Photosyntheses are transported through phloem by symplastic or apoplastic transport systems. Give a comprehensive characterization of the two transport systems

Answer 1

The transport of photosynthetic products, such as sugars, by phloem can occur through two main transport systems: symplastic and apoplastic. Both systems play a crucial role in facilitating the movement of these important molecules throughout the plant.

1. Symplastic Transport System:
- In the symplastic transport system, photosynthetic products move through the symplast, which includes the interconnected cytoplasmic network of living plant cells.
- This transport pathway utilizes plasmodesmata, narrow channels that connect adjacent plant cells. These channels enable the exchange of molecules and ions between cells.
- In the phloem, these plasmodesmata form sieve tubes, specialized structures where the phloem sap containing sugars flows.
- The movement of photosynthetic products through the symplast is relatively slow and relies on diffusion or active transport mechanisms.
- Symplastic transport is bidirectional, allowing the movement of sugars from source (photosynthetic tissues) to sink (non-photosynthetic tissues) and vice versa.
- It is an energy-consuming process and requires metabolic energy for transport.

2. Apoplastic Transport System:
- In the apoplastic transport system, photosynthetic products move through the non-living spaces outside the cells, mainly through the cell walls.
- The transport occurs through the apoplast, which is the extracellular matrix of the plant tissue composed of cellulose, hemicellulose, and pectin.
- Sugars can freely diffuse or be actively transported in the apoplast, as there are no cellular membranes to cross.
- The apoplastic pathway is relatively faster compared to the symplastic pathway due to the absence of cellular barriers.
- However, apoplastic transport is unidirectional, primarily moving sugars from source to sink cells.
- It does not require metabolic energy for transport, as it relies on passive diffusion or facilitated transport through specific carriers.

It is important to note that the symplastic and apoplastic transport systems can work in conjunction with each other and complement the movement of photosynthetic products. The relative contribution of each system can vary depending on the plant's developmental stage, environmental conditions, and the specific tissues involved.