Phloem Transport — Core Principles
Core Principles
Phloem transport, or translocation, is the movement of organic solutes, primarily sucrose, throughout a plant. This vital process distributes food from 'sources' (e.g., mature leaves where photosynthesis occurs) to 'sinks' (e.
g., roots, fruits, growing tips where food is used or stored). The phloem tissue comprises sieve tube elements (the main conduits), companion cells (metabolically active, aiding loading/unloading), phloem parenchyma, and phloem fibers.
The widely accepted mechanism is the Pressure Flow Hypothesis. At the source, sugars are actively loaded into sieve tubes, increasing solute concentration. Water then moves in from the xylem by osmosis, building high turgor pressure.
This pressure drives the 'phloem sap' (sugary solution) through the sieve tubes. At the sink, sugars are actively unloaded, decreasing solute concentration. Water then moves out of the sieve tubes, reducing pressure.
This continuous pressure gradient ensures efficient, energy-dependent distribution of nutrients, crucial for plant growth and survival.
Important Differences
vs Xylem Transport
| Aspect | This Topic | Xylem Transport |
|---|---|---|
| Tissue involved | Phloem | Xylem |
| Substances transported | Organic solutes (sugars, amino acids, hormones) | Water and mineral ions |
| Direction of transport | Bidirectional (source to sink, can be up or down) | Unidirectional (roots to shoots) |
| Driving force | Positive hydrostatic pressure gradient (Pressure Flow Hypothesis) | Negative pressure/tension (Transpiration Pull) |
| Energy requirement | Energy-dependent (active loading/unloading) | Passive (driven by transpiration, no metabolic energy required by xylem cells) |
| Conducting cells | Sieve tube elements (living, anucleate) | Tracheids and vessel elements (dead at maturity) |
| Associated cells | Companion cells | Xylem parenchyma, xylem fibers |