restricted access Chapter Two: Plants Put the Oxygen in the Air

From: Green Planet

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[28] chapter two PLANTS PUT THE OXYGEN IN THE AIR If there be a sapling in your hand When they say to you: Behold the Messiah! First plant the sapling, then go out to greet the Messiah. —TALMUD XXXI Take a Deep Breath In order to feel gratitude for the silent, clean, tireless work that plants perform, it is necessary to do no more than to take a deep breath. Better yet, go outside and look at the blue sky, and take a deep breath. Nearly all of the oxygen in the air came from the photosynthesis of land plants, aquatic plants, and other green aquatic organisms. Photosynthesis is the process that uses light (photo-) to synthesize food molecules. But photosynthesis does much more than that. It is a process so vitally important that it will take three chapters of this book to give it even the briefest overview. Before photosynthesis became widespread on this planet, there was no oxygen in the atmosphere. Three and a half billion years ago, when Earth cooled off enough for oceans to form, it did not take very long for the first photosynthetic bacteria to appear. They began putting oxygen gas into the oceans and atmosphere. At first, the oxygen that these cells released reacted with minerals such as iron, causing it to rust, and therefore oxygen was trapped in ocean water and sediments and did not accumulate in the air. It took a couple of billion years for oxygen to begin its buildup in the atmosphere. We know that this was happening because the iron that rusted left bands of red CH002.qxd 11/13/08 12:28 PM Page 28 Plants Put the Oxygen in the Air [29] deposits in sedimentary rocks. It was not until about 600 million years ago, after the melting of the most recent of three global ice ages, that oxygen gas reached the concentration that it now has in the global atmosphere, about 21 percent by volume. Photosynthetic oxygen also made the diversity of life possible. Every cell uses a molecule called ATP as a source of energy for its metabolic reactions, and the cells of large, active organisms need a lot of ATP. A process called cellular respiration produces a large amount of ATP, and nearly all organisms obtain their ATP from this process. Cellular respiration cannot take place without oxygen. Without oxygen in the air, large organisms could not exist. New photosynthetic organisms evolved in the oceans, most notably the seaweeds. One group, a certain lineage of green algae, evolved into the first land plants. The first land plants were small and confined to wetlands, but by 350 million years ago there were large trees (which were ancient gigantic relatives of today’s relatively small club mosses), ferns, and horsetails. They carried out so much photosynthesis that, according to some researchers, the atmosphere contained even more oxygen 300 million years ago than it does today.1 Processes such as decomposition of dead plant material would have removed much of this oxygen, but instead of breaking down, much of this dead matter was buried in the bottom of wetlands under sediments. Single-celled photosynthetic organisms accumulated there, and today their remains are oil deposits.The remains of large plants also mounted up, and today their remains are coal. Single-celled organisms in the oceans, when they died, settled to the bottom of the sea, where they were eventually drawn into the crust of the earth by the movement of the ocean floor plates. This process also diminished decomposition, allowing photosynthesis to fill the atmosphere with oxygen. Planet Earth is, as far as we know, unique in the universe—certainly in the solar system—because of its oxygen atmosphere. The gas giant planets (Jupiter, Saturn, Uranus, and Neptune) are really nothing but atmosphere , though much of it is liquid or frozen. Their atmospheres contain mostly carbon dioxide, methane, ammonia, water, and hydrogen. The atmospheres of Venus and Mars are primarily carbon dioxide. Astronomy has now advanced enough that the light from planets that revolve around CH002.qxd 11/13/08 12:28 PM Page 29 other stars can be analyzed in order to determine the composition of their surfaces or of their atmospheres—so far, none have been found with oxygen .What this indicates is that the other planets have no photosynthesis. Because oxygen gas reacts with nitrogen gas and with minerals, it would not be able to persist in the atmosphere of a planet unless it...


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