Phloem Transport: Pressure Flow Hypothesis
Describes the pressure flow (mass flow) hypothesis, which explains the translocation of sugars from source to sink in the phloem.
What is Phloem Transport: Pressure Flow Hypothesis?
The movement of organic solutes (like sugars) from one part of a plant to another.
Key points
- Explain the mechanism of translocation in phloem.
- Differentiate between source and sink tissues.
- Describe the role of turgor pressure in phloem transport.
- Understand the process of sugar loading and unloading.
Common exam trap
Confusing phloem transport with xylem transport (unidirectional, water and minerals).
Definitions
- Term
Translocation
- Meaning
The movement of organic solutes (like sugars) from one part of a plant to another.
- Term
Source
- Meaning
Plant tissue where sugars are produced or stored and then exported (e.g., mature leaf).
- Term
Sink
- Meaning
Plant tissue that imports sugars for metabolism, growth, or storage (e.g., root, fruit).
- Term
Turgor Pressure
- Meaning
The pressure exerted by the cell contents against the cell wall, caused by the uptake of water.
- Term
Hydrostatic Pressure Gradient
- Meaning
A difference in pressure between two points, which drives the flow of fluid.
Learning objectives
Explain the mechanism of translocation in phloem.
Differentiate between source and sink tissues.
Describe the role of turgor pressure in phloem transport.
Understand the process of sugar loading and unloading.
Relate water movement by osmosis to pressure changes in sieve tubes.
Prerequisites
Photosynthesis
Water Potential
Osmosis
Structure of Phloem (Sieve tube elements, companion cells)
Active and Passive Transport
Common mistakes
Confusing phloem transport with xylem transport (unidirectional, water and minerals).
Assuming passive loading/unloading of sugars.
Not understanding the role of osmosis in creating pressure gradients.
Forgetting that transport is bidirectional.
Keywords
Phloem
Translocation
Pressure Flow Hypothesis
Mass Flow
Source
Sink
Sucrose
Turgor Pressure
Osmosis
Hydrostatic Pressure
Sieve Tube Elements
Companion Cells
Practice preview
In the pressure flow hypothesis, the phloem loading at the source leads to:…
easy
Which of the following is a characteristic of the sink in the pressure flow hypothesis?…
easy
According to the pressure flow hypothesis, the direction of movement of sugars in the phloem is determined by:…
easy