Mechanism of apoplast and symplasmic
The apoplast and symplasmic pathways are two mechanisms involved in the movement of water and solutes in plants.
The apoplast pathway is the extracellular pathway that involves the movement of water and solutes through cell walls and intercellular spaces. In this pathway, water and solutes move freely through the apoplastic continuum without crossing plasma membranes. This pathway is driven by the pressure gradient created by transpiration, which pulls water from roots to leaves. The apoplast pathway is generally fast and allows for efficient long-distance transport of water and solutes. However, it does not allow for selective uptake or regulation of solute movements.
The symplasmic pathway, on the other hand, involves the movement of water and solutes through the cytoplasm of living cells. In this pathway, water and solutes move through plasmodesmata, which are channels that connect the cytoplasm of adjacent cells. This pathway allows for more selective uptake and regulation of solutes, as they must pass through the plasma membrane of cells before entering the symplasm. The symplasmic pathway is slower than the apoplast pathway but allows for greater control over the movement of solutes.
Both pathways are interconnected and complement each other in plant physiology. Water and solutes can move through both the apoplast and symplasmic pathways depending on the specific needs of the plant. The apoplast pathway allows for the fast and efficient transport of water and solutes over long distances, while the symplasmic pathway allows for selective uptake and regulation of solutes within individual cells.
characteristics of ordinary and intermediary companion cells
Ordinary Companion Cells:
1. Location: Ordinary companion cells are found in close association with sieve elements in the phloem tissue.
2. Cell Structure: They have a dense cytoplasm and a prominent nucleus. They are smaller in size compared to sieve elements.
3. Function: Ordinary companion cells play a crucial role in supporting the sieve elements. They provide metabolic support to sieve elements by supplying energy and nutrients.
4. Plasmodesmata: Ordinary companion cells have numerous plasmodesmata, which are intercellular channels connecting them with sieve elements. These channels facilitate the exchange of substances between the companion cells and sieve elements.
5. Transport Function: Ordinary companion cells are involved in loading and unloading of organic solutes, such as sugars, into and out of the sieve elements. They help maintain the pressure gradient necessary for the movement of phloem sap.
6. Metabolic Activities: Ordinary companion cells are highly metabolically active. They perform various metabolic functions, such as protein synthesis and ATP synthesis, to support the energy requirements of sieve elements.
Intermediary Companion Cells (Transfer Cells):
1. Location: Intermediary companion cells, also known as transfer cells, are typically found in specialized regions of the plant, such as the vascular tissues of developing seeds, young leaves, and growing roots.
2. Cell Structure: Transfer cells have an irregular shape with numerous finger-like projections called ingrowths. These ingrowths increase the surface area of the cell, facilitating enhanced transport.
3. Function: Transfer cells are specialized for enhancing nutrient uptake and transport. They play a key role in the transfer of solutes between the symplasm of the parenchyma cells and the adjacent phloem or xylem tissue.
4. Plasmodesmata: Transfer cells have an abundance of plasmodesmata that connect them with neighboring cells, allowing for rapid exchange of solutes.
5. Transport Function: Transfer cells play a crucial role in facilitating the movement of specific solutes, such as sugars and ions, across various plant tissues. They actively upregulate transport proteins to enhance nutrient uptake and transport.
6. Metabolic Activities: Transfer cells are highly metabolically active and have a high concentration of mitochondria to meet their energy demands. They have a high rate of ATP production to support the active transport processes involved in nutrient uptake and transport.
In summary, ordinary companion cells primarily provide metabolic support to sieve elements in the phloem, while intermediary companion cells (transfer cells) are specialized for enhancing nutrient uptake and transport in specific regions of the plant.
The apoplast and symplasm are two interconnected systems within plant cells that play important roles in transport processes.
1. Apoplast:
The apoplast is the space outside the plasma membrane in plant cells that includes the cell walls, intercellular spaces, and the extracellular matrix. It provides a continuous network for the movement of water, ions, and other molecules through the cell walls of adjacent cells.
- Passive movement: In the apoplast pathway, substances can passively move through the cell walls and intercellular spaces by diffusion. This is the primary route for the movement of water and solutes within the plant.
- Water movement: Water crosses the apoplast via the apoplastic route, often facilitated by hydrostatic pressure gradients. It moves through the porous cell walls and intercellular spaces, following a gradient established by transpiration from the leaves.
- Nutrient uptake: Nutrients, such as minerals, ions, and small organic molecules, can enter the root cells by crossing the apoplast. These substances may be later transferred to the symplast for further transport and distribution within the plant.
2. Symplasm:
The symplasm is the interconnected network of living cytoplasm within plant cells that is linked by plasmodesmata - small channels that traverse the cell walls. It allows for the selective movement of molecules and communication between neighboring cells.
- Controlled movement: In the symplasmic pathway, substances selectively move through plasmodesmata and are transported from one cell to another within the interconnected cytoplasm. This route allows for control of substances being transported and plays a role in regulating plant growth and development.
- Signal transmission: Plasmodesmata enable the transmission of signaling molecules, such as hormones and secondary messengers, between cells. This allows for coordinated responses to environmental cues and the coordination of developmental processes.
- Nutrient sharing: The symplastic pathway also facilitates the sharing of nutrients and metabolites between cells. This is especially important in plants with specialized tissues, such as the phloem, which transports sugars from photosynthetic tissues to non-photosynthetic tissues.
Overall, the apoplast and symplasm are interconnected systems that work together to facilitate the movement of water, nutrients, and signals within plant cells and between adjacent cells. The apoplast primarily involves the movement through cell walls, while the symplasm primarily involves movement via plasmodesmata and interconnected cytoplasm.