By the beginning of the nineteenth century, scientists understood the broad outlines of photosynthesis. It was known to use three principal ingredients—water, carbon dioxide (CO2), and light—and to produce not only carbohydrates but also oxygen gas (O2). Scientists had learned several things:
► The water for photosynthesis in land plants comes primarily from the soil and must travel from the roots to the leaves.
► Carbon dioxide is taken in, and water and O2 are released, through tiny openings in leaves, called stomata (singular, stoma) (Figure 8.1).
► Light is absolutely necessary for the production of oxygen and carbohydrates.
By 1804, scientists could summarize photosynthesis as follows:
carbon dioxide + water + light energy ^ sugar + oxygen which turns into an equation that is the reverse of the overall equation for cellular respiration given in Chapter 7:
Although correct, these statements say nothing about the details of the process, and in fact, in detail, photosynthesis is not the reverse of cellular respiration. What are the reactions of photosynthesis? What role does light play in these reactions?
How do the carbons become linked to form sugars? And does the oxygen gas come from the CO2 or from the H2O?
Almost a century and a half passed before the source of the O2 released during photosynthesis was determined. One of the first uses of an isotopic tracer in biological research resulted in its identification. In these experiments, two groups of green plants were allowed to carry on photosynthesis. Plants in the first group were supplied with water containing the oxygen isotope 18O and with CO2 containing only the common oxygen isotope 16O; plants in the second group were supplied with CO2 labeled with 18O and water containing only 16O.
When oxygen gas was collected from each group of plants and analyzed, it was found that O2 containing 18O was produced in abundance by the plants that had been given 18O-labeled water, but not by the plants given 18O-labeled CO2. These results showed that all the oxygen gas produced during photosynthesis comes from water (Figure 8.2). This discovery is reflected in a revised balanced equation:
Water appears on both sides of the equation because water is both used as a reactant (the twelve molecules on the left) and released as a product (the six new ones on the right). In this revised equation, there are now sufficient water molecules to account for all the oxygen gas produced.
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