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How Does Photosynthesis Create Plant Mass?

In the mid-1600s, scientist Jan van Helmont was surprised to discover that plant mass, or biomass, did not come from soil. He planted a 5-lb. willow tree in 200 lbs. of soil. Five years later, the tree had grown to a mass of 164 lbs. while the soil remained the same weight. He concluded that plant mass derives from water. However, experiments in the late 1700s by Joseph Priestly proved a plant's mass comes from the air, not water or soil. It is the product of photosynthesis, the chemical reaction that allows plants to take carbon dioxide and water and produce oxygen and carbohydrates.

Photosynthesis Basics
- Photosynthesis is a light-dependent, reduction-oxidation reaction. During it, hydrogen reduces carbon dioxide. Plants and algae need sunlight to produce oxygen. In this chemical reaction, plants combine the energy from light with carbon dioxide from the air and water. They then synthesize oxygen and glucose, or carbohydrates. Approximately 50 percent of a plant's mass is composed of carbon.
Chlorophyll in Plants
- The substance chlorophyll within plant cells reflects green light, giving plants their characteristic color. Chlorophyll absorbs blue-violet and red light, and this light energy "excites" electrons within chlorophyll molecules to fuel the photosynthesis reaction. The majority of photosynthesis takes place within the leaves, not the stems, of a plant.
Inside the Chloroplasts
- Within the chloroplast membranes of a plant cell water separates into component oxygen and hydrogen, and two chemicals emerge: ATP (adenosine triphosphate), which stores energy, and NADPH (nicotinamide adenine dinucleotide phosphate hydrogen), a key ingredient in the cell's life cycle. ATP and NADPH in turn give fuel to the reaction wherein carbon molecules bond together to produce carbohydrates.
Plant Mass Components
- Plant mass takes the form mainly of cellulose and lignin. It also has other, trace ingredients. The nutrients nitrogen, magnesium, sulfur and phosphorus are necessary for plant growth. Chlorophyll requires magnesium; plant proteins require nitrogen and sulfur. Nitrogen comprises almost 5 percent of plant dry mass.
Biotechnology
- Biotechnologists have tried to modify the process of photosynthesis to increase plant mass, and thus, crop yields. However, they have not succeeded in genetically modifying plants such as soybeans, wheat and corn. Also, environmental scientists are hoping plant-based biofuels, using biomass converted to energy, will replace fossil fuels in the future. Instead of emitting carbon dioxide during combustion, these fuels would absorb extra carbon dioxide in the atmosphere.

Nature