Difference between Apoplast and Symplast

Water is essential for all living organisms. It provides the medium for transporting solutes to various parts. In plants water is absorbed through roots. There are three methods for the transportation of water in plants. They are diffusion, root pressure and transpiration pull.

Some of the plants are shorter, hence water can be translocated through diffusion and root pressure. But in order to transport them to longer distances, diffusion and root pressure are not enough. Diffusion is a very slow process and root pressure can’t uplift water to more heights. Hence, for long distance transportation of water, transpiration pull mechanism is used. Here, when water is evaporated from leaves, tension is created between the water molecules that pulls each other. This transpiration pull occurs in xylem vessels.

In order to transport water up to the xylem vessels, generally there are two pathways. They are apoplastic pathway and symplastic pathway.

What is Apoplastic Pathway?

The term "apoplast" was coined by a scientist named "Munch" in the year 1930. In the word apoplast, "apo" means "away" and "plast" means cytoplasm. Apoplast refers to the outer space surrounding the cell membrane away from the cytoplasm. It is the dead part of plant cells that play a crucial role in transporting substances.

Water and several gases are transported through apoplastic pathway. Once water is absorbed by roots, it passes through the intercellular spaces between the cells and they continue to move like this until they reach endodermis. As endodermis is impermeable to water, water from here on must follow symplastic pathway to enter into the pericycle.

As the water is not passed within the cells, their movement is not affected by the metabolic activities of the cell. Movement is through simple diffusion and relatively faster. It is a passive process. In xylems, water movement is only through apoplastic pathway.

Recent discoveries showed that not only water, many nutrients and other substances are also passed in this pathway. Not all of them are simply passed. Some of them are deposited within these cell walls during transportation. Phytohormones are also present in apoplasts of xylem that affects the absorption of ions.

Movement of water through apoplastic pathway is divided into two parts as given below −

• First region is the passage of water through epidermis, cortex up to endodermis

• Second region is the transfer from pericycle into the stele i.e. xylem

Endodermis is the barrier between these two regions.

What is Symplastic Pathway?

Symplast represents the inner parts of the cell. They refer to the cytoplasm of the cells that are interconnected with the help of plasmodesmata. It is the living part of the plant tissue through which movement of water and other solutes occurs. Water is entered into the cell through plasma membrane. The membrane of the cells many carrier proteins embedded within the lipid bi layer. The proteins that carry water are known as "Aquaporins". Aquaporins are involved especially in the movement of water through the membrane. They promote the movement through symplastic pathway.

Here, the water is entered into the cell through plasma membrane which is selectively permeable. It allows only certain substances to pass through it. Once it is entered into the cell sap, movement of those substances is aided by cytoplasmic streaming. Water is moved to the next cell with the help of plasmodesmatal connections between those two cells.

As the water is moving through the membrane through osmosis, it is active transport. It requires energy for transportation. As the membrane restricts the flow, movement through symplast is relatively slower.

Transportation of Water through Apoplast and Symplast

Initially water present in the soil is absorbed by root hairs through the process of osmosis. This water reaches the outer layer of the root i.e. epidermis. From epidermis, most of the water is passed through intercellular spaces between the cell walls (apoplastic pathway). Some of the water is passed inside the cell through the membrane (symplastic pathway). This water travels to the cortex and then reaches the endodermal layer.

The walls of endodermis has suberin deposition and are referred as casparian strips. This layer acts as a barrier for the movement through apoplastic pathway. Suberin is hydrophobic hence doesn’t allow water to pass through them. So, in order to cross the endodermal layer, water must follow only symplastic pathway. Once it reaches the pericycle, water is transported only through apoplast in xylem.

Water travels faster in apoplastic pathway. This is due to that it can easily pass through the intercellular spaces. Whereas in symplastic pathway, movement of water is relatively slower. As the water need to enter inside the cell whose membrane is selectively permeable and inside the cell movement of water is through cytoplasmic streaming. These are slow processes. Also it requires energy for movement through symplasts.

Apoplast vs Symplast

The following table highlights the major differences between Apoplast and Symplast −

Apoplast	Symplast
Apoplast refers to the space outside the cell	Symplast refers to the inner cytoplasmic part of the plant
It is the dead part of plant tissue	It is the living part of plant tissue
Movement of water is through intercellular spaces	Movement of water is through plasmodesmatal connections and cytoplasmic streaming
Water movement is through diffusion	Water movement is through the process of osmosis
It is completely permeable	It is selectively permeable
Movement of water is relatively faster	Movement of water is slower
Passive absorption of water occurs	Active absorption of water occurs
Here, the movement is not affected by any metabolic activities of cell	Cellular metabolic activities have impact on the movement of water
In xylem, water is moved only through apoplast	In endodermal cells, movement of water is only through symplast

Conclusion

Apoplast and symplast pathways are used for translocation of water to short distances. They carry water from the surrounding soil into the xylem. Apoplast is the passive transport of water through simple diffusion based on hydrostatic pressure and symplast is the active transport of water through osmosis based on osmotic gradient.

Most of the water and ions are transported through apoplastic pathway up to endodermis. From endodermis, the movement is mostly through symplast.