Water and Sugar Transportation in Plants
Root hairs increase the surface area where the plant has to absorb the nutrients and water. To take water, hair cells increase concentration of organic chemicals (the process which needs ATP) and then use osmosis. There are two ways that water transport may go: apoplastic or symplastic. Apoplastic transport moves water through the cell walls of cortex: from the rhizodermis to the endodermis. Endodermis cell walls bear Casparian strips (rich of hydrophobic suberin and lignin) which prevent the water from passing through the cell wall and force symplastic transport (Fig 5.28). Symplastic transport there is directed to the center of root only and requires ATP to be spend.
By pumping water inside vascular cylinder and not letting it out, endodermis cells create the root pressure. It is easy to observe on tall herbaceous plants cut near the ground: drops of water will immediately appear on the cutting. Inside tracheary elements of xylem, water moves with the root pressure, capillary force and the sucking pressure of transpiration. The latter means that water column does not want to break and if water disappears from the top (stomata on leaves), it will move water inside plant. The main direction of water movement is from roots to leaves, i.e. upwards.
Products of photosynthesis (sugars) are moving inside living cells of phloem; these cells (sieve tubes) use only symplastic transport to distribute glucose and other organic compounds among all organs of plants. In fact, phloem transports these components in all directions: to the flowers (usually upwards), and at the same time to the roots (usually downwards).
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