# The Mass Flow Hypothesis ## Mass flow in the phloem - Phloem sap moves by **mass flow** up and down the plant - Phloem sap contains sucrose and other organic solutes - Carbohydrates are generally transported in the form of sucrose because: - Sucrose is a disaccharide and therefore packs more energy than a monosaccharide like glucose - Sucrose is less reactive than glucose as it is a non-reducing sugar

Mass flow is the bulk movement of phloem sap from areas of high hydrostatic pressure to areas of low hydrostatic pressure

## The hydrostatic pressure gradient - Mass flow occurs due to the presence of a **hydrostatic pressure gradient** between the source and the sink ### At the source - Sucrose is **actively loaded** into the sieve elements at the source - This lowers the water potential in the sieve tube - Water moves into the sieve elements by **osmosis** from the xylem - This increases the hydrostatic pressure at the source ### At the sink - Solutes are **unloaded** from the sieve elements at the sink - Water follows by osmosis out of the sieve tube and into the xylem - This lowers the hydrostatic pressure at the sink ### The pressure gradient drives flow - The difference in hydrostatic pressure between the source and the sink creates a **hydrostatic pressure gradient** - The pressure difference results in the **mass flow** of phloem sap from the high hydrostatic pressure area (source) to the low hydrostatic pressure area (sink) - The graph below illustrates how hydrostatic pressure decreases along the phloem from source to sink, creating the gradient that drives mass flow:

Mass flow means that solutes move faster than they could move by diffusion alone

## Movement of water and solutes | Location | Process | Effect on pressure | |----------|---------|-------------------| | **Source** | Sucrose actively loaded into sieve tubes | Water potential decreases | | **Source** | Water moves into sieve tubes by osmosis from xylem | Hydrostatic pressure increases | | **Sink** | Sucrose unloaded from sieve tubes | Water potential increases | | **Sink** | Water moves out of sieve tubes by osmosis into xylem | Hydrostatic pressure decreases | ## The complete process 1. At the **source** (e.g. photosynthesising leaf), sucrose is actively loaded into companion cells and then moves into sieve tube elements 2. This **lowers the water potential** inside the sieve tubes at the source 3. Water moves from the xylem into the phloem by **osmosis**, increasing the **hydrostatic pressure** at the source 4. At the **sink** (e.g. growing root tip), sucrose is actively unloaded from the sieve tube elements 5. This **raises the water potential** inside the sieve tubes at the sink 6. Water moves from the phloem into the xylem by **osmosis**, decreasing the **hydrostatic pressure** at the sink 7. The **hydrostatic pressure gradient** between source (high pressure) and sink (low pressure) causes **mass flow** of phloem sap down the pressure gradient

The source is not always the leaves and the sink is not always the roots; phloem sap can move either up or down the plant depending on the time of year

## Key points about mass flow - The process requires **metabolic energy** (ATP) for the active loading and unloading of sucrose - The hydrostatic pressure gradient is dependent on water moving in and out of the xylem vessels by osmosis - Mass flow enables rapid transport of organic solutes throughout the plant - The direction of flow can reverse depending on where sources and sinks are located - E.g. in spring, storage organs (tubers) act as sources, while developing leaves and flowers act as sinks

The hydrostatic pressure gradient is maintained by continuous loading at sources and unloading at sinks

## Evidence supporting the mass flow hypothesis - Phloem sap flows out under pressure when phloem tissue is cut, demonstrating high hydrostatic pressure - Radioactively labelled sugars can be tracked moving from source to sink in the phloem - Removal of a ring of bark (containing phloem) causes sugars to accumulate above the ring and deplete below it - The rate of flow is much faster than could be achieved by diffusion alone - Metabolic inhibitors (which stop ATP production) prevent translocation, showing the process requires energy