Phloem Transport Practice Questions
20 free Phloem Transport practice questions for the NCERT Biology. Tap an option to answer — you get instant feedback, the correct answer, and a detailed explanation for every question.
In the process of phloem transport, the 'source' and 'sink' relationship is described as:
- A Variable and reversible
- B Fixed and unchanging
- C Directional only from root to leaf
- D Dependent solely on gravity
Correct answer: Variable and reversible
The source and sink relationship is variable depending on the season or the plant's needs. For example, sugar stored in roots may become a source in early spring to support new bud growth.
Which of the following describes the direction of movement in the phloem?
- A Unidirectional (upwards)
- B Unidirectional (downwards)
- C Radial only
- D Bi-directional
Correct answer: Bi-directional
Unlike xylem transport which is always unidirectional (upwards), phloem transport is bi-directional. This allows food to move from any source to any sink, regardless of orientation.
The primary sugar transported through the phloem is:
- A Glucose
- B Fructose
- C Sucrose
- D Starch
Correct answer: Sucrose
Sucrose is a non-reducing disaccharide, making it chemically stable and ideal for long-distance transport. Glucose produced during photosynthesis is converted to sucrose before entering the phloem.
The accepted mechanism for the translocation of sugars from source to sink is the:
- A Cohesion-tension theory
- B Root pressure hypothesis
- C Pressure-flow hypothesis
- D Imbibition theory
Correct answer: Pressure-flow hypothesis
The pressure-flow hypothesis explains that sugar movement occurs due to a turgor pressure gradient. High osmotic pressure at the source and low pressure at the sink drive the mass flow.
During 'phloem loading' at the source, sucrose is moved into the companion cells and sieve tubes by:
- A Passive diffusion
- B Active transport
- C Facilitated diffusion
- D Osmosis
Correct answer: Active transport
Loading of sucrose into the phloem requires energy in the form of ATP. This active transport creates a hypertonic condition within the sieve tubes.
What happens to the water potential in the sieve tube when sucrose is actively loaded into it?
- A It increases
- B It decreases
- C It remains unchanged
- D It becomes zero
Correct answer: It decreases
The high concentration of sucrose lowers the water potential inside the sieve tube. This causes water to move from the adjacent xylem into the phloem by osmosis.
At the sink, the process of 'unloading' sucrose from the phloem involves:
- A Active transport
- B Passive movement only
- C Xylem suction
- D Evaporation
Correct answer: Active transport
Unloading of sucrose at the sink is an active process. Once sucrose is removed, the osmotic pressure decreases and water moves back into the xylem.
In the pressure-flow hypothesis, the movement of the entire solution through the phloem is driven by:
- A A concentration gradient of water
- B A hydrostatic pressure gradient
- C Atmospheric pressure
- D Capillary action
Correct answer: A hydrostatic pressure gradient
The accumulation of sugars at the source creates high turgor pressure, while the removal at the sink creates low pressure. The solution flows from high pressure to low pressure areas.
Which specialized cells of the phloem form the long columns with holes in their end walls for transport?
- A Xylem tracheids
- B Sieve tube elements
- C Phloem parenchyma cells
- D Albuminous cells
Correct answer: Sieve tube elements
Sieve tube elements are elongated cells joined end-to-end. Their end walls, called sieve plates, have pores that allow the continuous flow of cytoplasmic strands.
The 'girdling' or 'ringing' experiment was performed to identify:
- A The tissue responsible for water transport
- B The tissue responsible for food transport
- C The rate of transpiration
- D The location of stomata
Correct answer: The tissue responsible for food transport
By removing a ring of bark (including phloem) while leaving the xylem intact, researchers observed that the portion above the ring swelled. This proved that phloem is the tissue responsible for downward food transport.
In the girdling experiment, which part of the plant dies first?
- A The leaves
- B The stem above the ring
- C The root system
- D The flowers
Correct answer: The root system
When the phloem is removed, the roots are deprived of sugars coming from the leaves. Consequently, the roots die first, eventually leading to the death of the entire plant.
Which of the following is NOT typically found in the phloem sap?
- A Sucrose molecules
- B Free amino acids
- C Plant hormones
- D Starch granules
Correct answer: Starch granules
Phloem sap is primarily water and sucrose, but it also contains other sugars, hormones, and amino acids. Starch is a storage polysaccharide and is too large and insoluble to be transported in the sap.
The movement of water from the xylem into the sieve tubes at the source is due to:
- A Transpiration pull
- B Osmotic movement
- C Active pumping
- D Low xylem pressure
Correct answer: Osmotic movement
As sucrose is actively loaded into the phloem, the solute concentration increases, creating a water potential gradient. Water follows naturally by osmosis from the higher potential in the xylem.
Sieve tube elements are living cells but lack which of the following at maturity?
- A Cytoplasm
- B Plasma membrane
- C Cell nucleus
- D Mitochondria
Correct answer: Cell nucleus
Mature sieve tube elements lack a nucleus, ribosomes, and vacuoles to allow for an unobstructed flow of sap. Their functions are controlled by the adjacent companion cells.
What is the physical state of the sap within the phloem during mass flow?
- A Under negative pressure (tension)
- B At atmospheric pressure
- C Completely frozen in place
- D Positive hydrostatic pressure
Correct answer: Positive hydrostatic pressure
Unlike the xylem sap, which is under tension (negative pressure), the phloem sap is under positive hydrostatic pressure. This is why sap exudes when the phloem is punctured.
Which cell type provides metabolic support and helps in the loading of sucrose into sieve tubes?
- A Vessel elements
- B Phloem fibers
- C Sclereids
- D Companion cells
Correct answer: Companion cells
Companion cells are specialized parenchyma cells connected to sieve tube elements by plasmodesmata. They contain numerous mitochondria to provide energy for active loading.
When sugars are removed from the sink and converted into starch or cellulose, the osmotic pressure in the phloem:
- A Increases
- B Stays the same
- C Becomes infinite
- D Decreases
Correct answer: Decreases
The removal of soluble sugars at the sink reduces the solute concentration. This leads to a decrease in osmotic pressure, allowing water to exit the phloem.
During the early spring, the 'source' in a deciduous tree is likely to be the:
- A The developing buds
- B Mature, fully expanded leaves
- C Flowers
- D Root and stem reserves
Correct answer: Root and stem reserves
In early spring, before leaves develop, the sugars stored in roots and stems are mobilized. These storage organs act as the source to provide energy to the developing buds (the sink).
The Pressure-Flow Hypothesis was originally proposed by:
- A Ernst Munch
- B Stephen Hales
- C Melvin Calvin
- D Hans Krebs
Correct answer: Ernst Munch
Ernst Munch proposed the mass flow or pressure-flow hypothesis in 1930. It remains the most widely accepted model for long-distance transport in the phloem.
The pores of the sieve plate are essentially large:
- A Plasmodesmata
- B Stomata
- C Lenticel pores
- D Hydathodes
Correct answer: Plasmodesmata
The pores in the sieve plates are modified and enlarged plasmodesmata. They facilitate the continuous movement of solutes and water between adjacent sieve tube elements.