→ Scientists are able to synthesise chemicals with implanted radio-isotopes which organisms use identically to the natural chemicals.

→ Autoradiography is the use of x-ray or photographic film to detect these radioactive materials. They produce permanent record of positions and relative intensities which scientists can analyse.

→ \(\ce{^3H}\) can be used to track the transport of \(\ce{H}\) across the thylakoid membrane.

→ \(\ce{^{32}P}\) can be used to show that water moves in xylem vessels. When roots were surrounded by water containing \(\ce{^{32}P}\), it shown it was taken up by the roots, then through the xylem vessels into other plant organs.

→ The intricate nature of photosynthesis has posed many barriers when scientists attempt to study it. To overcome this scientists have used radioactive tracers, synthetic chemicals which are taken up by the plant and act as normal organic chemicals except they contain a radioactive component.

→ Radioactive atoms release radiation that can be seen by technologies like X-ray film, geiger counters etc. The pathway of a radioactive substance through the living thing can be followed, as can the biochemical pathways that the molecule/atom is involved in.

→ \(\ce{C^{14}O2}\) and \(\ce{H2O^{18}}\) are both radioactive tracers that can be used to study photosynthesis.

→ If plants are surrounded by \(\ce{C^{14}O2 (g)}\) the radioactivity is soon seen in starch granules in the leaves of the plant. This shows that the starch is formed from the \(\ce{CO2}\) in the air, and is composed of carbon atoms from the air. None of the \(\ce{C^{14}}\) in the \(\ce{C^{14}O2 (g)}\) taken in by the plant is lost.

→ If plants are watered with \(\ce{H2O^{18} (l)}\) the radioactivity is seen in the \(\ce{O2}\) that the plant releases into the air around the plant and not in molecules constructed by photosynthesis contained within the leaf.

→ Therefore in photosynthesis the water is split and the oxygen released into the atmosphere, the \(\ce{H}\) incorporated into the plant within intermediate molecules in a biochemical pathway, and then finally into a starch molecule.

→ This knowledge is of benefit to society because we need to find ways of reducing the carbon in the atmosphere because of excess use of fossil fuel combustion and its resultant climate change. We can understand that land clearing with its removal of photosynthetic species will exacerbate the build up of carbon in the atmosphere because of the loss of photosynthesis it causes.

→ Society is also concerned about the need to generate oxygen such as in the context of massive amounts of fossil fuel combustion also removing oxygen from the atmosphere. Understanding that plants release oxygen in photosynthesis is part of the offsets for fossil fuel use in re-forestation projects as the carbon is locked up in the plant and oxygen is released into the atmosphere.