Exchange and Transport Part Three

Movement of Water through Roots: The vast majority of plants are Terrestrial organisms. As a result they need to conserve water and are covered in a waterproof layer.They cannot exchange water over their whole body surface and so they have specialised exchange surfaces in the soil, called Root hairs. Root of a dicotyledonous plant(Cortex). Uptake of Water by Root Hairs: Plants constantly lose water by a process called Transpiration. All the water lost must be replaced with water that is absorbed by the root hairs. Each root hair is a long, thin extension of a root epidermal cell. These root hairs remain functional for a few weeks before dying back, to replaced by others nearer the growing tip. They are sufficient surfaces for exchange of water and miner ions because: They provide a large surface area as they are very long extensions and occur in thousands on each of the branched of a root. They have a thin surface layer (cell-surface membrane and cellulose cell wall) across which materials can easily move. The soil solution that surrounds the root hair cells contains mostly water and therefore has a very high water potential. In contrast to this the root hairs and other cells of the root have sugars, amino acids and mineral ions dissolved in them giving them a much lower water potential. As a result water moves by osmosis from the soil solution into the root-hair cells down this water potential gradient. After being absorbed into the root hair cell, water continues its journey across the root in two ways: The Apoplastic pathway and The Symplastic pathway. The Apoplastic Pathway: As water is drawn into endodermal cells, it pulls more water along behind it, due to the cohesive properties of water molecules. This creates a tension that draws water along the cell walls of the cells of the root cortex. The mesh-like structure of the cellulose cell walls of these cells has many water-filled spaces and so there is little resistance to this pull of water along the cell walls. The Symplastic Pathway: This takes place across…