Question 1

Which of the following describes a key functional and structural difference between xylem and phloem in vascular plants?

Accepted Answer

Xylem cells are dead at functional maturity and transport water via negative pressure, whereas phloem cells are living and transport organic solutes via positive pressure.

Answer

Xylem sap moves from sugar sources to sugar sinks using active transport, while phloem sap moves from roots to leaves using capillary action.

Answer

Xylem consists of sieve-tube elements that require ATP for water movement, whereas phloem consists of vessel elements that rely on transpiration pull.

Answer

Xylem utilizes companion cells to load minerals into vessels, while phloem relies on tracheids to move sucrose through sieve-tube elements.

Answer

Xylem provides structural support through lignin and moves substances bidirectionally, while phloem is primarily for protection and moves substances unidirectionally.

Question 2

The 'Pressure-Flow Hypothesis' explains how phloem sap moves through a plant. Which of the following events is necessary to initiate the bulk flow of phloem sap from a source, such as a mature leaf, to a sink?

Accepted Answer

The active transport of sugars into sieve-tube elements, which lowers water potential and causes water to enter from the xylem via osmosis.

Answer

The use of lignin-reinforced cell walls to generate a vacuum that pulls organic solutes toward areas of high metabolic activity.

Answer

The passive diffusion of glucose into tracheids, which creates a high-pressure gradient that pushes sap toward the roots.

Answer

The hydrolysis of ATP in companion cells to pump protons out of the xylem, allowing water to flow into the phloem by facilitated diffusion.

Answer

The evaporation of water through stomata, which creates a pulling force that drags sugar molecules upward through the sieve-tube elements.

Question 3

Water potential (

ψ

) plays a critical role in both xylem and phloem transport. How does the water potential of phloem sap at the source compare to the water potential of the adjacent xylem, and what is the resulting effect?

Accepted Answer

The phloem sap has a lower water potential due to high solute concentration, causing water to move from the xylem into the phloem.

Answer

The phloem sap has a higher water potential due to active transport of water, causing solutes to move from the phloem into the xylem.

Answer

The water potential is equal in both tissues, allowing for the passive exchange of nutrients and gases via plasmodesmata.

Answer

The xylem has a lower water potential due to the transpiration pull, which forces sugars to move out of the phloem and into the vessel elements.

Question 4

In the Cohesion-Tension theory of xylem transport, what is the primary role of the lignin found in the secondary cell walls of vessel elements and tracheids?

Accepted Answer

It provides structural support to prevent the cells from collapsing under the extreme negative pressure generated by transpiration.

Answer

It acts as a semi-permeable membrane that regulates the active transport of ions into the transpiration stream.

Answer

It provides the metabolic energy necessary to pump water vertically against the force of gravity.

Answer

It facilitates the loading of sucrose from companion cells into the xylem for distribution to the roots.

Question 5

Which of the following best explains why the flow of xylem sap is largely unaffected by metabolic inhibitors that stop ATP production, while the flow of phloem sap is severely restricted?

Accepted Answer

Xylem transport is driven by solar-powered evaporation and the physical properties of water, whereas phloem transport requires active transport for sugar loading at the source.

Answer

Xylem cells have sieve plates that facilitate movement without energy, while phloem tracheids are blocked by lignified walls.

Answer

Xylem vessels are living cells that utilize passive diffusion, while phloem sieve-tube elements are dead and require external ATP pumps.

Answer

Xylem sap consists of organic molecules that move via gravity, while phloem sap consists of minerals that move against a concentration gradient.

Question 6

Which of the following describes the correct direction and mechanism of transport in a vascular plant during the spring, when a deciduous tree is developing new leaves from stored starch in its roots?

Accepted Answer

Phloem sap moves upward from the roots (source) to the developing buds (sink) via positive pressure flow.

Answer

Phloem sap moves downward from the leaves to the roots via active transport in vessel elements.

Answer

Xylem sap moves downward from the leaves to the roots via the cohesion-tension mechanism.

Answer

Xylem sap moves upward from the roots to the leaves solely through the action of companion cells and ATP.

Question 7

Which of the following observations best supports the claim that phloem transport requires living cells, whereas xylem transport can occur in a non-living system?

Accepted Answer

Treatment of a leaf with a metabolic poison like cyanide stops the movement of sugars through the phloem but does not immediately stop the movement of water through the xylem.

Answer

The rate of xylem transport is highest during the night when the plant is metabolically active, while phloem transport peaks during the day.

Answer

Water molecules move through the xylem via osmosis across plasma membranes, while sugars move through the phloem via bulk flow through dead tracheids.

Answer

Xylem vessels contain cytoplasm and nuclei at functional maturity, while phloem sieve-tube elements are completely hollowed out.

Question 8

Which of the following describes the structural adaptations of sieve-tube elements and how they relate to the function of phloem?

Accepted Answer

They lack a nucleus, ribosomes, and a distinct vacuole to reduce internal resistance to the bulk flow of sap, while remaining alive to maintain the plasma membrane.

Answer

They are heavily lignified and lack a plasma membrane to allow for the unimpeded upward movement of water under negative pressure.

Answer

They possess a high density of mitochondria to provide the ATP required for the physical pulling of sugars from the roots to the leaves.

Answer

They are joined end-to-end to form a continuous hollow pipe of dead cells, preventing the collapse of the tissue during high rates of transpiration.

Question 9

Which of the following describes the relationship between physical forces and water potential that drives the movement of sap in the xylem versus the phloem?

Accepted Answer

Xylem transport is driven by a difference in negative pressure (tension) potential (

ψ

), while phloem transport is driven by a difference in positive pressure (turgor) potential (

ψ_p

).

Answer

Xylem transport utilizes active transport to move water against its concentration gradient, whereas phloem transport relies on gravity and capillary action.

Answer

Xylem transport depends on a gradient of solute potential (

ψ

) from leaves to roots, while phloem transport depends on a gradient of tension potential from roots to leaves.

Answer

Xylem transport is facilitated by the living protoplasts of vessel elements, while phloem transport is facilitated by the lignified secondary walls of sieve-tube elements.

Question 10

An aphid is an insect that feeds on plant sap by inserting its stylet (mouthpart) directly into a single plant cell. If the aphid's body is removed, sap continues to drip out of the stylet due to high internal hydrostatic pressure. Which tissue is the aphid feeding from, and what is the primary component of this sap?

Accepted Answer

Phloem; an aqueous solution of sucrose.

Answer

Xylem; an aqueous solution of minerals and nitrates.

Answer

Xylem; a concentrated solution of glucose and starch.

Answer

Phloem; a mixture of lipids and amino acids.

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