THE BIOLOGY BOOK

Photosynthesis

1845

Photosynthesis is of critical importance for the survival of living organisms because it captures the energy of the sun and converts it into the chemical energy required to carry out biological processes. In its absence there would be little food or organic matter, and most organisms would cease to exist in an atmosphere devoid of oxygen. The chemical equation that summarizes the process of photosynthesis is:

6 CO₂ + 12 H₂O + Light → C₆H₁₂O₆ + 6 O₂

In the process, carbon dioxide (CO₂) from the environment enters the stomata (tiny pores) on the underside of leaves, where it is joined by water that has traveled from the roots of plants and is transported up to the leaves through vascular bundles (veins). Sunlight is absorbed by chlorophyll, a green pigment located in chloroplasts, cell structures that are the locus of photosynthesis. Photosynthesis occurs in two stages: light reactions and dark reactions. In the light reaction, sunlight is converted to chemical energy and stored in the form of adenosine triphosphate (ATP) and NADPH, a high-energy electron-carrying molecule. In the dark reaction stage, carbon dioxide, ATP and NADPH are converted to the sugar glucose (C₆H₁₂O₆), which is stored in plant leaves, and oxygen is released through the stomata into the environment.

Discovery of the process of photosynthesis began in 1771 with the studies by the English clergyman-scientist Joseph Priestley, who burned a candle in a closed container until the air (later found to be oxygen) within the container could no longer support combustion. Priestley then placed a sprig of mint in the container and, after several days, the candle could, once again, burn. In 1779, the Dutch physician Jan Ingenhousz repeated Priestley’s experiment and showed that light and tissues from a green plant were required to restore the oxygen. The German physician-physicist Julius Robert Mayer in 1845 formulated the concept that solar energy is stored as chemical energy in organic products formed during photosynthesis. (Mayer was also the earliest to state the first law of thermodynamics dealing with the conservation of matter.)

SEE ALSO: Algae (c. 2.5 Billion BCE), Land Plants (c. 450 Million BCE), Metabolism (1614), Gas Exchange (1789), Plant Nutrition (1840), Phototropism (1880), Mitochondria and Cellular Respiration (1925).

The survival of living organisms depends upon the process of photosynthesis, which provides organic food from inorganic molecules in the presence of sunlight and oxygen. Chlorophyll, a green pigment, gives leaves their color and is critical in this process.