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Difficulty: HardTransport Mechanisms in Plants

During active translocation in angiosperms, sucrose synthesized in photosynthetic mesophyll cells is actively loaded into companion cells and sieve tube elements. Which of the following best explains the immediate physical mechanism that drives the bulk flow of phloem sap from source to sink tissues?

  1. The osmotic influx of water into sieve tubes generates high hydrostatic pressure at the source regionAnswer
  2. B
    The continuous transpiration pull creates a negative tension gradient along sieve elements
  3. C
    Active transport pumps within xylem vessels push organic solutes upward against a concentration gradient
  4. D
    Capillary action combined with root pressure directly pushes the viscous sap through sieve pores

Answer

The osmotic influx of water into sieve tubes generates high hydrostatic pressure at the source region
According to the Pressure Flow (Mass Flow) Hypothesis, active loading of sucrose into sieve tubes at the source tissue significantly decreases the water potential. This causes water to enter the sieve tubes from adjacent xylem vessels by osmosis. The resulting increase in volume generates high hydrostatic pressure at the source. At the sink end, sucrose is actively unloaded, increasing water potential and causing water to leave the sieve tube, leading to low hydrostatic pressure. This hydrostatic pressure gradient drives the bulk movement of phloem sap from source to sink.

Step-by-Step Solution

1
Identify the primary mechanism of phloem transport described by the Münch Pressure Flow Hypothesis
Active loading of sucrose into sieve tubes at the source lowers water potential inside the sieve elements
Accumulation of solutes increases osmotic concentration inside sieve tube elements
2
Determine the movement of water resulting from the water potential gradient
Water moves by osmosis from adjacent xylem vessels into the sieve tubes at the source
Water naturally moves down its water potential gradient into regions of higher solute concentration
3
Analyze how water entry creates the driving force for sap transport
The entry of water causes a buildup of high hydrostatic (turgor) pressure at the source, pushing sap toward sink regions of lower hydrostatic pressure where sucrose is unloaded
Bulk flow is driven by hydrostatic pressure differences between source and sink

Key Concept

Pressure Flow (Mass Flow) Hypothesis of Phloem Translocation
Estimated Time:1m 30s
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Topics

Transport Mechanisms in Plants

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