What is Photosynthesis?*
Photosynthesis (literally “synthesis from light”) is a metabolic process by which the energy of sunlight is captured and used to convert carbon dioxide (CO2) and water (H2O) into carbohydrates (which we represent as a six-carbon sugar, C6H12O6) and oxygen gas (O2). By early in the nineteenth century, scientists had grasped these broad outlines of photosynthesis and had established several facts about the way the process works.
By 1804, scientists had summarized photosynthesis as follows:
Carbon dioxide + water + light energy → sugar + oxygen In molecular terms, this equation seems to be the reverse of the overall equation for cellular respiration. More precisely, photosynthesis can be written as: 6 CO2 + 6 H2O → C6H12O6 + 6 O2 While this equation and the one for cellular respiration are essentially correct, they are too general for a real understanding of the processes involved. A number of questions arise: What are the precise chemical reactions of photosynthesis? What role does light play in these reactions? How do carbons become linked to form carbohydrates? What carbohydrates are formed? And where does the oxygen gas come from: CO2 or H2O? |
Experiments with isotopes show that in photosynthesis O2 comes from H2O
In 1941 Samuel Ruben and Martin Kamen, at the University of California, Berkeley, performed experiments using the isotopes 18O and 16O to identify the source of the O2 produced during photosynthesis (Figure 10.2). Their results showed that all the oxygen gas produced during photosynthesis comes from water, as is reflected in the revised balanced equation:
6 CO2 + 12 H2O → C6H12O6 + 6 O2 + 6 H2O
Water appears on both sides of the equation because it is both used as a reactant (the twelve molecules on the left) and released as a product (the six new ones on the right). This revised equation accounts for all the water molecules needed for all the oxygen gas produced.
The realization that water was the source of photosynthetic O2 led to an understanding of photosynthesis in terms of oxidation and reduction. As we describe in Chapter 9, oxidation-reduction (redox) reactions are coupled; when one molecule becomes oxidized in a reaction, another gets reduced. In this case, oxygen atoms in the reduced state in H2O get oxidized to O2:
12 H2O → 24 H+ + 24 e- + 6 O2
while carbon atoms in the oxidized state in CO2 get reduced to carbohydrate, with the simultaneous production of water:
6 CO2 + 24 H+ + 24 e- → C6H12O6 + 6 O2
6 CO2 + 12 H2O → C6H12O6 + 6 O2 + 6 H2O
Water appears on both sides of the equation because it is both used as a reactant (the twelve molecules on the left) and released as a product (the six new ones on the right). This revised equation accounts for all the water molecules needed for all the oxygen gas produced.
The realization that water was the source of photosynthetic O2 led to an understanding of photosynthesis in terms of oxidation and reduction. As we describe in Chapter 9, oxidation-reduction (redox) reactions are coupled; when one molecule becomes oxidized in a reaction, another gets reduced. In this case, oxygen atoms in the reduced state in H2O get oxidized to O2:
12 H2O → 24 H+ + 24 e- + 6 O2
while carbon atoms in the oxidized state in CO2 get reduced to carbohydrate, with the simultaneous production of water:
6 CO2 + 24 H+ + 24 e- → C6H12O6 + 6 O2
Adding these two equations (chemistry students will recognize them as half-cell reactions) gives the overall equation shown above. As you will see, there is an intermediary carrier of the H+ and electrons between these two processes – the redox coenzyme, nicotinamide adenine dinucleotide phosphate (NADP+).
Photosynthesis involves two pathways
The equations above summarize the overall process of photosynthesis, but not the stages by which it is completed. Like glycolysis and other metabolic pathways that harvest energy in cells, photosynthesis is a process consisting of many reactions. These reactions are commonly divided into two main pathways:
Photosynthesis involves two pathways
The equations above summarize the overall process of photosynthesis, but not the stages by which it is completed. Like glycolysis and other metabolic pathways that harvest energy in cells, photosynthesis is a process consisting of many reactions. These reactions are commonly divided into two main pathways:
- The light reactions convert light energy into chemical energy in the form of ATP and the reduced electron carrier NADPH. This molecule is similar to NADH but with an additional phosphate group attached to the sugar of its adenosine. In general, NADPH acts as a reducing agent in photosynthesis and other anabolic reactions.
- The light-independent reactions (carbon fixation reactions) do not use light directly, but instead use ATP, NADPH (made by the light reactions), and CO2 to produce carbohydrate.
* Berenbaum, May R., et al. Life: the Science of Biology. Sinauer Associated, Inc., 2011.
This text has 39 sentences, with 715 words (23.06 per sentence) with 1.67 syllables per word.
Flesch-Kincaid Grade Level 12.7
This text has 39 sentences, with 715 words (23.06 per sentence) with 1.67 syllables per word.
Flesch-Kincaid Grade Level 12.7
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