Phloem primarily carries nitrogen- and carbon-containing compounds throughout a plant.
Understanding Phloem Transport
The phloem is a vital tissue in plants, acting as a transport system responsible for moving essential substances from areas where they are produced or stored to areas where they are needed for growth, metabolism, or storage. According to the provided reference, the phloem is specifically tasked with the transport of nitrogen- and carbon-containing compounds from source tissues, such as leaves, to various sink tissues [82].
What Specific Compounds Are Carried?
While the reference broadly states "nitrogen- and carbon-containing compounds," the main substances transported by the phloem are sugars (like sucrose, a key carbon-containing compound produced during photosynthesis) and amino acids (nitrogen-containing compounds essential for building proteins).
- Sugars: Provide energy for plant cells and serve as building blocks for other organic molecules. Sucrose is the most common form transported.
- Amino Acids: Transported to growing regions and storage organs to synthesize proteins and other nitrogen-containing molecules.
These dissolved organic compounds are transported in a watery solution called phloem sap.
Source vs. Sink Tissues
The direction of transport in the phloem is typically from a source to a sink.
- Source Tissues: Areas where sugars and other compounds are produced or stored in excess.
- Mature leaves (photosynthesis)
- Storage organs like roots or tubers (when mobilizing stored reserves)
- Sink Tissues: Areas where sugars and compounds are needed for growth, metabolism, or storage.
- Roots
- Developing fruits and seeds
- Growing shoots and leaves
- Flowers
- Storage organs like tubers or bulbs (during storage)
Phloem's Key Components
The phloem tissue itself is composed of specialized cells that facilitate this transport. The reference highlights two primary cell types [82]:
| Cell Type | Primary Function | Support Provided By |
|---|---|---|
| Sieve Element (SE) | Responsible for the actual conduction of nutrients. | Metabolic support from companion cells. |
| Companion Cell (CC) | Provides metabolic support for the sieve element. | Directly associated with sieve elements. |
Sieve elements are the conducting cells, forming long tubes throughout the plant. They rely heavily on the adjacent companion cells, which provide metabolic energy and regulate the loading and unloading of sugars into the sieve elements.
In summary, phloem serves as the plant's internal pipeline for moving energy-rich sugars and other essential organic molecules from where they are made to where they are needed.
