Phloem transports organic solutes (mainly sugars such as sucrose, plus amino acids, hormones, RNAs and other organic molecules) and moves them from source to sink — i.e., from tissues that produce or release assimilates (usually mature leaves) to tissues that consume or store them (growing tips, roots, fruits, storage organs). Transport can be upward, downward or lateral depending on where sources and sinks are.
Explanation
- What is transported:
- Primarily sugars (sucrose) produced by photosynthesis.
- Also amino acids, organic acids, hormones, small RNAs, proteins, some ions and sometimes pathogens (e.g., plant viruses).
- Direction of transport:
- Phloem flow is source → sink. Typical examples: leaves → roots (downward), leaves → developing fruits/flowers (upward), leaves → young growing leaves or shoot tips (upward/lateral).
- Because sources and sinks can change with development and season, phloem transport is effectively bidirectional at the whole-plant level (different sieve tubes can carry sap in different directions).
- How it works (brief—pressure-flow hypothesis):
- Sugars are actively loaded into sieve-element/companion-cell complexes at the source, increasing solute concentration.
- Water enters the phloem by osmosis from the xylem, creating high turgor pressure at the source.
- Sap flows down a pressure gradient toward the sink, where sugars are unloaded and used or stored, lowering solute concentration and pressure.
- Water returns to the xylem and circulates back to sources.
This source-to-sink mechanism explains why flow direction depends on which tissues are acting as sources or sinks at a given time.