Question

Photosynthesis in plants can be represented by the following overall reaction: $$6 \mathrm{CO}_{2}(g)+6 \mathrm{H}_{2} \mathrm{O}(l) \stackrel{\text { Light }}{\longrightarrow} \mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6}(s)+6 \mathrm{O}_{2}(g)$$ Algae grown in water containing some \({ }^{18} \mathrm{O}\) (in \(\mathrm{H}_{2}{ }^{18} \mathrm{O}\) ) evolve oxygen gas with the same isotopic composition as the oxygen in the water. When algae growing in water containing only \({ }^{16} \mathrm{O}\) were furnished carbon dioxide containing \({ }^{18} \mathrm{O}\), no \({ }^{18} \mathrm{O}\) was found to be evolved from the oxygen gas produced. What conclusions about photosynthesis can be drawn from these experiments?

Short answer

Based on the results of the two experiments, we can conclude that the source of the evolved oxygen in photosynthesis is water and not carbon dioxide. This is supported by the evidence that the isotopic composition of oxygen in water affects the evolved oxygen's isotopic composition, while the isotopic composition of carbon dioxide has no impact on the evolved oxygen's isotopic composition.

Step-by-step solution

Understanding the Process of Photosynthesis

In this process, plants convert carbon dioxide and water into glucose and oxygen using solar energy: \[ 6 \mathrm{CO}_{2}(g) + 6 \mathrm{H}_{2} \mathrm{O}(l) \stackrel{\text { Light }}{\longrightarrow} \mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6}(s) + 6\mathrm{O}_{2}(g) \] The source of the evolved \(\mathrm{O}_{2}(g)\) in photosynthesis can be determined by studying the isotopic composition of the oxygen in the evolved gases. To do that, we have two experiments.

Experiment 1: Algae grown in water containing \(^{18}O\).

In this experiment, algae are grown in water containing some \(^{18} \mathrm{O}\) (in \(\mathrm{H}_{2}{ }^{18} \mathrm{O}\)). The evolved oxygen gas has the same isotopic composition as the oxygen in the water. This indicates that the oxygen present in the evolved \(\mathrm{O}_{2}\) comes from the water molecule, which means water is the source of the evolved oxygen in this case.

Experiment 2: Algae grown in water containing \(^{16}O\), supplied with carbon dioxide containing \(^{18}O\).

In this experiment, algae are grown in water containing only \({ }^{16} \mathrm{O}\), and they are provided with carbon dioxide containing \({ }^{18} \mathrm{O}\). No \({ }^{18} \mathrm{O}\) is found in the evolved oxygen gas. This result indicates that the carbon dioxide's oxygen atoms are not the source of the evolved oxygen in photosynthesis.

Drawing Conclusions

Based on the results of both experiments, the following conclusions can be drawn: 1. The source of the evolved oxygen in photosynthesis is water, not carbon dioxide. 2. The evidence supporting this conclusion is that the isotopic composition of oxygen in water affects the evolved oxygen's isotopic composition, while carbon dioxide's isotopic composition has no impact on the evolved oxygen's isotopic composition.

Key concepts

Isotopic Composition

The term isotopic composition refers to the distribution of different isotopes of an element within a sample. Each isotope of an element has the same number of protons but a different number of neutrons, resulting in varying atomic masses. In the context of photosynthesis experiments, scientists use isotopes like \( ^{18}O \) and \( ^{16}O \) to trace the movement of oxygen atoms.
By analyzing isotopic composition, researchers can determine the origin of oxygen produced during photosynthesis. In photosynthesis, \( O_2 \) is evolved as a gas, and understanding its isotopic makeup tells us which molecules were split to release this oxygen. This insight is pivotal in unraveling the photosynthesis process and understanding its chemical pathways. Since water (H\( _2^{18}O \)) introduces \( ^{18}O \) into the system, observing this isotope in the evolved oxygen gas proves that water molecules are indeed the source of this oxygen.

Experiments with Algae

Scientists often use algae to study photosynthesis due to their simplicity and rapid growth. Algae, being aquatic and photosynthetic organisms, are ideal for controlled experiments with different isotopic compositions.
In the first experiment, algae were placed in water containing \( ^{18}O \); subsequent analysis showed that the released oxygen gas mirrored the isotopic composition of the water. This confirmed that the oxygen in the evolved gas comes from water molecules.
In a separate experiment, algae exposed to carbon dioxide containing \( ^{18}O \) but water with only \( ^{16}O \) showed no \( ^{18}O \) in their evolved oxygen gas. These experiments with algae effectively demonstrate the origins of oxygen gas in photosynthesis and reinforce the role of water as the source of evolved oxygen.

Oxygen Evolution in Photosynthesis

Photosynthesis, a process used by plants and algae, converts light energy into chemical energy, releasing oxygen as a byproduct. The phenomenon of oxygen evolution is integral to this process.
During photosynthesis, water molecules are split to release electrons; this procedure also leads to the release of oxygen gas. This stage is known as the oxygen-evolving process and occurs within the chloroplasts under light conditions.
Through isotopic experiments, scientists have discerned that the oxygen comes specifically from water and not carbon dioxide. Despite carbon dioxide's involvement in photosynthesis to form glucose, it is the water molecules that are primarily responsible for the oxygen evolved during the process. The precise mechanism and pathway through which this occurs have been an area of focused research, key in understanding plant life and ecosystem energy cycles globally.