Part 4 — Phloem Transport (Translocation of Food) (Q76–100)

Part 4 — Phloem Transport (Translocation of Food) (Q76–100)

Q76.

Which tissue is responsible for long-distance transport of organic solutes in plants?
A. Xylem
B. Phloem ✅
C. Cortex
D. Epidermis

Explanation:

• A. Xylem mainly conducts water and minerals.
• B. (Correct) Phloem transports sugars (mainly sucrose), amino acids, hormones.
• C/D. Cortex/epidermis are not conducting tissues.

Q77.

According to the pressure-flow hypothesis, translocation of sugars occurs due to:
A. Root pressure
B. Turgor pressure gradient between source and sink ✅
C. Active pumping by sieve elements
D. Simple diffusion along concentration gradient

Explanation:

• A. Root pressure applies to xylem, not phloem.
• B. (Correct) Source cells load sugars → water enters → high turgor; at sink, sugars unloaded → low turgor. Gradient drives mass flow.
• C. Sieve tubes lack nuclei and don’t actively pump.
• D. Diffusion is too slow for long-distance transport.

Q78.

Which is the principal form of carbohydrate transported in phloem sap?
A. Glucose
B. Sucrose ✅
C. Fructose
D. Starch

Explanation:

• A/C. Glucose/fructose are present but unstable in transport.
• B. (Correct) Sucrose is the primary transport sugar in phloem.
• D. Starch is insoluble, not transported.

Q79.

The functional units of phloem responsible for translocation are:
A. Tracheids
B. Vessel elements
C. Sieve tube elements ✅
D. Companion cells only

Explanation:

• A/B. Xylem elements.
• C. (Correct) Sieve tube elements form continuous tubes, assisted by companion cells.
• D. Companion cells help in loading/unloading, but sieve tubes conduct.

Q80.

Phloem loading at the source is achieved by:
A. Passive diffusion of sugars only
B. Active transport of sucrose into sieve elements via companion cells ✅
C. Root pressure
D. Osmosis alone

Explanation:

• A. Diffusion alone insufficient.
• B. (Correct) Active transport pumps sucrose into sieve tubes, creating osmotic gradient.
• C. Root pressure is for water.
• D. Osmosis follows sugar loading, not the cause.

Q81.

At the sink, sucrose is:
A. Actively loaded into phloem
B. Unloaded into sink tissues, reducing turgor pressure ✅
C. Hydrolyzed into starch in phloem
D. Pumped back into leaves

Explanation:

• A. Loading happens at source.
• B. (Correct) At sink, sucrose is removed → water potential increases → water exits → pressure gradient maintained.
• C. Starch is formed in storage tissue, not phloem itself.
• D. Flow is source → sink, not reverse.

Q82.

Which structure connects companion cells with sieve tube elements for transport?
A. Pits
B. Sieve pores ✅
C. Plasmodesmata
D. Xylem vessels

Explanation:

• A. Pits connect xylem.
• B. (Correct) Sieve plates with sieve pores connect sieve elements.
• C. Plasmodesmata connect companion and sieve cells laterally, but translocation occurs via sieve pores.
• D. Xylem vessels not involved.

Q83.

Which of the following is correct about phloem transport?
A. It is unidirectional like xylem
B. It is bidirectional depending on source-sink relations ✅
C. It is always upward
D. It is passive diffusion

Explanation:

• A. Unlike xylem, phloem is not unidirectional.
• B. (Correct) Phloem flow can be upward or downward depending on source (leaf) and sink (roots, fruits, shoots).
• C. Not always upward.
• D. It is pressure-driven bulk flow, not passive diffusion.

Q84.

Mass flow hypothesis of phloem transport was proposed by:
A. Dixon and Joly
B. Munch ✅
C. Strasburger
D. Priestley

Explanation:

• A. Cohesion-tension theory of xylem.
• B. (Correct) Ernst Munch proposed pressure-flow (mass flow) hypothesis.
• C/D. Not related to phloem transport theory.

Q85.

The sugar loading into phloem causes:
A. Increase in water potential of sieve tubes
B. Decrease in water potential, leading to osmotic water entry ✅
C. Immediate unloading at the source
D. Closure of stomata

Explanation:

• A. Wrong direction — loading decreases ψ.
• B. (Correct) Sucrose loading reduces ψ, water enters osmotically, creating pressure.
• C. Unloading is at sink, not source.
• D. Not related to stomata.

Q86.

Which experiment demonstrated girdling effect in phloem transport?
A. Darwin’s phototropism experiment
B. Munch’s pressure-flow experiment
C. Ringing experiment ✅
D. Meselson-Stahl experiment

Explanation:

• A. Tropism study.
• B. Pressure-flow hypothesis.
• C. (Correct) Removing a ring of bark (phloem) stops downward transport → swelling above girdle shows food accumulates in phloem.
• D. DNA replication study.

Q87.

Why does phloem transport require living cells?
A. Xylem cells provide active pumping
B. Active loading/unloading of sugars needs ATP ✅
C. Translocation is passive
D. Sieve plates are impermeable

Explanation:

• A. Xylem = dead cells.
• B. (Correct) Active loading/unloading requires ATP from companion cells.
• C. Translocation is pressure-driven but initiated by active processes.
• D. Sieve plates allow flow, not impermeable.

Q88.

Which of the following molecules is transported in phloem along with sugars?
A. Mineral ions ✅
B. Oxygen gas
C. Water vapor
D. Carbon dioxide

Explanation:

• A. (Correct) Along with sucrose, amino acids, hormones, and some mineral ions are transported.
• B. Oxygen is via diffusion, not phloem.
• C. Water vapor lost via stomata.
• D. CO₂ diffuses for photosynthesis.

Q89.

In plants, the source for phloem transport is generally:
A. Roots
B. Mature leaves ✅
C. Young stems
D. Flowers

Explanation:

• A. Roots are usually sinks.
• B. (Correct) Mature photosynthesizing leaves act as sources of sucrose.
• C/D. Young tissues are sinks, not sources.

Q90.

The sink in phloem transport can be:
A. Actively photosynthesizing leaf
B. Developing fruits, roots, and storage tissues ✅
C. Mature stem tissues
D. Fully expanded leaves

Explanation:

• A. Source, not sink.
• B. (Correct) Sinks = non-photosynthetic or growing tissues requiring assimilates.
• C. Mature stem tissues often passive.
• D. Expanded leaves = source.

Q91.

Which evidence supports pressure-flow hypothesis?
A. Sap exudes from phloem when cut ✅
B. Root pressure experiments
C. Capillarity in xylem
D. Guttation in leaves

Explanation:

• A. (Correct) Phloem is under positive pressure; cutting causes sap to exude.
• B. Root pressure relates to xylem.
• C. Capillarity in xylem, not phloem.
• D. Guttation is due to root pressure.

Q92.

Which element is directly involved in phloem loading?
A. Potassium ✅
B. Manganese
C. Magnesium
D. Chlorine

Explanation:

• A. (Correct) K⁺ ions maintain charge balance and aid sucrose transport.
• B/C/D. Important for photosynthesis, not directly phloem loading.

Q93.

Which statement about phloem is FALSE?
A. Phloem transport is source-to-sink driven
B. Companion cells supply ATP to sieve tubes
C. Transport can be bidirectional
D. Transport is slower than diffusion ✅

Explanation:

• A. True.
• B. True.
• C. True.
• D. (Correct) Phloem translocation is very fast (up to 1 m/hr), faster than diffusion.

Q94.

Which part of phloem lacks a nucleus but is functional?
A. Companion cells
B. Sieve tube elements ✅
C. Phloem fibers
D. Phloem parenchyma

Explanation:

• A. Companion cells have nuclei.
• B. (Correct) Sieve tube elements lose nucleus at maturity but remain alive with help of companion cells.
• C. Fibers are dead.
• D. Parenchyma cells are living.

Q95.

Which is NOT transported by phloem?
A. Sucrose
B. Amino acids
C. Hormones
D. Water vapor ✅

Explanation:

• A-C. All are transported by phloem.
• D. (Correct) Water vapor exits via stomata, not transported in phloem.

Q96.

In ringing experiment, swelling occurs:
A. Above the ring ✅
B. Below the ring
C. At the ring
D. No swelling

Explanation:

• A. (Correct) Sugars accumulate above the removed bark because downward transport is blocked.
• B. Below the ring, tissues starve.
• C. Not at ring (removed area).
• D. Swelling is seen.

Q97.

Which phloem cell type provides metabolic support to sieve tube elements?
A. Phloem parenchyma
B. Phloem fibers
C. Companion cells ✅
D. Xylem vessels

Explanation:

• A. Parenchyma stores.
• B. Fibers provide strength.
• C. (Correct) Companion cells maintain sieve tube elements.
• D. Xylem is unrelated.

Q98.

Translocation of sugars in phloem is mainly:
A. Active diffusion
B. Mass flow driven by pressure gradient ✅
C. Facilitated diffusion
D. Osmosis only

Explanation:

• A. Active diffusion is incorrect concept.
• B. (Correct) Pressure-flow (mass flow) is the accepted mechanism.
• C/D. Play partial roles but not the main process.

Q99.

Sucrose is transported in phloem because:
A. It is insoluble and stable
B. It is soluble, non-reducing, and less reactive ✅
C. It is the only sugar in plants
D. It is produced in mitochondria

Explanation:

• A. Sucrose is soluble.
• B. (Correct) Sucrose is soluble, stable, non-reducing → ideal for transport.
• C. Not the only sugar, but main transport form.
• D. Produced in cytoplasm of leaves, not mitochondria.

Q100.

Phloem transport differs from xylem transport in that phloem transport:
A. Is always unidirectional
B. Requires living cells and ATP for loading/unloading ✅
C. Transports only water
D. Is driven solely by root pressure

Explanation:

• A. Xylem is unidirectional; phloem is bidirectional.
• B. (Correct) Phloem transport depends on living companion cells for active loading/unloading.
• C. Water is mainly transported by xylem, not phloem.
• D. Root pressure is for xylem, not phloem.

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