photosynthases are transported through phloem by symplatic or apoplastic transport system. Gve a comprehensive characterization of the two transport systems
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.
The phloem is responsible for the transport of various substances, including photosynthates (such as sugars), throughout the plant. This transport can occur through two different systems: symplastic transport and apoplastic transport. Here is a comprehensive characterization of the two transport systems:
1. Symplastic Transport:
- Symplastic transport refers to the movement of substances through the symplast, which refers to the continuous cytoplasm of plant cells interconnected by plasmodesmata.
- In this transport system, photosynthates move from cell to cell through plasmodesmata, which are small channels connecting adjacent cells.
- This transport mechanism allows for direct communication between cells, as well as the exchange of nutrients and signaling molecules.
- The movement of photosynthates through the symplast is regulated by the activity of transport proteins, enzymes, and the osmotic gradient between cells.
- Symplastic transport is relatively slow compared to apoplastic transport but provides tighter control over the movement of substances.
2. Apoplastic Transport:
- Apoplastic transport refers to the movement of substances through the apoplast, which refers to the extracellular spaces, cell walls, and intercellular spaces.
- In this transport system, photosynthates can move freely through the cell walls and intercellular spaces without entering the cytoplasm of cells as long as there are no physical barriers.
- This transport mechanism relies on the water potential gradient between cells, which facilitates the movement of solutes along with the bulk flow of water.
- In the apoplastic transport system, photosynthates can bypass certain cells or tissues, enabling faster long-distance transport.
- However, apoplastic transport lacks the selectivity and regulation provided by the symplastic transport system, as it does not involve direct cell-to-cell communication.
In summary, symplastic transport involves the movement of substances through plasmodesmata and allows for direct cell-to-cell communication, while apoplastic transport involves movement through the extracellular spaces and provides faster long-distance transport but lacks selectivity and regulation. Both systems contribute to the transport of photosynthates through the phloem.