Photosynthesis
Posted: 25 May 2007 12:03 AM   [ Ignore ]
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How does photosynthesis work? Wikipedia says that: “Photosynthesis is the synthesis of glucose from sunlight, carbon dioxide and water, with oxygen as a waste product.” And then it says that: “Photosynthesis uses light energy and carbon dioxide to make sugars like glucose.” After that it gets very complicated for me to understand…

Do the plants actually absorb the light energy? Because if they do, shouldn’t the weight of Earth increase with every breath of a plant?

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Posted: 25 May 2007 07:29 AM   [ Ignore ]   [ # 1 ]
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I’m not a chemist, but IIRC the photons (light) supply the energy that allows the binding of CO2 and H2O into glucose. In that process the oxygen is stripped off the H2O.

Photons don’t have mass, so when they are absorbed they don’t increase the mass of the plant or of the earth. What they do do is to increase the energy of the atoms that absorb them.

For ease of checking, the Wikipedia entry on photosynthesis is HERE.

One little thing: “weight” is a term that only refers to mass under gravitational influence. Hence it’s probably more accurate to ask if the mass of the earth would increase ... etc.

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Posted: 25 May 2007 09:34 AM   [ Ignore ]   [ # 2 ]
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I failed to realize that light doesn’t have any mass. Thanks.

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Posted: 25 May 2007 09:43 AM   [ Ignore ]   [ # 3 ]
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... what makes things more complex is that Einstein showed that mass and energy are inter-transferable. (Via E = MC^2). The real concept is “mass/energy”. And so photon absorption does increase the mass/energy of the earth. But I believe it does this by increasing the energy of the earth, basically by heating it.

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Posted: 25 May 2007 10:14 AM   [ Ignore ]   [ # 4 ]
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Does Earth lose the heat? If the heat energy from photons is expressed in a form of kinetic energy in our atmosphere, what would happen to the heat if t was to leave Earth? If heat cannot be transmitted into vaccum, what happens to it? Or does the Earth not lose its heat?

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Posted: 25 May 2007 10:31 AM   [ Ignore ]   [ # 5 ]
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Sure, the earth radiates heat into space. All bodies do. That’s why things left in space freeze solid.

I believe the heat would be radiated in the form of infrared radiation (mediated by photons). Of course, the earth also radiates reflected light into space, radioactive decay elements from the core, etc.

The heat is transmitted through the vacuum ... again, in the form of radiated energy.

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Posted: 25 May 2007 09:56 PM   [ Ignore ]   [ # 6 ]
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Right, as you may remember from high school physics heat is tranferred by three modes:  Radiation, convection, and conduction.

And you have to remember, George that everything’s a part of a large equilibrium.  The tree absorbs sunlight energy and carbon dioxide and converts it to a number of things, for example, a carbohydrate and releases oxygen.  Then you pick the apple, eat it, and your body breathes in oxygen, you oxidize the sugar (carbohydrate) in the apple, use the energy to walk around, and you breathe out carbon dioxide. 

Occam

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Posted: 25 May 2007 10:47 PM   [ Ignore ]   [ # 7 ]
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George,

As someone who loaths chemistry, I have to say photosynthesis is well worth putting some effort in to understand. It is a remarkably elegant series of interactions that makes almost all life on Earth possible. It’s the sort of thing that used to make me think there had to be a god, and that now convinces me that, as Sagan used to argue, the real truth once understood is far more beautiful and moving than the fairy-tale explanations the religious settle for. Basically, energy from light is captured in a chemical bond and then used, in a series of steps, to rearrange atoms to produce food for the plant and, incidentally, the oxygen all animal life requires. And, as a bonus, it uses up the carbon dioxide animals give off as a waste product of cellular respiration, the other side of the ecological see-saw from photosynthesis. Wehn you look into the details, you find such cool things. The chloroplasts in a plant, for example, use exactly the same proton pumping mechanism in their work and the mitochondria in animals use. Evolution makes such wonderful and, dare I say creative, use of what’s lying around to create things entirely new.

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