The Method of Labeled Atoms in Biology: Isotopes Tracing Photosynthesis

The labelled-atom method (also called the tracer method) is a technique in which an isotope is introduced into a compound so its path through a living organism can be tracked, and it is now widely used across biological research. In plant biology it lets scientists follow a single element as it moves through metabolic pathways, revealing how processes such as photosynthesis, respiration, and mineral nutrition actually work.

Why are isotopes used as labelled atoms?

Isotopes work as labelled atoms because they share the chemical behaviour of an element while remaining physically distinguishable. Many chemical elements are in fact a mixture of isotopes. Isotopes of the same element differ from one another in the number of neutrons in the nucleus — that is, in mass — whereas the chemical properties of an element depend on the number and arrangement of the electrons surrounding the nucleus.

Because all isotopes of a given element share the same chemical properties, they can be substituted for ordinary atoms without changing how a compound reacts. This is what makes isotopes usable as tracers. A compound labelled with an isotope is introduced into the plant, and the presence of the labelled atoms in the plant's tissues is then detected either by their radioactivity or with specialised instruments such as mass spectrometers.

Whether an isotope can be used depends on how long it survives, which is defined by its half-life — the period over which half the atoms of a radioactive element decay. The half-lives of the isotopes commonly used in plant research vary enormously, from minutes to thousands of years:

• nitrogen N¹³ — a half-life of under 10 minutes;
• phosphorus P³² — 14.3 days;
• sulphur S³⁵ — 87.1 days;
• hydrogen H³ (tritium) — 12.3 years;
• carbon C¹⁴ — 5,600 years.

A short half-life means the tracer must be applied and measured quickly, while a long-lived isotope such as carbon C¹⁴ can be followed over extended experiments. Using labelled atoms makes it possible to trace the transformation of a particular element inside plants, which is what makes the method indispensable for studying photosynthesis, respiration, mineral nutrition, and other processes.

How did the labelled-atom method reveal the source of oxygen in photosynthesis?

The labelled-atom method showed that the oxygen released during photosynthesis comes from water, not from carbon dioxide. In the experiments of A. P. Vinogradov, the isotope of oxygen O¹⁸ was introduced separately into molecules of CO₂ and H₂O. When the heavy isotope was placed in the CO₂ used in photosynthesis, the oxygen given off had an atomic weight of 16 — the ordinary isotope.

This result demonstrated that the oxygen released in photosynthesis does not originate from CO₂. A plant supplied instead with water containing O¹⁸ released precisely that labelled oxygen during photosynthesis. Tracer method in biology

The conclusion is that in the process of photosynthesis oxygen is split out of water rather than from carbon dioxide, overturning the earlier assumption. The discovery settled a long-standing question about where the oxygen in the atmosphere produced by green plants actually comes from.

What intermediate products of photosynthesis did the tracer method uncover?

Labelling carbon with the isotope C¹⁴ made it possible to identify the intermediate products formed during photosynthesis. By feeding plants radioactive carbon and then analysing the leaves after very brief exposures to light, researchers could catch compounds at the earliest moments of carbon fixation.

Even after only 5–10 seconds of exposure to light, the leaves were found to contain many organic acids — including pyruvic, malic, succinic, and oxaloacetic acids — along with the amino acids alanine, aspartic acid, and glutamic acid, and their amides, (see this: Benefits of corn oil). The method of labeled atoms made it possible to study the products formed during photosynthesis

The labelled-atom method also showed that the products of photosynthesis change depending on the species of plant, its age, and the surrounding conditions of light, temperature, and mineral nutrition, (detailed: How do external factors influence the process of photosynthesis?). By tracing carbon through these variables, scientists could see how a plant's chemistry shifts in response to its environment — something that would have been invisible without isotopic tracers.