Question

If photosynthesizing plants are grown in the presence of \(^{14} \mathrm{CO}_{2}\), is every carbon atom of the glucose that is produced labeled with the radioactive carbon? Why or why not?

Short answer

Yes, every carbon atom in the glucose produced is labeled with \(^{14} \text{C}\). This happens because all the carbon atoms in glucose originate from \(^{14} \text{CO}_2\).

Step-by-step solution

Understanding Photosynthesis

Photosynthesis is the process by which plants convert carbon dioxide and water into glucose and oxygen using sunlight. The general equation for photosynthesis is: \[\text{6} \text{CO}_{2} + \text{6} \text{H}_{2}\text{O} + \text{light energy} \rightarrow \text{C}_6\text{H}_{12}\text{O}_{6} + \text{6} \text{O}_{2}\]

Radioactive Carbon in Photosynthesis

When plants are provided with \(^{14} \text{CO}_2\), the radioactive carbon isotope \(^{14} \text{C}\) is used instead of the regular \(^{12} \text{C}\) during the process of photosynthesis. This radioactive carbon gets incorporated into the glucose molecule being synthesized.

Formation of Glucose Molecule

The glucose molecule \( \text{C}_6\text{H}_{12}\text{O}_6 \) formed during photosynthesis contains six carbon atoms. The carbon atoms in the glucose molecule come from the carbon dioxide used in the reaction.

Distribution of Radioactive Carbon

Every carbon atom in glucose comes directly from carbon dioxide molecules fixed during the Calvin cycle. Since \(^{14} \text{CO}_2\) is the only carbon source, each carbon in the resulting glucose will be labeled with radioactive \(^{14} \text{C}\).

Key concepts

Photosynthesis Process

Photosynthesis is an essential process in which plants convert light energy into chemical energy, producing glucose and oxygen. This takes place primarily in the chloroplasts of plant cells and involves two main stages: the light-dependent reactions and the Calvin cycle.

In the light-dependent reactions, sunlight is absorbed by chlorophyll, and its energy is used to split water molecules, releasing oxygen and transferring energy to produce ATP and NADPH. The overall equation for photosynthesis is:

\[\text{6} \text{CO}_2 + \text{6} \text{H}_2\text{O} + \text{light energy} \rightarrow \text{C}_6\text{H}_{12}\text{O}_{6} + \text{6} \text{O}_2\]

The second stage, known as the Calvin cycle, utilizes the ATP and NADPH from the light-dependent reactions to convert carbon dioxide into glucose, a process that will be further explained in detail below.

Radioactive Carbon

Radioactive carbon, often symbolized as \(^{14}\text{C}\), is an isotope of carbon with two extra neutrons compared to the regular \(^{12}\text{C}\) isotope. This makes it unstable and radioactive. \(^{14}\text{C}\) is used in many scientific studies to track carbon pathways because it behaves similarly to \(^{12}\text{C}\) in chemical processes but is detectable due to its radioactive properties.

In experiments involving photosynthesis, plants exposed to \(^{14} \text{CO}_2\) will incorporate this radioactive carbon into their metabolic processes, particularly photosynthesis, where it will be used to synthesize glucose. By using \(^{14} \text{CO}_2\), scientists can trace the incorporation of carbon atoms into various organic molecules, such as glucose.

Calvin Cycle

The Calvin cycle, also known as the dark reactions or light-independent reactions, is a series of biochemical reactions that occur in the stroma of chloroplasts in photosynthetic organisms. This cycle does not require light and is responsible for fixing carbon dioxide into organic molecules.

The Calvin cycle is divided into three main phases:

• Carbon fixation: The enzyme RuBisCO catalyzes the attachment of carbon dioxide to ribulose-1,5-bisphosphate (\text{RuBP}, a five-carbon compound), forming a six-carbon intermediate that immediately splits into two molecules of 3-phosphoglycerate (\text{PGA}).
• Reduction phase: ATP and NADPH produced in the light-dependent reactions are used to convert \text{PGA} into glyceraldehyde-3-phosphate (\text{G3P}), a three-carbon sugar.
• Regeneration of \text{RuBP}: Some of the \text{G3P} molecules are used to regenerate \text{RuBP}, allowing the cycle to continue, while others are used to synthesize glucose and other carbohydrates.

Given that the carbon atoms involved in the Calvin cycle come from carbon dioxide (in this case, \(^{14} \text{CO}_2\)), each carbon in the glucose produced during photosynthesis would indeed be labeled with radioactive \(^{14}\text{C}\).

Glucose Synthesis

Glucose synthesis is a crucial outcome of the photosynthesis process, particularly during the Calvin cycle. The produced \(\text{G3P}\) molecules are eventually converted into glucose through a series of chemical pathways. Every glucose molecule (\(\text{C}_6\text{H}_{12}\text{O}_6\)) consists of six carbon atoms, twelve hydrogen atoms, and six oxygen atoms.

When plants photosynthesize in the presence of \(^{14}\text{CO}_2\), the carbon atoms in the resulting glucose molecule come from carbon dioxide molecules fixed during the Calvin cycle. Therefore, if the only carbon source is \(^{14} \text{CO}_2\), all six carbon atoms in the glucose molecule produced will be labeled with radioactive \(^{14} \text{C}\).

This means that by tracking the incorporation of \(^{14} \text{C}\) into glucose, scientists can study and understand carbon fixation and the synthesis of organic compounds during photosynthesis. It offers valuable insights into the metabolic processes of plants and the flow of energy in ecosystems.