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| Introduction |
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| The first photosynthetic organism probably appeared almost three billion years ago. With the evolution of photosynthesis, however, organisms began to change the face of our planet and, as a consequence, to exert strong influences on each other. Organisms have continued to change the environment, at an ever increasing rate, up to the present day. |
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| Primitive earth had reducing atmosphere (an-oxygenic). The atmosphere in which the first cells evolved consisted largely of ammonia, water, methane and other carbon gases. Gradually methane and ammonia were used up in the formation of the organic molecules on which the first cells were dependent. Their energy probably came from anaerobic glycolysis. |
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| Then, there slowly evolved photosynthetic organisms that used carbon dioxide as their carbon source and released oxygen. As these photosynthetic organisms multiplied, they provided a new supply of organic molecules and free oxygen began to accumulate. Oxygen - consuming organisms have an advantage over those that do not use oxygen and are active and complex. |
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| Life on earth continues to be dependent on photosynthesis both for its oxygen and for its carbon containing fuel (food) molecules. Photosynthetic organisms capture light energy and use it to form carbohydrates and free oxygen from carbon dioxide and water, in a complex series of reactions. |
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| Historical perspective |
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| Aristotle and other Greeks thought that plants get their food from the soil. Later, a Belgian physician, Jan Baptista an Helmont concluded that all the substance of the plant was produced from water. In the end of 18th century, Joseph Priestly, showed that plants have the ability to take up CO2 from the atmosphere and release O2. Later in 1780, Jan Ingenhousz discovered that releases of O2 by plants was possible only in sunlight and only by the green parts of the plants. |
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| Photosynthesis is a process by which plants and other organisms (like algae and some bacteria) synthesize their own food in the presence of light. |
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| Or |
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| Photosynthesis is a multi-stage process consisting of a series of light requiring reactions and a series of chemical reactions. |
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| The former, collectively called the light reaction involves light energy, absorbed by chlorophyll and carotenoids. This light energy 'splits' water molecules (photolysis) into hydrogen and oxygen. The latter is released as gaseous oxygen. In the non-light dependent reactions, or dark reactions, hydrogen combines with (reduces) carbon dioxide to form carbohydrate. The overall equation can be represented as shown below: |
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