The Pressure Flow Hypothesis, also known as the Mass Flow Hypothesis, is the most widely accepted scientific explanation for how plants move sugars from where they are made to where they are needed for growth or storage. This process, called translocation, is fundamental to a plant’s survival, ensuring that energy produced in the leaves reaches all other parts, such as the roots, fruits, and developing buds. Proposed in 1930, the hypothesis details a physical mechanism driven by internal pressure differences, rather than simple diffusion, to achieve rapid, long-distance transport of these organic compounds. It describes the movement of a sugary solution, called phloem sap, through specialized tissue.
Key Structures and Roles
Transport occurs within the phloem, a specialized vascular tissue that distributes food throughout the plant. The primary conducting cells are the sieve tube elements, which are long, cylindrical cells joined end-to-end to form continuous tubes. These elements are largely hollow, having lost their nucleus and most organelles at maturity to maximize space for sap flow. Their end walls feature perforated sieve plates that allow the sugary solution to pass through.
Adjacent to every sieve tube element is a companion cell, which provides metabolic support because the sieve tube lacks many functional components. Companion cells are densely packed with mitochondria and play an active role in regulating the movement of sugars into and out of the sieve tube.
The movement is organized around two functional locations known as the “source” and the “sink.” A source is any plant part that produces or releases sugars in excess of its own needs, such as a mature leaf performing photosynthesis. Conversely, a sink is any part that consumes or stores sugars, like developing fruit, growing root tips, or storage organs. This relationship is dynamic; for example, a root can be a sink in the summer (storing sugar) and a source in the spring (releasing stored sugar for new growth).
The Mechanism of Mass Movement
The process begins with phloem loading, where sugar molecules, primarily sucrose, move from the source cells into the sieve tube elements. This loading significantly increases the concentration of solutes within the sieve tube, creating a low water potential at the source end. Since the phloem runs parallel to the xylem, this low water potential causes water to move by osmosis from the adjacent xylem vessels into the sieve tubes.
The influx of water generates a high hydrostatic pressure, also known as turgor pressure, at the source end of the phloem pathway. This pressure acts as the driving force for the mechanism. The accumulated volume of sugary solution (phloem sap) is then physically pushed through the sieve tubes toward areas of lower pressure.
As the sap moves toward the sink, phloem unloading occurs, where sugar molecules exit the sieve tube elements for use or storage by the sink cells. This removal of solutes causes the concentration within the phloem to drop. In response to the resulting higher water potential, water moves out of the sieve tubes, often returning to the neighboring xylem vessels.
The water movement out of the phloem at the sink reduces the turgor pressure, completing the pressure gradient. The continuous difference in hydrostatic pressure—high at the source and low at the sink—drives the steady, unidirectional mass movement of the phloem sap. This pressure-driven flow allows the plant to transport large amounts of sugar rapidly over long distances.
Energy Use and Directional Control
While the physical movement of the phloem sap between the source and the sink is a passive process driven by the pressure gradient, the creation and maintenance of this gradient require energy expenditure. The loading of sucrose into the sieve tube elements at the source is achieved through active transport. Specialized membrane proteins, often proton-sucrose symporters, use energy (ATP) to actively pump the sugar against a concentration gradient.
Similarly, the unloading of sugars at the sink tissues often requires active transport to move the sucrose out of the phloem for storage or metabolism. This use of metabolic energy at both ends of the transport pathway explains why the companion cells are so metabolically active and why the overall process cannot be sustained by passive forces alone.
The direction of the sap flow is entirely determined by the location of the source and the sink, as the flow is always from the high-pressure area (source) to the low-pressure area (sink). A plant regulates nutrient distribution by changing which tissues act as a sink based on its developmental needs. For example, in the spring, developing leaves and flowers are strong sinks, drawing sugars from storage roots. In the summer, mature leaves become the primary source, feeding storage roots and developing fruits that become the new sinks.