No, phloem transports from source to sink, not the other way around.
Understanding Phloem Transport
Phloem is a crucial vascular tissue in plants responsible for transporting sugars, primarily sucrose, produced during photosynthesis. This transport process is vital for distributing energy and nutrients throughout the plant. The direction of phloem flow is from a source to a sink, but what exactly are these?
Defining Source and Sink
- Source: A source is any part of the plant that produces more photosynthate (sugars) than it uses. The primary example of a source is a mature leaf.
- Sink: A sink is any part of the plant that consumes or stores photosynthate. Examples of sinks include roots, developing fruits, young leaves, and storage organs.
Phloem Flow Direction
According to the reference, "Sucrose is transported by the vascular tissue - phloem, from source to a sink." The primary direction of phloem transport is from where sugars are produced (source) to where they are needed or stored (sink). This ensures all parts of the plant have access to necessary nutrients.
Reversible Roles of Source and Sink
Importantly, the roles of source and sink are not fixed. The provided reference notes, "Source and Sink may reversed depending on the season." This means that a part of the plant that is a source at one time can become a sink and vice versa at different times of year or during the plant's lifecycle. For instance, storage organs like roots might become sources when a plant is producing new shoots in the spring.
Examples of Source and Sink:
- Leaves (Mature): These are often the primary source due to their high rate of photosynthesis.
- Roots: Usually function as a sink by taking in sugar for growth and storage.
- Developing fruits/flowers: These are considered as sinks as they require a large supply of sugars for their development.
- Growing shoots/new leaves: These generally function as sinks as they need energy to grow.
Table: Source vs Sink
| Feature | Source | Sink |
|---|---|---|
| Function | Sugar production | Sugar consumption/storage |
| Example | Mature leaves | Roots, fruits, young leaves |
| Direction | Supplies sugars to the phloem | Receives sugars from the phloem |
