The Structural Nutrients
Seventeen nutrients have been confirmed as essential by plants for growth. Three of these nutrients – carbon, hydrogen and oxygen – constitute 90 to 95% of plant dry weight of most plants, and are classed as the “structural nutrients”. They are used to form all carbohydrates, proteins and other organic compounds. These complex organic compounds are derived from simple carbohydrate building blocks formed as a result of photosynthesis. The sources hydrogen and oxygen are water that is taken into the plant by the root, and carbon from carbon dioxide that is absorbed by leaves from the atmosphere that surrounds the plant. Carbon dioxide enters the plant almost entirely through stomata on the leaves. Water also is absorbed through stomata, but the relative amount is small in comparison to the water that enters the plant through the roots. Elemental oxygen (O2) is also absorbed from air through stomata, roots, or other plant parts. Photosynthesis is the process by which light energy is converted into chemical energy, and a reductant derived from water is formed. The overall process leading to formation of carbohydrates is illustrated by the following equation:
CO2 + H2O + light energy → (CH2O) + O2
Photosynthesis in this equation occurs in two steps: (1) the Light Reaction and (2) the Dark Reaction. Both reactions are regulated by enzymes, which require plant nutrient elements as either structural components or cofactors (catalysts required for the reactions to take place, but are not consumed).
The Light Reaction involves the conversion of light energy into stored chemical energy in high-energy bonds as adenosine triphosphate (ATP) and the formation of a chemical reducing agent (reductant), nicotinamide adenine dinucleotide phosphate (NADPH). A reducing agent is able to give up electrons in a redox chemical reaction and is thereby Plant Nutrition Overview oxidized, but in the process NADPH gives up energy to fix carbon from carbon dioxide, (simultaneously releasing oxygen gas), and thus drives the formation of glucose (sugar) in the Calvin Cycle. The Light Reaction occurs in the part of the chloroplast known as grana, which are stacks of membranes called thylakoids. Two photosystems participate in The Light Reaction, Photosystem I absorbs light at 700 nm (red light), and transferring the energy as electrons to NADP forming NADPH. Photosystem II absorbs light at 680 nm (red light), and in this system, water is split to release oxygen gas and H+ or electrons. The electrons flow to Photosystem I replacing electrons that were released by absorption of light. The H+ is used to bind to NADP to form NADPH. Also, the flow of electrons from Photosystem II back to Photosystem I provides the energy to add an inorganic phosphate (Pi) to adenosine diphosphate (ADP) to form the high-energy phosphate bond in ATP. That is, ADP + Pi + energy from electron flow ATP.
The Calvin Cycle and the formation of glucose is part of the Dark Reaction, which occurs in the gel-like matrix known as the stroma of chloroplasts. In the Calvin Cycle carbon dioxide is first fix as a part of phosphoglyceric acid, which is then metabolized to form glucose. Thus, the carbon and oxygen of organic molecules in plants are derived from carbon dioxide, and the hydrogen is derived from water.