Mass flow hypothesis
Laura Armstrong
Teacher
Recall Questions
This topic requires prior knowledge of phloem structure. You can test your knowledge on this topic below.
What are the two main types of cells in the phloem, and what are their functions?
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Sieve tube elements: Form the transport channels for solutes but lack a nucleus and most organelles to allow efficient flow.
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Companion cells: Provide metabolic support to sieve tube elements and use ATP for active transport of sucrose into the phloem.
What is the function of the sieve plates in sieve tube elements?
Sieve plates have perforations that allow sucrose and other solutes to pass between sieve tube elements while maintaining structural support.
In what direction do solutes such as sucrose travel in the phloem?
From source to sink, both up and down the plant.
Topic Explainer Video
Check out this @LauraDoesBiology video that explains mass flow hypothesis or read the full notes below. Once you've gone through the whole note, try out the practice questions!
What is Translocation?
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Translocation is the movement of organic solutes (mainly sucrose) from source to sink through the phloem.
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It is an active process that occurs in both directions (unlike xylem transport, which is one-way, from roots to leaves only).
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The Mass Flow Hypothesis is the most widely accepted explanation for how translocation occurs.
The Mass Flow Hypothesis Explained
1. Loading Sucrose at the Source (e.g., Leaves)
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Companion cells actively pump hydrogen ions (H+) into their cell walls.
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Large concentrations of hydrogen ions in the cell wall of the companion cell results in the H+ moving down the concentration gradient back to the cytoplasm of the companion cell.
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The H+ move through a cotransport protein. While transporting the hydrogen ions, this protein also carries sucrose molecules into the companion cell against their concentration gradient.
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The sucrose molecules then move into the sieve tubes via the plasmodesmata from the companion cells.
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This loading of sucrose lowers the water potential in the sieve tube elements.
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Water then enters by osmosis from the xylem, down the water potential gradient. Water will also enter from the adjacent companion cells.
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This increases the hydrostatic pressure at the source end of the phloem.
2. Mass Flow of Phloem Sap
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Due to the high hydrostatic pressure at the source and low hydrostatic pressure at the sink, a pressure gradient is created.
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The phloem sap (sucrose and water) moves by mass flow down this pressure gradient.
3. Unloading at the Sink (e.g., Roots, Growing Tissues, Storage Organs)
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Sucrose is actively removed from sieve tube elements, via the companion cells and used for respiration or storage in the sink cells (e.g., converted to starch in roots).
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This increases the water potential in the sieve tubes, so water leaves by osmosis back into the xylem, reducing hydrostatic pressure at the sink.
Note: This can often be simplified in an exam question to state that sucrose is actively loaded into the sieve tube elements via the companion cells.
You could also be asked why companion cells have many mitochondria. The mitochondria produce ATP during aerobic respiration which is needed for the active transport of sucrose into the sieve tube elements.
No answer provided.
Key Terms:
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Translocation: The movement of sucrose and other organic solutes through the phloem from source to sink.
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Mass Flow Hypothesis: The theory that translocation occurs by pressure-driven mass flow of solutes.
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Source: A part of the plant that produces or releases sucrose (e.g., photosynthesising leaves, storage organs in spring).
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Sink: A part of the plant that uses or stores sucrose (e.g., roots, fruits, growing tissues).
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Hydrostatic Pressure: The pressure created when water enters sieve tubes by osmosis, driving mass flow.
No answer provided.
Exam Tips
When describing the Mass Flow Hypothesis, ensure you include:
- Sucrose loading at the source via companion cells (active transport of sucrose lowers water potential, causing water entry from the xylem by osmosis).
- Pressure gradient driving mass flow.
- Sucrose unloading at the sink (active transport raises water potential, causing water loss back into the xylem).
Using terms like active transport, osmosis, hydrostatic pressure, and mass flow will help secure full marks!
No answer provided.
Explain how the Mass Flow Hypothesis accounts for the movement of sucrose in plants. (6 marks)
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Sucrose is actively transported via companion cells into sieve tube elements at the source (e.g., leaves). (1)
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This lowers water potential, causing water to enter by osmosis, from the xylem. (1)
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This increases hydrostatic pressure at the source end of the phloem. (1)
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A pressure gradient is established, with high pressure at the source and low pressure at the sink. (1)
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Sucrose moves by mass flow down the pressure gradient from source to sink. (1)
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At the sink (e.g., roots or growing tissues), sucrose is actively removed from sieve tubes for respiration or storage. (1)
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This increases water potential at the sink end of the phloem, so water leaves the phloem by osmosis. (1)
Practice Question 1
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!
Practice Question 2
If you want to try out another one, check this video out and see how you do!