Question

Photosynthesis in plants can be represented by the following overall equation: $$6 \mathrm{CO}_{2}(g)+6 \mathrm{H}_{2} \mathrm{O}(l) \stackrel{\text { Light }}{\longrightarrow} C_{6} \mathrm{H}_{12} \mathrm{O}_{6}(s)+6 \mathrm{O}_{2}(g)$$ 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 two experiments, we can conclude that the oxygen produced during photosynthesis primarily comes from the water molecules and not from the carbon dioxide molecules. The isotopic composition of the oxygen in the experiments supports this conclusion. This indicates that water molecules play a more significant role in contributing the oxygen released during photosynthesis.

Step-by-step solution

Review the Photosynthesis Equation

Begin by reviewing the overall equation for photosynthesis: $6 \mathrm{CO}_{2}(g)+6 \mathrm{H}_{2} \mathrm{O}(l) \stackrel{\text { Light }}{\longrightarrow} C_{6} \mathrm{H}_{12} \mathrm{O}_{6}(s)+6 \mathrm{O}_{2}(g)$ This equation shows that 6 water molecules and 6 carbon dioxide molecules react to form one glucose molecule and 6 oxygen molecules. From this equation, we will analyze the results of the two experiments provided.

Analyze the First Experiment

Experiment 1 states that algae grown in water containing some \(^{18}O\) (in \(H_2^{18}O\)) evolve oxygen gas with the same isotopic composition as the oxygen in the water. This indicates that the oxygen produced during photosynthesis in this experiment comes from the water molecules.

Analyze the Second Experiment

Experiment 2 states that when algae growing in water containing only \(^{16}O\) were furnished carbon dioxide containing \(^{18}O\), no \(^{18}O\) was found to be evolved from the oxygen gas produced. This means that despite the presence of \(^{18}O\) in the carbon dioxide provided, the oxygen released during photosynthesis did not contain \(^{18}O\). This suggests that the oxygen produced during photosynthesis in this experiment did not come from carbon dioxide.

Draw Conclusions About Photosynthesis

Based on the analysis of the two experiments, we can conclude the following: The oxygen produced during photosynthesis primarily comes from the water molecules and not from the carbon dioxide molecules. This is evident from the fact that the isotopic composition of the oxygen produced in Experiment 1 matches the isotopic composition of the oxygen in the water, while in Experiment 2, the isotopic composition of the oxygen produced did not match the isotopic composition of the oxygen in the provided carbon dioxide. This indicates that the water molecules play a more significant role in contributing the oxygen released during photosynthesis.

Key concepts

Oxygen Evolution

In the process of photosynthesis, oxygen evolution refers to the generation of oxygen gas as a byproduct. This oxygen is released into the atmosphere—a crucial step in maintaining life on Earth. When plants engage in photosynthesis, they utilize water molecules.
In this process, these water molecules are split during light-dependent reactions. The splitting of water releases electrons, which are essential for driving further biochemical reactions necessary for energy production and glucose formation.
As a result of this splitting, oxygen molecules are released as a byproduct. This phenomenon is essential because it explains why oxygen is produced in this process, originating from water rather than carbon dioxide.

• Oxygen is derived from water molecules.
• It is important for balancing atmospheric oxygen levels.

Isotopic Labeling

Isotopic labeling is a useful technique in biochemical experiments. It helps trace the path of specific atoms through chemical reactions. In the context of photosynthesis, isotopic labeling is used to understand the origins of the oxygen gas evolved.
This technique involves substituting a standard oxygen isotope in either water or carbon dioxide with a heavier isotope, such as \( ^{18}O \). By observing where this heavier isotope appears in the products of photosynthesis, scientists can determine the original source of specific atoms in the final chemical products.

• It involves the substitution of standard elements with isotopic variants.
• Helps identify the source of elements in biochemical reactions.

Water Molecules

Water molecules (\( H_2O \)) are fundamental to the process of photosynthesis. They provide electrons and protons necessary for the photosynthetic reactions, especially in the light-dependent phase.
When light is absorbed by chlorophyll, it energizes electrons that initiate a chain of reactions. During this, water molecules are split in a process known as photolysis. This splitting results in the production of oxygen (\( O_2 \)) molecules and hydrogen ions.
Understanding the role of water in photosynthesis helps clarify why oxygen evolution happens. It's because oxygen in the new molecules originates directly from the water, rather than from other sources like carbon dioxide.

• Involved in electron transport.
• Source of oxygen and hydrogen ions.

Carbon Dioxide

Carbon dioxide (\( CO_2 \)) is another critical reactant in photosynthesis. It provides the carbon atoms needed to form glucose during the light-independent reactions, often referred to as the Calvin cycle.
While carbon dioxide is essential for producing glucose, it does not contribute to the oxygen gas released during photosynthesis. This is evident from experiments using isotopic labeling, showing that oxygen atoms in the released oxygen gas stem from water molecules rather than from carbon dioxide.
The primary role of carbon dioxide is synthesizing glucose, highlighting the division of roles between carbon dioxide and water in photosynthesis.

• Provides carbon for glucose formation.
• Does not contribute to the oxygen evolution process.

Glucose Formation

Glucose formation occurs in the Calvin cycle, a series of light-independent reactions during photosynthesis. In this cycle, the carbon atoms from \( CO_2 \) are fixed into larger organic molecules, eventually forming glucose (\( C_6H_{12}O_6 \)).
The energy required for this process is derived from ATP and NADPH, generated during the light-dependent reactions. This makes glucose vital as an energy source and an essential building block for other organic compounds that form plant structures.
Glucose formation underscores the significance of carbon dioxide in photosynthesis and the broader biosphere, acting as a primary energy storage for plants and a key energy source for other living beings.

• Glucose is a fundamental energy storage molecule.
• Produced from carbon atoms in carbon dioxide.