The Mass Flow Hypothesis explains that the movement of phloem sap is driven by a hydrostatic pressure gradient between the source and the sink.
At the source, the high concentration of sucrose lowers the water potential () of the phloem sap, causing water to move from the adjacent xylem into the phloem by osmosis.
This influx of water increases the hydrostatic pressure at the source end of the sieve tube.
At the sink, sucrose is actively or passively removed for use or storage, which increases the water potential of the sap, causing water to leave the phloem and return to the xylem.
The resulting pressure difference () forces the entire column of sap to flow en masse from the high-pressure source to the low-pressure sink.
Active Loading is the primary method for moving sucrose into the phloem against its concentration gradient, requiring energy in the form of ATP.
Step 1: Proton Pumping: Companion cells use ATP to actively pump hydrogen ions () out of their cytoplasm and into the cell wall space.
Step 2: Co-transport: A high concentration of in the cell wall creates an electrochemical gradient. ions diffuse back into the companion cell through a co-transporter protein, bringing a sucrose molecule with them against its gradient.
Step 3: Diffusion to Sieve Tubes: Once inside the companion cell, sucrose moves into the sieve tube elements via plasmodesmata (cytoplasmic channels connecting the cells).
Avoid the 'Diffusion' Trap: Never describe the movement of sap within the sieve tube as diffusion. While loading involves diffusion/active transport, the movement through the tube is mass flow (bulk movement) driven by pressure.
Identify Source and Sink: Remember that sources and sinks can change. In summer, leaves are sources and roots are sinks. In early spring, storage organs like tubers act as sources to provide energy for new leaf growth (the new sink).
Link to Water Potential: Always explain the movement of water in terms of water potential gradients. Sucrose loading lowers water potential, which is the trigger for water entry and pressure increase.
Evidence Check: Be prepared to discuss evidence for mass flow (e.g., sap oozing from punctured phloem indicates pressure) and evidence against it (e.g., different solutes moving at different speeds).
Gravity Misconception: Students often assume phloem only moves downwards because of gravity. In reality, it moves to wherever the demand (sink) is, which can be upwards to a developing fruit or flower.
Sieve Tube Vitality: Do not mistake sieve tube elements for dead cells. Although they lack a nucleus and many organelles to facilitate flow, they must remain living to maintain the cell membrane required for osmotic gradients.
ATP Usage: ATP is not used to 'push' the sap directly; it is used by companion cells to load sucrose, which then creates the physical pressure gradient that moves the sap.
