History of the Universe

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Photosynthesis in Detail

We described photosynthesis briefly in the main part of this website. Our lives depend upon this process, so we describe it in more detail here.

First a molecule called chlorophyll is able to absorb the energy of light and some of its electrons become excited, that is, they move faster than before. Eventually one of these electrons leaves the chlorophyll and passes to an electron acceptor (or oxidizing agent). We will follow its progress in a moment. Meanwhile the chlorophyll is able to get an electron back by taking one from a water molecule. There is good evidence that two or more manganese atoms in association with protein can take four electrons from two water molecules. This releases four protons, which we will also follow in a moment. The by-product of this is that two oxygen atoms, which originally formed part of the water molecules, are left without their hydrogen. These can be joined together, by enzymes, to form a molecule of oxygen O2.

Some of these freed protons are captured, by the electrons, and tied to a molecule called NADPH. The rest of the electrons are carried by floating molecules across an oily membrane to the other side, so forming an electrical field across the membrane. Embedded within the membrane are molecules shaped like tunnels. The electrical field makes protons flow down this tunnel, attracted by the electrons on the other side, and as they pass the protons generate molecules of ATP, the main energy carrying molecule in life.

In a series of reactions known as the Calvin cycle, the energy of these ATP molecules is used to join carbon dioxide molecules together into sugars. This requires the addition of protons, which are carried by the NADPH molecules which we saw formed earlier in the photosynthesis process. The Calvin cycle does not require light, only the ATP and NADPH formed earlier by using light energy.

That, then, is the fantastic and beautiful molecular process we all rely upon for our food, called photosynthesis.

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History of the Universe eBook
History of the Universe eBook
Only £5.99

Written by Wyken Seagrave
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