Phloem is alive because its cells, specifically sieve elements, need to be metabolically active to actively transport sugars and other nutrients throughout the plant.
The Vital Role of Living Cells in Phloem Transport
The phloem is the vascular tissue responsible for translocating photosynthates (sugars produced during photosynthesis) from source tissues (e.g., leaves) to sink tissues (e.g., roots, developing fruits). This long-distance transport requires living cells.
Sieve Elements: The Conductors
Sieve elements are the primary conducting cells of the phloem. Although they lack nuclei and other organelles at maturity to minimize obstruction to flow, they are alive and depend on companion cells for metabolic support.
- Sieve Plates: Sieve elements are connected end-to-end, forming sieve tubes. The end walls where they connect are perforated, forming sieve plates. These plates facilitate the flow of phloem sap.
- Cytoplasmic Streaming: The cytoplasm within sieve elements, while simplified, facilitates the movement of sugars and other substances. This process requires energy and metabolic activity.
Companion Cells: The Metabolic Support
Companion cells are specialized parenchyma cells closely associated with sieve elements. They provide essential metabolic support for the enucleate sieve elements.
- ATP Production: Companion cells contain organelles like mitochondria that produce ATP (energy) needed for phloem loading and unloading.
- Protein Synthesis: They synthesize proteins and other essential molecules that are then transported to the sieve elements.
- Regulation: Companion cells also regulate the environment of the sieve elements, ensuring optimal conditions for transport.
Why Living Cells are Necessary:
The translocation of sugars through the phloem relies on active transport mechanisms, particularly at the source (where sugars are loaded into the phloem) and the sink (where sugars are unloaded). These processes require energy expenditure, which is dependent on living cells. A proposed mechanism is the pressure-flow hypothesis. While technically passive in the sieve tube, the loading and unloading necessitate living cells.
Contrast with Xylem
It is important to note that the xylem, the other major vascular tissue in plants, is not alive at maturity. Xylem cells, such as tracheids and vessel elements, die, and their cell walls form hollow conduits for water transport. The structural rigidity imparted by these dead cells also provides support to the plant. The differing structures and functions account for these differences.
