• 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