How Does the Symplast Pathway Facilitate Internal Transport in Plants?
All plants consist of symplast which is the interior of the plasma membrane where water and other low- molecular solutes like amino acids, sugars, and other ions between cells are freely disseminated. It is also called the protoplast in the plant. Simplast’ cells consist of more than one nucleus. It is the plasmodesmata that make sure that all the small molecules freely move and flow whereas the molecules larger in sizes such as plant viruses and transcription factors can be dispersed with acting structures. This ensures that there is a cytoplasm-to-cytoplasm transport of water and minerals which also include concentration gradients. Mainly it is used to bring all the nutrients from the soil through the root systems. It transports these solutes with the help of cortex from the epidermis cells inside the endodermis.
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When the transportation starts, the solutes extend to the endodermal cells by the way of apoplastic transport. Then these solutes are pushed into the Symplastic pathway because of the occupancy of the Casparian strip. When all the solutes are peaceably processed then they lastly reach the pericycle. From the pericycle, they are shifted to the xylem where the long-distance transport happens. Xylem transport is very much different from the apoplastic flow because it uses cell wall transport.
Explain Transportation in Plants: Xylem and Phloem Transport
The transportation in plants is carried out by two main tissues which carry out transport of food and minerals in plants. In this next section we will explain xylem and phloem.
Xylem
The work of xylem transport is the transport of water and minerals from the roots upwards to the different parts of the plant. Xylem transport water to all the parts of the plant. On the other hand, all the amino acids and sucrose is transported using the phloem transport to the other parts of the plant. Mature xylem incorporates many elongated dead cells which are positioned from end to end to form a continuous vessel or tube. To transport water in plants various tubes are used.
Mature Xylem Tubes:
Accommodates no cytoplasm.
Are resistant to water.
Have strong walls which have lignin that is like a woody material.
Xylem transport in plants is very prominent since a single tree consists of many xylem tissues. This way you can define transportation in plants.
Phloem
Now, as compared to xylem the phloem has living cells that are positioned end to end. Phloem consists of cytoplasm that moves from one cell to another through different holes.
The phloem transports different amino acids and sucrose up and down in the plant. This process takes place between the sources and the sinks. It is like where the substances are produced and where they are utilized or deposited. This whole process is called translocation. The xylem transport and phloem transport is essential in the growth of the plant.
This means, for example,
The sucrose is transported from sources that are inside the root to the sinks which are inside the leaf. This usually happens during springtime
In summer, the sucrose is transported from the sources that are in the leaves to the sinks that are in the roots.
When chemicals are applied like the pesticides, they move inside the plant by translocation. Phloem is also used to transport water in plants.
Vascular Bundle
During the whole process of transportation in plants, Xylem and Phloem tissues are found in clusters also known as vascular bundles. The position of the vascular bundles is different in different plants. Xylem and phloem in leaf, for example, can usually be found closer to the lower part of the plant.
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The Apoplast, Symplast, and Transmembrane Route
Now, the transport of water and minerals in plants is done through the Xylem and Phloem vessels as we saw above, various pathways that are used to pass the water to the xylem vessels.
The right transportation of food in plants decides the growth of a plant. The root of every plant is made in such a way that it helps in the transport of water in plants along with other solutes. To transport minerals in plants along with water, two major compartments are involving – the apoplastic and symplastic pathways.
What is the Apoplast Pathway?
The apoplastic pathway allows the transport of materials in plants into the cell walls and extracellular spaces. Xylem transport plays its part in the transportation process. This is exactly what is meant by apoplast pathway.
What is the Symplast Pathway?
In the symplastic pathway, the solutes and water progress along the cytosol. When the transport of substances in plants reaches this pathway, the materials are required to traverse the plasma membrane when moving from one cell to the other. This process is done via the plasmodesmata.
What is the Transmembrane Route?
In the transmembrane route, the water and dissolved minerals disperse from one cell to another by going through the cell wall to move out of one cell and enter into another. Phloem transport plays its part in the transportation process.
These 3 pathways or routes are not totally exclusive from one another, the transport of minerals in plants may use more than one pathway if required.
Fun Fact
Though symplast and apoplast are mutually existing parts of the plants, the symplast is the living part of the plant whereas the apoplast is the nonliving part of the plant.
And even though the apoplast has non-living parts, it's a faster pathway as compared to the apoplast. Also, Xylem and Phloem differ in various ways, still they both transport water in plants.
FAQs on Symplast: Meaning, Structure & Importance
1. What is the symplast pathway in plants?
The symplast pathway refers to the continuous network of living protoplasts within a plant. It consists of the cytoplasm of adjacent cells connected by microscopic channels called plasmodesmata, which pass through the cell walls. This creates a living, uninterrupted route for the movement of water and solutes from one cell to another without crossing the plasma membrane.
2. What is the main difference between the symplast and apoplast pathways?
The main difference between the symplast and apoplast pathways lies in whether they are living or non-living routes for transport in plants. The key distinctions are:
- Pathway Components: The symplast is the living pathway, consisting of the interconnected protoplasts of cells. In contrast, the apoplast is the non-living pathway, comprising the cell walls and the intercellular spaces.
- Transport Mechanism: Movement through the symplast is a slower, active process as it occurs through the cytoplasm and is regulated by the cell's metabolic state. Apoplastic movement is faster and passive, occurring mainly by diffusion through the non-living cell walls.
- Barrier Crossing: In the symplast, substances move between cells via plasmodesmata. In the apoplast, movement occurs outside the cell membrane until it is blocked by the Casparian strip in the roots.
3. How are plasmodesmata crucial for the symplastic pathway?
Plasmodesmata are the foundational structures of the symplastic pathway. They are narrow, membrane-lined channels that directly connect the cytoplasm of neighbouring plant cells, passing through the intervening cell walls. Their crucial role is to create a continuous cytoplasmic bridge, or 'symplast', across the plant tissue. This allows water, nutrients, hormones, and other signalling molecules to move directly from cell to cell, enabling intercellular communication and transport without having to cross the plasma membrane and cell wall repeatedly.
4. Why is the symplastic pathway essential for transport into the plant's vascular system?
The symplastic pathway is essential because the apoplastic route is blocked at the root's endodermis by a waxy layer called the Casparian strip. This strip is impermeable to water. Therefore, any water and minerals travelling through the apoplast must switch to the symplastic pathway to cross the endodermal cells. This forces the substances to pass through the living plasma membrane, giving the plant selective control over what enters the xylem, the main water-conducting tissue.
5. Is transport through the symplast faster or slower than the apoplast, and why?
Transport through the symplast is generally slower than transport through the apoplast. The reason for this difference in speed is the resistance faced. In the symplastic pathway, movement is hindered by the viscosity of the cytoplasm and the narrowness of the plasmodesmata channels. In contrast, the apoplastic pathway involves bulk flow through the porous cell walls, which offers much less resistance. However, the advantage of the slower symplastic route is that it allows for metabolic regulation and selective transport.
6. What types of substances are transported via the symplastic pathway?
While it is a primary route for water and mineral ions absorbed by roots, the symplastic pathway is also used to transport a variety of other essential substances. These include organic molecules like sugars and amino acids, as well as crucial signalling molecules such as plant hormones. This cell-to-cell communication is vital for coordinating growth, development, and responses to environmental changes throughout the plant.
