Chapter 37

• Every organism is an open system linked to its environment by a continuous exchange of energy and materials.
 In ecosystems, plants and other photosynthetic autotrophs perform the crucial step of transforming inorganic compounds into organic ones.
 Plants need sunlight as the energy source for photosynthesis.
 They also need inorganic raw materials such as water, CO2, and inorganic ions to synthesize organic molecules.
 Plants obtain CO2 from the air. Most vascular plants obtain water and minerals from the soil through their roots.
 The branching root and shoot systems of vascular plants allow them to draw from soil and air reservoirs of inorganic nutrients.
• Roots, through fungal mycorrhizae and root hairs, absorb water and minerals from the soil.
• CO2 diffuses into leaves from the surrounding air through stomata. Concept 37.1 Plants require certain chemical elements to complete their life cycle
• Early ideas about plant nutrition were not entirely correct and included:
 Aristotle’s hypothesis that soil provided the substance for plant growth.
 van Helmont’s conclusion from his experiments that plants grow mainly from water.
 Hale’s postulate that plants are nourished mostly by air.
• In fact, soil, water, and air all contribute to plant growth.
• Plants extract mineral nutrients from the soil. Mineral nutrients are essential chemical elements absorbed from soil in the form of inorganic ions.
 For example, many plants acquire nitrogen in the form of nitrate ions (NO3−).
 However, as van Helmont’s data suggested, mineral nutrients from the soil contribute little to the overall mass of a plant.
• About 80–90% of a plant is water. Because water contributes most of the hydrogen ions and some of the oxygen atoms that are incorporated into organic atoms, one can consider water a nutrient.
 However, only a small fraction of the water entering a plant contributes to organic molecules.
 More than 90% of the water absorbed by a field of corn is lost by transpiration.
 Most of the water retained by a plant functions as a solvent, provides most of the mass for cell elongation, and helps maintain the form of soft tissues by keeping cells turgid.
• By weight, the bulk of the organic material of a plant is derived not from water or soil minerals, but from the CO2 assimilated from the atmosphere.
• The dry weight of an organism can be determined by drying it to remove all water. About 95% of the dry weight of a plant consists of organic molecules. The remaining 5% consists of inorganic molecules.
 Most of the organic material is carbohydrate, including cellulose in cell walls.
• Carbon, hydrogen, and oxygen are the most abundant elements in the dry weight of a plant.
• Because some organic molecules contain nitrogen, sulfur, and phosphorus, these elements are also relatively abundant in plants.
• More than 50 chemical elements have been identified among the inorganic substances present in plants.
 However, not all of these 50 are essential elements, required for the plant to complete its life cycle and reproduce.
• Roots are able to absorb minerals somewhat selectively, enabling the plant to accumulate essential elements that may be present in low concentrations in the soil.
 However, the minerals in a plant also reflect the composition of the soil in which the plant is growing.
 Some elements are taken up by plant roots even though they do not have any function in the plant.
Plants require nine macronutrients and at least eight micronutrients.
• Plants can be grown in hydroponic culture to determine which mineral elements are actually essential nutrients.
 Plants are grown in solutions of various minerals in known concentrations.
 If the absence of a particular mineral, such as potassium, causes a plant to become abnormal in appearance when compared to controls grown in a complete mineral medium, then that element is essential.
 Such studies have identified 17