photosynthases are transported through phloem by symplatic or apoplastic transport system. Gve a comprehensive characterization of the two transport systems

Answer 1

The transport of photosynthates, such as sugars and amino acids, through the plant's phloem can occur via either the symplastic or apoplastic transport system.

1. Symplastic Transport System:
- The symplastic transport system involves movement of substances through the living cytoplasmic continuum, which includes plasmodesmata.
- Plasmodesmata are microscopic channels that connect the cytoplasm of adjacent plant cells, allowing direct communication and transport between cells.
- In this system, photosynthates are loaded into the sieve elements of the phloem, which are specialized cells responsible for long-distance transport in the plant.
- During loading, photosynthates enter the companion cells surrounding the sieve elements through plasmodesmata.
- From the companion cells, the photosynthates move into the sieve elements via further plasmodesmatal connections.
- Once inside the sieve elements, the photosynthates are transported from source cells (photosynthetic tissues) to sink cells (storage organs like roots, fruits, or growing tissues) through pressure flow.
- Symplastic transport relies on the movement of substances through plasmodesmata, thus allowing direct cytoplasmic connectivity between different plant cells.

2. Apoplastic Transport System:
- The apoplastic transport system involves the movement of substances outside the living cytoplasmic continuum, primarily through the cell walls, intercellular spaces, and the xylem.
- In this system, photosynthates move into the apoplast (extracellular space) of the source cells.
- These photosynthates travel through the cell walls and intercellular spaces until they reach the phloem in the sieve elements.
- Sieve elements have specialized pores, called sieve plates, that allow apoplastic transport substances to enter.
- Once within the sieve elements, the photosynthates are transported through sieve tubes to the sink cells via pressure flow, similar to the symplastic transport system.
- Unlike the symplastic transport system, the apoplastic transport system does not rely on direct cytoplasmic connectivity between cells, instead using extracellular spaces for transport.

In summary, both the symplastic and apoplastic transport systems play roles in the movement of photosynthates through the phloem. The symplastic system involves transport through plasmodesmata and direct cytoplasmic connectivity, while the apoplastic system primarily utilizes extracellular spaces and cell walls.