Study of Photosynthesis


Learning Objective

  • Describe the use of radioactive isotopes in the study of photosynthesis

Key Points

    • The quantities of different isotopes can be measured by mass spectrometry and compared to a standard.
    • Mass spectrometry has been used to study the ratio of isotopes in various plants to understand the mechanisms of photosynthesis.
    • Plants that follow C3 photosynthetic pathways have ratios of carbon isotopes that differ from those that follow C4 pathways.


  • isotopeAny of two or more forms of an element where the atoms have the same number of protons but a different number of neutrons; forms have the same atomic number but a different mass number.
  • photosynthesisThe process by which plants and other photoautotrophs generate carbohydrates and oxygen from carbon dioxide, water, and light energy; performed in chloroplasts.
  • mass spectrometryAn analytical technique that measures the mass:charge ratio of the ions formed when a molecule or atom is ionized, vaporized, and introduced into a vacuum; may also involve breaking molecules into fragments, enabling structure to be determined.

Isotopes in Photosynthesis

Mass spectrometry has been used to study the ratio of isotopes in various plants to understand the mechanisms of photosynthesis. For example, in laboratory experiments, labeling the atmosphere with oxygen-18 allows us to measure the oxygen uptake by the photorespiration pathway.

Photosynthetic equationOxygenic photosynthesis, explained in an equation.

Stable carbon isotopes in carbon dioxide are utilized differentially by plants during photosynthesis. C3 carbon fixation is a metabolic pathway that converts carbon dioxide and ribulose bisphosphate into 3-phosphoglycerate. This reaction occurs in all plants as the first step of the Calvin cycle. In C4 plants, carbon dioxide is drawn out of malate and into this reaction rather than being drawn directly from the air.

Grasses in temperate environments, such as barley, rice, and wheat, follow a C3 photosynthetic pathway that yields distinctive isotopic ratios. Grasses in hot, arid environments, specifically maize, but also millet, sorghum, sugar cane, and crabgrass, follow a C4 photosynthetic pathway that produces higher ratios of 13C to 12C.

Eating different plants affects the 13C values in the consumer’s body tissues. Case studies show that millet and maize eaters can easily be distinguished from rice and wheat eaters. Studying the geographical distribution of dietary preferences through time can illuminate migration paths of people and dispersal paths of different agricultural crops.