Question

The conversion of ${\mathrm{CO}}_2$ into carbohydrates by plants via photosynthesis can be represented by the reaction $$6{\mathrm{CO}}_2(\mathrm{g})+6{\mathrm{H}}_2\mathrm{O}\stackrel{\text{ light }}{⟶}{\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6+6{\mathrm{O}}_2(\mathrm{g}).$$ To study the mechanism of photosynthesis, algae were grown in water containing ${}^{18}$ O, that is, ${\mathrm{H}}_2^{18}\mathrm{O}$ The oxygen evolved contained oxygen-18 in the same ratio to the other oxygen isotopes as the water in which the reaction was carried out. In another experiment, algae were grown in water containing only ${}^{16}\mathrm{O}$,but with oxygen-18 present in the ${\mathrm{CO}}_2$. The oxygen evolved in this experiment contained no oxygen-18. What conclusion can you draw about the mechanism of photosynthesis from these experiments?

Short answer

The oxygen that plants produce in photosynthesis comes from water and not from carbon dioxide.

Step-by-step solution

Understanding the problem

The question presents two different experiments, both involving the photosynthesis process using algae. In the first experiment, algae were grown in water containing oxygen-18, and the result was that the oxygen product also contained oxygen-18. In the second experiment, the water contained only oxygen-16, but the carbon dioxide contained oxygen-18. The oxygen produced from this experiment had no traces of oxygen-18.

Analyzing the first experiment

In the first experiment, both the reactants - water (H_2^{18}O) and carbon dioxide (CO_{2}) - had the possibility of contributing the oxygen-18 to the product. However, it clearly stated that the ratio of oxygen-18 to the other oxygen isotopes in the product was the same as the ratio in the water used for the reaction. This implies that, in this first experiment, the oxygen-18 in the product came from the water.

Analyzing the second experiment

In the second experiment, the water contained only oxygen-16, but carbon dioxide contained oxygen-18. Yet, it was stated that the oxygen produed during this experiment contained no traces of oxygen-18. This means that the oxygen-18 present in the carbon dioxide did not contribute to the oxygen in the final product.

Drawing conclusion

The results of these two experiments indicate that the oxygen released during photosynthesis comes from water (H_{2}O) and not from carbon dioxide (CO_{2}). This conclusion is consistent with experimental evidence showing that the oxygen in carbon dioxide is incorporated into glucose (C_{6}H_{12}O_{6}), the other product of photosynthesis, and not released as oxygen gas.

Key concepts

Oxygen Isotopes

Oxygen isotopes played a crucial role in understanding the process of photosynthesis. Isotopes are different forms of the same element that contain the same number of protons but a different number of neutrons. This results in different atomic weights. In the context of photosynthesis studies, oxygen isotopes like oxygen-18 (^{18}O) and oxygen-16 (^{16}O) help us trace the path of oxygen molecules in chemical reactions.

• Oxygen-18 ( ^{18}O): A heavier isotope of oxygen, often used as a tracer in biochemical experiments.
• Oxygen-16 ( ^{16}O): The most commonly found isotope of oxygen, making up about 99.76% of natural oxygen.

The experiments involving algae and these isotopes provided insight into which molecules contribute oxygen to the photosynthesis process. By identifying whether (^{18}O) or (^{16}O) was present in the oxygen gas produced, scientists could determine that the oxygen released during photosynthesis comes from the water and not from carbon dioxide. This finding was crucial in mapping the reactions involved and identifying the sources and destinations of oxygen atoms in photosynthesis.

Mechanism of Photosynthesis

The mechanism of photosynthesis is an intricate process that converts light energy into chemical energy stored in glucose, a sugar. Plants, algae, and some bacteria perform photosynthesis to create food, with oxygen as a by-product. The photosynthesis reaction can be simplified with the equation: $$6{\text{CO}}_2+6{\text{H}}_2\text{O}+extlight\to {\text{C}}_6{\text{H}}_{12}{\text{O}}_6+6{\text{O}}_2$$“The work of scientists using oxygen isotopes has been instrumental in uncovering the details of the photosynthesis mechanism.”
The photosynthesis process occurs mainly in two stages:

• Light-dependent Reactions: Occurs in the thylakoid membranes, where sunlight is harvested to split water molecules. This process releases oxygen and generates energy-carrying molecules, like ATP and NADPH.
• Calvin Cycle (Light-independent Reactions): Happens in the stroma of chloroplasts where the energy from ATP and NADPH is used to convert carbon dioxide into glucose.

Studies involving oxygen isotopes showed that the released oxygen is a result of the splitting of water molecules in the light-dependent reactions.This understanding has helped clarify many details of the photosynthesis process that were once theories, such as identifying that oxygen in the produced O2 comes exclusively from water, not carbon dioxide.

Algae Experiments

Algae experiments have been pivotal in understanding the photosynthesis process and its details. Algae are simple aquatic organisms that, like plants, can conduct photosynthesis. They are excellent subjects for experiments aimed at deciphering photosynthesis due to their simple structure and rapid growth.
These experiments generally involve altering certain variables such as isotopic composition, light intensity, or carbon dioxide concentrations to observe changes in photosynthesis products.
In the experiment with algae grown in water containing (^{18}O), scientists observed that the oxygen released matched the isotopic composition of the water. This finding indicates that the source of oxygen in photosynthesis is water.

• Why use algae? Algae have straightforward biological processes, making any changes easier to observe and understand, and they exhibit clear responses to experimental conditions.

In contrast, when algae were grown in water with only (^{16}O) but with carbon dioxide containing (^{18}O), the oxygen produced did not contain (^{18}O). This further confirmed that the oxygen produced during photosynthesis comes from the water, reinforcing our understanding of the photosynthesis mechanism. Using algae experiments, scientists have managed to isolate and understand specific pathways and reactions within photosynthesis, making these simple organisms incredibly valuable to biological research.