37.1 Water and minerals move upward through the xylem.
Overview of Water and Mineral Movement Through Plants
• Water enters the roots and moves through the xylem because of multiple factors. (p. 768)
• Aquaporins enhance the transport of water at the cellular level, and such movement plays a significant role in bulk water transport. (p. 768)
• Because water molecules are cohesive to each other, and adhesive to the tracheids of vessel walls, transpiration through the stomata exerts most of the pulling force that moves water up the plant. (p. 768)
• Water potential is the combination of a plant's pressure potential and its solute potential. (p. 769)
• Water will move from a solution with higher water potential to a solution with lower water potential. (p. 769)
Water and Mineral Absorption
• Most of a plant's absorbed water enters by root hairs, which are usually turgid because their solute potential is greater than that of their surroundings. (p. 770)
• Plasma membranes of root hair cells contain protein transport channels through which protein pumps transport specific ions against large concentration gradients. (p. 770)
• Root pressure causes the movement of water into the plant and up the xylem despite the absence of transpiration. (pp. 770-771)
Water and Mineral Movement
• The tensile strength of a water column varies inversely with the column diameter. (p. 772)
• The formation of air bubbles in the column would cause it to fail. (p. 772)
• More than 90% of water taken in by the roots is ultimately lost to the atmosphere through transpiration from the leaves. (p. 772)
• Stomata open and close due to changes in guard cell turgor pressure resulting from the active uptake of potassium, which is regulated by abscisic acid. (p. 773)
• Other factors, such as CO2 concentration, light, and temperature, can also affect and control stomatal opening. (pp. 773-774)
• Plants have evolved many mechanisms to conserve water loss. These include dormancy during dry periods, deciduousness, thick hard leaves, and the presence of trichomes. (p. 774)
• Oxygen deprivation is among the most significant problems facing plants in flooded areas because standing water has much less oxygen than moving water. (p. 774)
37.2 Dissolved sugars and hormones are transported in the phloem.
Phloem Transport Is Bidirectional
• Translocation distributes carbohydrates manufactured in the leaves to other parts of the plant. (p. 776)
• The mass-flow hypothesis suggests that dissolved carbohydrates flow from a source and are released at a sink. (p. 776)
• Carbohydrates enter sieve tubes by osmosis, and the resultant increased turgor pressure drives the water throughout the sieve-tube system. At the sink, carbohydrates are actively removed. (pp. 776-777)