For many plants, the initial carbon dioxide fixation product in the Calvin cycle is phosphoglycerate (with three carbon atoms), whereas in other plants, carbon dioxide is fixed as malic acid or aspartic acid (with four carbon atoms). Those in the former group, which includes rice, spinach and trees, are called C3 plants, and those in the latter, which includes corn, are known as C4 plants.
In C4 plants, photosynthesis occurs in a roundabout way in which phosphoenolpyruvate carboxylase with a bicarbonate ion (HCO3-) - against which oxygen does not compete - as a substrate fixes carbon dioxide, other enzymes are then released in the cell, and finally RuBisCO refixes carbon dioxide. Although this C4 photosynthetic reaction uses extra energy (ATP), oxygenase activity is not induced under conditions of low extracellular carbon dioxide concentration, thus allowing efficient photosynthesis. These C4 plants have thrived over tens of millions of years, reducing the carbon dioxide concentration in the atmosphere from higher levels to the current 0.037%. Practical application of C4 photosynthesis has been investigated with the aim of increasing the productivity of crops such as rice.
Photosynthesis and Changes in Carbon Dioxide Concentration in the Earth’s Atmosphere
When the first organisms appeared on earth 3.8 billion years ago, it is believed that the carbon dioxide concentration as a ratio of atmospheric pressure was much higher than today. However, this level was greatly reduced by the emergence of phototrophs to just a few percent in the Paleozoic era, and the growth of terrestrial plants along with the emergence of C4 plants further reduced it to today’s levels. Additionally, significant short-term changes have occurred in connection with human activity and climate change. As an example, the carbon dioxide level, which stood at 0.027% in the 17th century, rose sharply after the Industrial Revolution to the current figure of 0.037%.