Research

Research into Photosynthesis

During the 18th century, the roles of water, air and sunlight were independently announced as important in the survival of plants by many scientists. By the end of that century, Ingenhousz reported that plants used oxygen in the presence of sunlight and that it was the green parts of the plants that related these two factors. By 1796, he had discovered that it was carbon dioxide (CO₂) being taken from the air and used by plants to produce organic nourishment and oxygen that is released back to the air. In 1804, de Saussure further stated that water was also used in the growth of plants in conjunction with the decomposition of CO₂.

The chemists Pelletier and Caventou first isolated the green parts of plants where these chemical changes were occurring and named them chlorophyll. By 1845, the German physician Mayer was first able to write the basic photosynthesis equation for the conversion of light, water and CO₂ into chemical energy, organic matter, and released oxygen.

6CO₂ + 12 H₂O + light energy -> C₆H₁₂O₆ + 6O₂ +6H₂O
In 1880, Engelmann demonstrated that oxygen is evolved within chloroplasts, where the chlorophyll is located, using the spiral filament of the green alga Spirogyra . Engelmann exposed the algae to an oxygen-dependent bacteria in a closed chamber and watched the bacteria migrate to the location of the chloroplasts, where the oxygen is located. Engelmann also pioneered research about light spectrums of photosynthesis. He found that chloroplasts evolve more oxygen when they are exposed to blue and red light by shining light through a prism, thereby breaking up white light into its color spectrum.

By the 1900零, scientists knew that photosynthesizing organisms used both water and carbon dioxide, but it was believed that the O₂ released was from the splitting of the CO₂ molecule. It was not until 1931 that van Niel clarified that the O₂ released comes from split water molecules. Van Niel formulated his hypothesis from bacteria that use CO₂ for carbohydrate but also use H₂S instead of H₂O. These bacteria then release sulfur and not O₂. This led to the general formula:

General formula: CO₂ + 2H₂X == CH₂O + H₂0 + 2X
Sulfur bacteria: CO₂ +2H₂S == CH₂O + H₂O + 2S
Plants: CO₂ + 2H₂O == CH₂O + H₂O + O₂
Van Niel零 hypothesis was confirmed in 1941 by Ruben and Kamen using oxygen-18, an isotope that works as a tracer for the path of O₂ in photosynthesizing plants.

In the late 1930's, Hill found that isolated chloroplasts, suspended in solution, can evolve oxygen in the presence of light and a receptor that takes the place of the carbon dioxide. The light and the receptor, usually carbon dioxide in plants, create energy in the chloroplast through a process that is known as the Hill Reaction. This discovery also clarified that the oxygen released through photosynthesis is from the water and not from the carbon dioxide.

Another breakthrough that opened the door to better understanding photosynthesis was established by Calvin and his colleagues at the University of California at Berkeley in 1946. Calvin used the radioactive isotope carbon-14, which had been recently discovered by nuclear scientists at the soon-to-become Lawrence Berkeley National Laboratory, to trace the path of carbon from CO2 in the air to the glucose created for nutrition.

Calvin found that carbon is incorporated into organic compounds of plants through a cycle that is not dependent on light directly, and is therefore known as a dark reaction. This independent cycle is, however, dependent on the energy provided by sunlight to power the breakdown of carbon dioxide. Hence, Calvin clarified that photosynthesis is a two-part process, one of light absorption and another that converts carbon into carbohydrates for food and both are dependent on the other. Calvin received the Nobel Prize in Chemistry for his important contributions.

Ongoing Research: Endosymbiosis

Endosymbiosis, the theory of engulfment of one cell by another, has established almost universal credence among scientists, although the details are still undergoing intense research. This theory, proposed in 1883 by Schimper, seeks to explain how cells became eukaryotes, or cells with nuclei and other organs, and developed the ability to photosynthesize and become plants and algae as we know them today. Endosymbiosis is one explanation for the diversity and adaptability of life on earth.

There is strong evidence that 600 million years ago endosymbiosis began when large heterotrophic cells invaginated smaller autotrophic cells, probably bacteria. Instead of digesting them, however, the heterotrophic cells used the nutrition made by the autotrophs for their own survival, while lending proteins and support to the smaller cells. These mutually beneficial relationships persisted in the cells and became part of their genetic makeup.

The theory of endosymbiosis is supported in part by the observation that the genetic material of plastids, the general term for the photosynthetic organelles of plants, is often different from the genetic material of the "host cell", even after millions of years of evolving together. Research is being conducted to determine if the diversity of plastids is the result of one eukaryote engulfing a prokaryote, and then a bigger eukaryote engulfing that cell , or the result of many different engulfments of diverse forms of bacteria and host cells. To test this, scientists compare the genetic makeup, through DNA and RNA, of a plastid with that of various bacteria. Early results from research suggest that most photosynthesizing developed first from a single endosymbiosis of cyanobacteria, and that the diversity of photosynthesis is the result of developments by those cells. However, there are still some results that suggest discrepancies between plastids and cyanobacteria and that many individual endosymbiotic events occurred with diverse prokaryotes and eukaryotes millions of years ago.