Transporting organic substances
Phloem transports organic substances through a plant. These substances, known as assimilates, include sucrose, amino acids and plant hormones.
Phloem contains specialised living cells. Two important cell types are sieve tube elements and companion cells.
Sieve tube elements and companion cells
| Structure | Adaptation and role |
|---|---|
| Sieve tube elements | Join to form tubes through which assimilates can move. Reduced cell contents provide less resistance to movement through the tube. |
| Sieve plates | Perforated regions between successive sieve tube elements allow assimilates to pass from one element to another. |
| Companion cells | Contain many mitochondria that supply ATP for active processes such as sucrose loading. |
| Plasmodesmata | Channels linking sieve tube elements with companion cells allow substances to move between their cytoplasm. |
This arrangement provides a low-resistance pathway for transported assimilates alongside companion cells that support energy-requiring transport processes.
The direction shown in the structural diagram represents one possible flow direction in a particular transport pathway. Across the plant, assimilates may be transported upwards or downwards depending on the locations of the source and sink.
Exam Tip: Make the structure–function links explicit: reduced sieve-tube contents → less resistance to flow; sieve plates → passage between adjacent elements; many companion-cell mitochondria → ATP for active loading.