Transport in Plants — BIOLOGY STD 11 Science — Question

The responsibility of absorption of water and minerals is more specifically the function of the root hairs that are present in millions at the tips of the roots. Root hairs are thin-walled slender extensions of root epidermal cells that greatly increase the surface area for absorption. Water is absorbed along with mineral solutes, by the root hairs, purely by diffusion. Once water is absorbed by the root hairs, it can move deeper into root layers by two distinct pathways:

• Apoplast pathway.
• Symplast pathway.

Within a plant, the apoplast is the free diffusional space outside the plasma membrane. It is interrupted by the Casparian strip in roots, air spaces between plant cells and the cuticula of the plant. Structurally, the apoplast is formed by the continuum of cell walls of adjacent cells as well as the extracellular spaces, forming a tissue level compartment comparable to the symplast. The apoplastic route facilitates the transport of water and solutes across a tissue or organ. This process is known as apoplastic transport. The symplast of a plant is the inner side of the plasma membrane in which water (and low-molecular solutes) can freely diffuse. The plasmodesmata allow the direct flow of small molecules such as sugars, amino acids, and ions between cells. Larger molecules, including transcription factors and plant viruses, can also be transported through with the help of actin structures. This allows direct cytoplasm to cytoplasm flow of water and other nutrients along concentration gradients. In particular, it is used in the root systems to bring in nutrients from soil. It moves these solutes from epidermis cells through the cortex into the endodermis and eventually the pericycle, where it can be moved into the xylem for long distance transport. It is contrasted with the apoplastic flow, which uses cell wall transport.