IB Biology Topic B3.2.18: Adaptations of Phloem Sieve Tubes and Companion Cells for Translocation of Sap Notes

Sieve Tubes and Companion Cells Work as a Team

1. The phloem carries sap: sugars (mainly sucrose), amino acids, and other nutrients, in a process called translocation.
2. Sap moves from sources such as mature leaves to sinks such as roots, fruits, and growing tissues.

Structural Adaptations for Efficient Flow

1. Sieve plates
   1. These are the end walls between neighbouring sieve tube elements.
   2. They carry pores that let sap flow from one element to the next with little resistance.
2. Little cytoplasm and few organelles
   1. Sieve tube elements keep only a thin layer of cytoplasm and few organelles, leaving space for sap to flow.
   2. This creates a clear, continuous column of sap with low internal resistance.
3. No nucleus
   1. Mature sieve tube elements have no nucleus, so it does not take up space or use energy that would slow sap movement.
   2. Because of this, the sieve tube elements rely on companion cells for metabolic support.

1. Companion cells with dense cytoplasm and many mitochondria
   1. Companion cells have a nucleus, dense cytoplasm, and many mitochondria.
   2. The mitochondria supply the ATP needed for the active transport of sugars into and out of the sieve tubes.
2. Plasmodesmata linking the two cells
   1. Plasmodesmata are channels that connect companion cells and sieve tube elements.
   2. They let the companion cell pass sugars, signals, and resources into the sieve tube, keeping it alive and active.

How Sugars Enter the Phloem

Pressure Flow Moves Sap from Source to Sink

1. Loading at the source
   1. At the source, sucrose is actively loaded into the sieve tubes via the companion cells.
   2. The high sugar concentration lowers the water potential, so water enters by osmosis and raises the pressure in the sieve tube.
2. Unloading at the sink
   1. At the sink, sucrose is unloaded from the sieve tubes into surrounding cells.
   2. As sugar leaves, water follows it out, lowering the pressure at the sink end.
3. The pressure difference between source (high) and sink (low) drives sap along the phloem by bulk flow.

Xylem and Phloem Compared