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Introduction to Photosynthesis

The Evolution of Photosynthesis

Life theoretically originated on Earth 3.5 to 4 billion years ago. The atmosphere was thin: composed of methane, carbon dioxide, and water vapour. Any gaseous oxygen had been used up in the combustion (or oxidation) of materials when the Earth was very hot.

The cooling water collected in pools, assimilating the nutrients from the rocks. As water evaporated, the nutrients concentrated, forming a rich soup. The first organisms would have made a good living off this food source, breaking down the complex molecules into water and carbon dioxide through respiration. Eventually, as life grew, the need arose to somehow resynthesize complex compounds, both to eat and to use for structure and function. Some organisms learned how to use the sun's energy to synthesize large molecules from small molecules. Other organisms learned to use other sources of reductive power. These organisms who have learned how to build the building blocks of life are called autotrophs, or self-feeders. Autotrophs are found in the bacterial and in the plant kingdom.

The Discovery of Photosynthesis

Joseph Priestly, a chemist and minister, discovered that when he isolated a volume of air under an inverted jar, and burned a candle in it, the candle would burn out very quickly, much before it ran out of wax. He further discovered that a mouse could similarly "injure" air. He then showed that the air that had been "injured" by the candle and the mouse could be restored by a plant. In 1778, Jan Ingenhousz, court physician to the Austrian Empress, repeated Priestly's experiments. He discovered that it was the influence of sun and light on the plant that could cause it to rescue a mouse in a matter of hours.

In 1796, Jean Senebier, a French pastor, showed that CO2 was the "fixed" or "injured" air and that it was taken up by plants in photosynthesis. Soon afterwards, Theodore de Saussure showed that the increase in mass of the plant as it grows could not be due only to uptake of CO2, but also to the incorporation of water.

Thus the basic reaction of photosynthesis was outlined:

CO2 + H2O + light energy ---> (CH2O)n + O2

Overview of Light and Dark Reactions

Early in the 20th Century, researchers took advantage of the use of isotopes to better understand the basic equation of photosynthesis. It was discovered that when carbon dioxide was labelled with a heavy isotope of oxygen, only the lighter isotope was emitted from the plant as oxygen gas. However, if the oxygen of the water was labelled, so was the oxygen gas emitted. This showed that the oxygen for photosynthesis was derived from the water.

Light energy entering the plant splits the water into hydrogen and oxygen:

H2O + light energy ---> ½ O2 + 2H+ + 2 electrons

These electrons travel through the mebrane much like the electrons in oxidative phosphorylation, using their energy to pump protons through the membrane. The proton gradient thus established can be used to synthesize ATP.

More importantly, that same electron reduces NADP+ to NADPH. This molecule plays the same role in synthesis as does NAD+ in the respiratory pathway, as a carrier of reductive power. This store of power serves to reduce carbon dioxide to the more complex carbon structure of glucose, the building block of life.

The reactions leading to the production of ATP and reduction of NADP+ are called the light reactions because they are initiated by the splitting of water by light energy. The reduction of carbon dioxide to glucose, using the NADPH produced by the light reactions, is governed by the dark reactions.

The Chloroplast

The chloroplast is the organelle of photosynthesis. In many ways, the chloroplast resembles the the mitochondrion.

The chloroplast has three membranes: inner, outer, and thylakoid . It has three compartments: stroma, thylakoid space, and inter-membrane space. These compartments and the membranes that separate them serve to isolate different aspects of photosynthesis. Dark reactions take place in the stroma. Light reactions take place on the thylakoid membranes.

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