discuss the apoplastic and symplastic mechanisms regulating sugar transport between mesophyll cells and phloem
The transport of sugars between mesophyll cells and the phloem, a process known as phloem loading, involves both apoplastic and symplastic mechanisms.
1. Apoplastic Mechanism: The apoplast refers to the extracellular space outside the plasma membrane. In the apoplastic mechanism of phloem loading, sugars move through the cell walls and intercellular spaces between mesophyll cells without crossing any membrane barriers. This occurs through the following steps:
- Sucrose Synthesis: In the mesophyll cells, sucrose is synthesized from the products of photosynthesis, primarily glucose and fructose. This process occurs in the cytoplasm.
- Sucrose Export: Once synthesized, sucrose is exported from the cytoplasm into the apoplast. This export is mediated by transporters located in the plasma membrane, such as sucrose/H+ symporters or ATPase proton pumps. The sucrose is released into the apoplast and can move freely through the cell walls and intercellular spaces.
- Loading into Sieve Tube Elements: The released sucrose then enters the conducting cells of the phloem, known as sieve tube elements. These cells contain specialized plasma membrane proteins called sucrose transporters, which facilitate the uptake of sucrose from the apoplast into the sieve tube elements. This step relies on diffusion or facilitated transport through the membranes.
- Translocation: Once loaded into the sieve tube elements, the sucrose is transported through the phloem to other parts of the plant. This movement is driven by an osmotic pressure gradient, as the accumulation of sugars in the sieve tube elements results in an increase in sap pressure.
2. Symplastic Mechanism: The symplast refers to the interconnected network of living plant cells through plasmodesmata, which are cytoplasmic connections between adjacent cells. In the symplastic mechanism of phloem loading, sugars move from mesophyll cells to sieve tube elements through the symplast. The steps involved in this mechanism are as follows:
- Sucrose Synthesis: Similar to the apoplastic mechanism, sucrose is synthesized from glucose and fructose in the mesophyll cells.
- Plasmodesmata Transport: Sucrose molecules are transported from the cytoplasm of mesophyll cells to adjacent cells through plasmodesmata. This transport occurs through small pores in the cell walls called sieve pores or modified plasmodesmata that allow for the passage of macromolecules like sucrose.
- Symplastic Loading: Once inside the neighboring cells, the sucrose is transported through the symplast via plasmodesmata until it reaches the sieve tube elements. This movement relies on diffusion or cytoplasmic streaming.
- Loading into Sieve Tube Elements and Translocation: Similar to the apoplastic mechanism, once in the sieve tube elements, the sucrose is loaded through specific transporters and translocated through the phloem.
Overall, both apoplastic and symplastic mechanisms play important roles in regulating sugar transport between mesophyll cells and the phloem, ultimately enabling the efficient distribution of photosynthates throughout the plant.