Difference between Apoplast and Symplast

The movement of material in bulk from one location to another due to pressure differences between two points is referred to as mass flow. Different substances move at different rates in diffusion due to concentration gradients. To achieve bulk flow, a (+ve) hydrostatic pressure gradient or a (–ve) hydrostatic pressure gradient is utilized.

Water and minerals are absorbed by the millions of root hairs at the root tips due to diffusion. Root hairs enhance the amount of surface area accessible for absorption. Two mechanisms, the Apoplast pathway and the Symplast pathway, transport absorbed water deeper into root layers. The apoplast is the region outside of the plasma membrane where material can freely spread.

Apoplast

Plant cell walls, as well as the water contained within them, create a connected system (the cell wall is composed of cellulose fibres, between which are spaces filled with water). Water (including dissolved ions and solutes) is transported through cell walls via the apoplast pathway. 

Water flows from a plant’s root cortex to the endodermis mostly through this channel. To distinguish the living symplast from the dead apoplast, Munch invented the term “apoplast” in 1930. Inside the plant, the apoplast is the outer space of the plasma membrane, which allows the material to freely diffuse and is interrupted by the Casparian strip in roots.

Apoplast Pathway

The apoplast pathway transports water from root hair to xylem by passing through the cell walls of adjacent cells. The apoplastic route is blocked by a Casparian band of endodermal cells. As a result, water and ions are transported across the cortex via the symplastic pathway. 

So because apoplast is made up of non-living components, it is least affected by the root’s metabolic condition.

Symplast

The symplast, which is positioned on the inner side of the plasma membrane, allows water and low-molecular-weight solutes to enter freely. Symplast cells have a large number of nuclei. Because water enters the cytoplasm of the cell through the plasma membrane, the symplastic pathway must cross cell membranes. 

Since it passes through the cell membrane, the symplastic pathway is also known as the transmembrane pathway. Water transport in the symplastic route is aided by cytoplasmic streaming.

Symplast Pathway

The ion and water paths produced by symplast are referred to as the symplastic pathway. This route provides resistance to water flow because the root cells’ selective plasma membrane manages ion and water intake. 

The root’s metabolic condition has an impact on symplasty. The symplastic pathway occurs beyond the endodermis in plants with secondary growth.

Water Movement in Apoplast

The root hair transports water up and around the plant, while the xylem and phloem move solutes through the root, stem, and plant.

Root Pressure

As ions from the soil are actively transported into the vascular tissues of the roots, water follows (its potential gradient) and increases the pressure inside the xylem.

This positive pressure is referred to as root pressure. It helps to carry water up the stem to small heights.

Experiment to prove the existence of Root Pressure

Cut a delicate plant stem horizontally towards the base early in the morning, while there is still some moisture in the air. Drops of solution seep from the cut stem. This is because the root pressure is positive. 

At night and early in the morning, evaporation is modest. Extra water collects as droplets along vein openings near the tips of grass blades and leaves of diverse herbaceous components as a result. Guttation is the word for this type of liquid-phase water loss. Only a small amount of root pressure can help in water transmission. It does not affect the transportation of water up tall trees.

Root pressure’s primary role is to re-establish continuous chains of water molecules in the xylem, which are regularly broken due to transpiration-induced stresses. In most plants, the majority of water is carried via transpiration pull.

Transpiration Pull

In plants, water flows upward at high rates (up to 15 m/hr) via the xylem. The major force that pushes water through the plant is the transpiration pull. As transpiration is a force that propels you forward. It’s known as the cohesion-tension-transpiration pull model of water movement.

Similarities between Apoplast and Symplast

Osmosis allows the root hair cells to absorb water or moisture from the soil. The water is subsequently transferred through the root cortex to the xylem and then to the root. Osmosis is used for transportation. 

The apoplast is the pathway by which water travels from the root cortex’s cell walls and intercellular space. The symplastic pathway transports water through the protoplasts of the root cortex.

The apoplast pathway is a completely permeable path where water moves by passive diffusion. The symplast, on the other hand, is a selectively permeable pathway in which water is moved through osmosis. The endodermis blocks the apoplast route, preventing water and any solutes dissolved in water from entering through this layer. 

Water can also travel through the endodermis by crossing the membrane of endodermal cells twice. Water movement in and out of the apoplast’s xylem, which is a component of the apoplast, may be controlled since it must enter the endodermis’ symplast.

Conclusion

Plant cell walls, as well as the water contained within them, create a connected system (the cell wall is composed of cellulose fibres, between which are spaces filled with water). Water (including dissolved ions and solutes) is transported through cell walls via the apoplast pathway. 

The symplast, which is positioned on the inner side of the plasma membrane, allows water and low-molecular-weight solutes to enter freely. Symplast cells have a large number of nuclei. Because water enters the cytoplasm of the cell through the plasma membrane, the symplastic pathway must cross cell membranes.