move upward in transpiration stream.
C/D. Local processes only.

Q91.

Which is NOT translocated via xylem?
A. Calcium
B. Potassium
C. Sucrose ✅
D. Nitrate

Explanation:

A/B/D. Mineral ions → xylem.
C. (Correct) Sucrose → phloem.

Q92.

Rate of transpiration influences mineral transport because:
A. More transpiration = stronger pull in xylem ✅
B. Transpiration only affects phloem
C. Minerals move passively without water
D. Stomata regulate sugar uptake

Explanation:

A. (Correct) Higher transpiration increases xylem sap flow, enhancing mineral transport.
B-D. Incorrect.

Q93.

Which hormone enhances absorption of minerals by stimulating root growth?
A. Auxin ✅
B. Cytokinin
C. Gibberellin
D. Ethylene

Explanation:

A. (Correct) Auxin promotes root hair formation → greater surface area for absorption.
B-D. Affect growth but not directly mineral uptake.

Q94.

Why is calcium relatively immobile in phloem?
A. Forms insoluble calcium pectate ✅
B. Is a component of ATP
C. Is gaseous in nature
D. Is stored in xylem

Explanation:

A. (Correct) Ca²⁺ binds in walls/middle lamella, immobilized, not easily translocated in phloem.
B/C/D. Incorrect.

Q95.

During passive absorption, ions enter root hairs primarily by:
A. Diffusion along electrochemical gradient ✅
B. ATP-driven pumps
C. Endocytosis of soil solution
D. Hydrolysis of starch

Explanation:

A. (Correct) Passive diffusion down gradient is main mechanism.
B. Active transport.
C. Not typical for ion uptake.
D. Unrelated.

Q96.

Which element is translocated almost exclusively through xylem and rarely via phloem?
A. Calcium ✅
B. Potassium
C. Magnesium
D. Sulphur

Explanation:

A. (Correct) Ca²⁺ is transported in xylem, immobile in phloem.
B/C/D. Can move via both pathways.

Q97.

In which form is nitrogen translocated in xylem sap?
A. Ammonia gas
B. Amino acids and nitrate ✅
C. Urea
D. ATP

Explanation:

A. NH₃ is toxic and not translocated freely.
B. (Correct) Amino acids (organic N) and nitrate (NO₃⁻) move in xylem sap.
C/D. Not primary forms.

Q98.

When transpiration is low, upward movement of minerals may still occur due to:
A. Capillary action only
B. Root pressure ✅
C. Bulk flow in phloem
D. Endocytosis in xylem

Explanation:

A. Minor effect.
B. (Correct) Root pressure can drive upward movement when transpiration is low (e.g., at night).
C. Phloem = organic transport.
D. Not a mechanism.

Q99.

Which experiment demonstrated xylem as the main pathway for mineral transport?
A. Girdling experiment
B. Dye uptake experiment ✅
C. Potometer
D. Hydroponic culture

Explanation:

A. Girdling shows phloem role.
B. (Correct) Dyes/salts (e.g., eosin) placed in water move through xylem vessels, staining them → proof of xylem transport.
C/D. Different purposes.

Q100.

If roots are killed by heat, which absorption is most affected?
A. Passive absorption
B. Active absorption ✅
C. Apoplastic movement
D. Osmotic diffusion

Explanation:

A/C/D. Passive uptake may continue briefly by physical forces.
B. (Correct) Active absorption requires living metabolism and is abolished by root death.

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Part 4 — Toxicity of Micronutrients, Absorption, and Translocation (Q76–100)