Summary

Concept 25.1: Plants Acquire Mineral Nutrients from the Soil

• Plants are photosynthetic autotrophs that require water and certain mineral nutrients to survive. They obtain most of these mineral nutrients as ions from the soil solution.
• The essential elements for plants include six macronutrients and several micronutrients. Plants that lack a particular nutrient show characteristic deficiency symptoms. Review Figure 25.1 and ANIMATED TUTORIAL 25.1
• The essential elements were discovered by growing plants hydroponically in solutions that lacked individual elements. Review Figure 25.2
• Soils supply plants with mechanical support, water and dissolved ions, air, and the services of other organisms. Review Figure 25.3
• Protons take the place of mineral nutrient cations bound to clay particles in soil in a process called ion exchange. Review Figure 25.4
• Farmers may use shifting agriculture or fertilizer to make up for nutrient deficiencies in soil.

Concept 25.2: Soil Organisms Contribute to Plant Nutrition

• Signaling molecules called strigolactones induce the hyphae of arbuscular mycorrhizal fungi to invade root cortical cells and form arbuscules, which serve as sites of nutrient exchange between fungus and plant. Review Figure 25.5A
• Legumes signal nitrogen-fixing bacteria (rhizobia) to form bacteroids within nodules that form on their roots. Review Figure 25.5B
• In nitrogen fixation, nitrogen gas (N₂) is reduced to ammonia in a reaction catalyzed by nitrogenase. Review Figure 25.6
• Carnivorous plants supplement their nutrient supplies by trapping and digesting arthropods. Parasitic plants obtain minerals, water, or products of photosynthesis from other plants.

Concept 25.3: Water and Solutes Are Transported in the Xylem by Transpiration–Cohesion–Tension

• Water moves through biological membranes by osmosis, always moving toward regions with a more negative water potential. The water potential (ψ) of a cell or solution is the sum of its solute potential (ψ_s) and its pressure potential (ψ_p). Review Figure 25.8 and ANIMATED TUTORIAL 25.2
• The physical structure of many plants is maintained by the positive pressure potential of their cells (turgor pressure); if the pressure potential drops, the plant wilts.
• Water moves into root cells by osmosis through aquaporins. Mineral ions move into root cells through ion channels, by facilitated diffusion, and by secondary active transport. Review Figure 25.10
• Water and ions may pass from the soil into the root by way of the apoplast or the symplast, but they must pass through the symplast to cross the endodermis and enter the xylem. The Casparian strip in the endodermis blocks further movement of water and ions through the apoplast. Review Figure 25.11 and ACTIVITY 25.1
• Water is transported in the xylem by the transpiration–cohesion–tension mechanism. Evaporation from the leaf produces tension in the mesophyll, which pulls a column of water—held together by cohesion—up through the xylem from the root. Review Figure 25.12 and ANIMATED TUTORIAL 25.3
• Stomata allow a balance between water retention and CO₂ uptake. Their opening and closing is regulated by guard cells. Review Figure 25.13

Concept 25.4: Solutes Are Transported in the Phloem by Pressure Flow

• Translocation is the movement of the products of photosynthesis, as well as some other small molecules, through sieve tubes in the phloem. The solutes move from sources to sinks.
• Translocation is explained by the pressure flow model: the difference in solute potential between sources and sinks creates a difference in pressure potential that pushes phloem sap along the sieve tubes. Review Figure 25.14 and ANIMATED TUTORIAL 25.4