Question

Which one of the following equations suggests that \(\mathrm{O}_{2}\) Ifeased during photosynthesis comes from water? (a) \(6\left(0_{2}^{18}+12 \mathrm{H}_{2} \mathrm{O} \rightarrow 6 \mathrm{O}_{2}^{18}+\mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6}+6 \mathrm{H}_{2} \mathrm{O}^{18}\right.\) (b) \(6 \mathrm{CO}_{2}+12 \mathrm{H}_{2} \mathrm{O}^{18} \rightarrow 6 \mathrm{O}_{2}+\mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6}+6 \mathrm{H}_{2} \mathrm{O}^{18}\) (d) \(6 \mathrm{CO}_{2}^{18}+12 \mathrm{H}_{2} \mathrm{O} \rightarrow 60_{2}^{18}+\mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6}+6 \mathrm{H}_{2} \mathrm{O}\) (c) \(6\left[0_{2}+12 \mathrm{H}_{2} 0^{18} \rightarrow 60_{2}^{18}+\mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6}+6 \mathrm{H}_{2} \mathrm{O}\right.\)

Short answer

Equation (b) indicates that the released O2 during photosynthesis comes from water, as it shows the labeled oxygen from water being incorporated into the oxygen gas produced.

Step-by-step solution

Understanding the Use of Isotopes

The isotopic labels (^18O) help determine the source of oxygen atoms in the products. ^18O is a stable isotope of oxygen, different from the most common isotope ^16O by having two extra neutrons. When used in experiments, it allows scientists to trace the movement of oxygen atoms from reactants to products.

Analyzing equation (a)

Equation (a) uses isotopically labeled oxygen in the carbon dioxide molecules, but the oxygen gas produced does not carry the label. This suggests that oxygen gas did not come from carbon dioxide in this scenario.

Analyzing equation (b)

Equation (b) uses isotopically labeled water, and the labeled oxygen is found in both the produced oxygen gas and the remaining water. This indicates that the oxygen in the produced O2 comes from water.

Analyzing equation (c)

Equation (c) seems incomplete and incorrectly notated with '0' instead of 'O'. However, it suggests that water with labeled oxygen leads to oxygen gas with the same label, similar to equation (b)

Analyzing equation (d)

Equation (d) also uses isotopically labeled carbon dioxide, and the resulting oxygen gas carries the ^18O label, implying that in this scenario, the oxygen gas came from carbon dioxide, which does not align with empirical findings in photosynthesis.

Key concepts

Photosynthetic Oxygen Source

Photosynthesis is an essential process that fuels life on Earth, with plants converting carbon dioxide and water into glucose and oxygen, using sunlight as energy. But a long-standing question in biology has been: where does the oxygen produced in photosynthesis come from? Through the ingenious use of oxygen isotopes, specifically oxygen-18, experiments have been able to uncover this mystery.

These experiments demonstrate that the oxygen released during photosynthesis comes from water, rather than carbon dioxide. How was this determined? By tracking the movement of oxygen atoms from water to the oxygen gas produced, using isotopic labels. This finding is not only fundamental to our understanding of photosynthesis but also to the entire carbon cycle and the way life operates on our planet.

Oxygen-18 Water Experiment

In scientific research, especially in studies like tracing the path of atoms in biochemical reactions, isotopic labeling is a pivotal tool. The oxygen-18 water experiment was a groundbreaking use of this technique to show that the oxygen produced during photosynthesis originates from water. During this experiment, water molecules containing the heavier isotope, oxygen-18, were used.

Scientists observed that the oxygen gas released by the plant contained the heavier isotope, directly implicating water as the source of oxygen. This type of experiment provided a definitive answer that was elegantly simple and scientifically robust, highlighting the importance of isotopic experiments in unraveling biological processes.

Photosynthesis Equations

To depict the biochemical reactions in photosynthesis, specific equations are used. These equations illustrate the transformation of carbon dioxide and water into glucose and oxygen. A generic photosynthesis equation looks like this: \[6 \text{CO}_2 + 12 \text{H}_2O + \text{light energy} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6 \text{O}_2 + 6 \text{H}_2O\]

The equation accounts for the six molecules of carbon dioxide and six molecules of water used to produce one molecule of glucose, six molecules of oxygen, and six molecules of water. These equations become particularly important when isotopes are included to trace the origin of oxygen, providing clear visual evidence that the oxygen evolved from photosynthesis comes from water, not from carbon dioxide.

Isotopic Labeling in Biology

Isotopic labeling involves the incorporation of an isotope, like oxygen-18, into a molecule to trace its path through a biological system. This is a crucial technique in biological research as it allows scientists to observe the detailed routes that atoms take during chemical reactions.

Isotopic labeling shines in experiments, such as determining the source of oxygen in photosynthesis, and is equally informative in understanding metabolic pathways, tracking the flow of nutrients, and the study of how drugs are metabolized in the body. By utilizing these techniques, scientists can gather precise data about complex biochemical processes, which was beautifully demonstrated in the photosynthesis isotopes experiment.