Absorption of water in plants

The plant absorbs water through the roots which are anchored in the soil. The water that is added to the soil will be transported through the roots. The main function of water and mineral absorption is done by the root hairs that are present at the tip of the root. The root hairs are the extensions to the epidermal cells of the root which enhance the surface area of absorption. The root hairs absorb the minerals mostly by the process of diffusion. The water that is taken by the root hairs will move further into the deep layers of the root by two separate ways such as symplast and apoplast pathways.

water absorption by roots
Apoplast system of water movement
This pathway involves movement of water through adjacent walls of the cells right from epidermis to the inner xylem vessels. The water movement does not occur through the casparian strips region of the endodermis. The movement of water through apoplastic pathway happens through the intercellular spaces and the cell walls. The continuous water flow that is maintained in this pathway is called apoplast. The water movement through the apoplast does not depend on the membrane of the cell as it occurs due to the presence of gradient. The apoplastic transport of water that creates the apoplast is not considered as the obstacle to the mass flow of the water. The water that evaporates into the atmosphere or into the intercellular spaces will generate certain pressure in the apoplast. The mass transport of water occurs because of the cohesive and adhesive features of water. Apoplast is the mass continuous water flow.
Symplast system of water movement
The protoplasts are interconnected in the symplast system of water movement. The water body in the neighbouring cells are connected through the cytoplasmic strands of each of the cells which happens due to the presence of plasmodesmata. The symplast way of water movement involves water transport through the cell cytoplasm via the plasmodesmata. The movement of water enters into the cells through the membranes of the cell which is observed to be a little slow. Symplastic water movement is observed to happen down the potential gradient. The symplastic water movement might be supported by cytoplasmic streaming. The chloroplast movement along with the movement of water is clearly observed in hydrilla leaf.
The movement of water in the roots takes place through the apoplast as the cortical cells are packed loosely. The cortical cells that are loosely arranged allow the movement of water without any resistance. The layer that is present as innermost region of the cortex is endodermis. Endodermis has the walls covered by suberized matrix which is called as casparian strip. When the movement of water cannot happen through the casparian strips, the water is made to travel through the membranes of the cells. The water movement occurs through the symplast and ultimately reach the xylem by crossing the cell membrane. Hence, the movement of water in the root hairs through the endodermis is symplastic. Symplast is the way of water movement through the xylem vessels.
In the xylem vessels, water moves across the cells or through the cells. In the young roots, xylem vessels receive water directly. The tracheids are non living cells and form the apoplast. There are some more structures that help in the transport of water or in absorption of minerals or water. The symbiotic association of the root system with the fungus is termed as mycorrhiza. The mycorrhiza occupy the entire young root as a fungal network and enter into the root cells. The hyphae of the fungus will spread onto a large area and absorb the water and mineral ions from the soil, which cannot be done by the root. The fungus and root mutually provide benefit. The fungus provides minerals and water to the root while the root provides nitrogen containing compounds and sugars to the mycorrhizal growth. Some plants like Pinus are known to have obligate association with the mycorrhizae.

symplast and apoplast



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