Photosyntheses are transported through phloem by symplastic or apoplastic transport systems. Give a comprehensive characterization of the two transport systems
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.
The transportation of photosynthates (products of photosynthesis) through phloem can occur via two main mechanisms: symplastic transport and apoplastic transport. Let's explore each of these systems in detail:
1. Symplastic Transport:
Symplastic transport refers to the movement of substances through the continuous cytoplasmic connections between plant cells, known as plasmodesmata. This interconnected network allows for the direct transfer of photosynthates from one cell to another, bypassing the extracellular space.
Characteristics of symplastic transport:
- Continuous pathway: The plasmodesmata create an unbroken network of cytoplasmic connections throughout the plant, ensuring a continuous pathway for the flow of photosynthates.
- Controlled movement: The symplastic transport system allows for a regulated movement of solutes, as it requires the opening and closing of specific plasmodesmata.
- Selectivity: Certain types of plasmodesmata have mechanisms to restrict the movement of specific substances, ensuring that only necessary solutes are transported.
2. Apoplastic Transport:
Apoplastic transport involves the movement of photosynthates through the extracellular spaces of plant tissues. In this mechanism, solutes follow a path outside the cells, utilizing the porous cell walls, intercellular spaces, and xylem vessels.
Characteristics of apoplastic transport:
- Extracellular pathway: In apoplastic transport, solutes move through the cell walls and intercellular spaces, maintaining the extracellular nature of the transport pathway.
- Rapid movement: The apoplastic system allows for a fast and unrestricted movement of solutes due to the lack of cell membrane regulation.
- Limited selectivity: Apoplastic transport does not have as strict selectivity mechanisms as symplastic transport, potentially enabling a broader range of solutes to move freely.
It's important to note that the symplastic and apoplastic systems are not mutually exclusive; they often work in conjunction to ensure the efficient transportation of photosynthates. The symplastic route is commonly used for selective long-distance transport, while apoplastic transport aids in the rapid movement of water and soluble substances within a localized area.
Overall, both symplastic and apoplastic transport systems play crucial roles in transporting photosynthates through the phloem, offering distinct characteristics and advantages that contribute to the functioning of the plant.