Reading6_Lane_PS - Lane Life Ascending PHOTOSYNTHESIS...

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Lane – Life Ascending 1 PHOTOSYNTHESIS Summoned by the Sun an excerpt from Lane, Nick (2009) Life Ascending: the Ten Great Inventions of Evolution . magine a world without photosynthesis. It wouldn't be green, for a start. Our emerald planet reflects the glory of plants and algae, and ultimately their green pigments, which absorb light for photosynthesis. First among pigments is the marvelous transducer that is chlorophyll, which steals a beam of light and conjures it into a quantum of chemical energy, driving the lives of both plants and animals. The world probably wouldn't be blue either, for the azures of the heavens and the marines of the oceans depend on clear skies and waters, cleansed of their haze and dust by the scouring power of oxygen. And without photosynthesis there would be no free oxygen. In fact there might not be any oceans either. Without oxygen there is no ozone; and without that, there is little to cut down the searing intensity of ultraviolet rays. These split water into oxygen and hydrogen. The oxygen is formed slowly and never builds up in the air; instead it reacts with iron in the rocks, turning them a rusty-red colour. And hydrogen, the lightest of gases, evades the tug of gravity and slips away into space. The process may be slow but it is also inexorable: the oceans bleed into space. Ultraviolet radiation cost Venus its oceans, and maybe Mars too. So we don't need much imagination to picture a world without photosynthesis: it would look a lot like Mars, a red dusty place, without oceans, and without any overt signs of life. Of course, there is life without photosynthesis, and many astrobiologists seek it on Mars. But even if a few bacteria are found hiding beneath the surface, or buried in an icecap, the planet itself is dead. It is in near-perfect equilibrium, a sure sign of inertia. It could never be mistaken for Gaia. Oxygen is the key to planetary life. No more than a waste product of photosynthesis, oxygen really is the molecule that makes a world. It is let loose by photosynthesis so fast that it finally overwhelms the capacity of a planet to swallow it up. In the end, all the dust and all the iron in the rocks, all the sulphur in the seas and methane in the air, anything that can be oxidised is oxidised, and free oxygen pours into the air and the oceans. Once there, oxygen puts a stop to the loss of water from the planet. Hydrogen, when released from water, inevitably bumps into more oxygen before it finds its way out into space. Swiftly it reacts to form water again, which now rains back down from the heavens, drawing to a halt the loss of the oceans. And when oxygen accumulates in the air, an ozone shield forms, ablating the searing intensity of the ultraviolet rays, and making the world a more habitable place. Oxygen doesn't just rescue a planet's life: it energises all [well, almost all - ed.] life,
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This note was uploaded on 03/08/2011 for the course BIS 2A taught by Professor Grossberg during the Fall '08 term at UC Davis.

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Reading6_Lane_PS - Lane Life Ascending PHOTOSYNTHESIS...

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