Question

Glucose is labeled with \(14 \mathrm{C}\) and followed as it is broken down to produce \(\mathrm{CO}_{2}, \mathrm{H}_{2} \mathrm{O},\) and \(\mathrm{ATP}\) in a human muscle cell. This label will be detectable in which of the following sites? (A) In the cytoplasm only (B) In the mitochondria only (C) In the cytoplasm and the mitochondria (D) In the nucleus and the mitochondria (E) In the nucleus, cytoplasm, and the mitochondria

Short answer

(C) In the cytoplasm and the mitochondria

Step-by-step solution

Identify the initial process of glucose metabolism

Glucose initially undergoes a process called glycolysis, which happens in the cytoplasm of the cell. During this process, glucose is split into two molecules of pyruvate, generating a small amount of ATP and reducing equivalents. Hence, parts of the \(14\mathrm{C}\) labeled glucose will be detectable here.

Recognize the subsequent processes of glucose metabolism

The pyruvate molecules produced in glycolysis are then transported into the mitochondria, where they undergo the Krebs cycle (also known as the citric acid cycle). During this process, more ATP, CO2, and reducing equivalences are produced from the pyruvate. Hence, the \(14\mathrm{C}\) label in glucose will also be detectable in the mitochondria.

Draw the conclusion

From Steps 1 and 2, we can see that the \(14C\) labeled glucose traverses both the cytoplasm (during glycolysis) and the mitochondria (during the Krebs cycle). Therefore, the \(14C\) label should be detectable in both

Key concepts

Glycolysis

Glycolysis is the first step in cellular respiration, which takes place in the cytoplasm of the cell. This process does not require oxygen, so it is considered anaerobic. Glycolysis involves breaking down one molecule of glucose, a six-carbon sugar, into two three-carbon molecules called pyruvate. During this transformation:

• Two molecules of ATP are used to start the process, but four molecules of ATP are produced, resulting in a net gain of two ATP molecules.
• Nicotinamide adenine dinucleotide (NAD+) is reduced to NADH, capturing energy to be used later in cellular respiration.

Glycolysis is crucial because it initiates the breakdown of glucose into energy that cells can use. This process is highly regulated to ensure that energy production meets the cell's needs. The products of glycolysis, pyruvate, and NADH, proceed to further metabolic pathways when oxygen is available.

Krebs Cycle

The Krebs Cycle, also known as the Citric Acid Cycle, is a series of chemical reactions used by all aerobic organisms to release stored energy. This process occurs in the mitochondria, following glycolysis in cellular respiration. Here's how it works:

• Pyruvate, the end product of glycolysis, undergoes a conversion to acetyl-CoA before entering the Krebs Cycle.
• During these cyclic reactions, each turn of the cycle processes an acetyl-CoA molecule, generating high-energy molecules such as ATP, NADH, and FADH2.
• Carbon dioxide (CO₂) is released as a byproduct of this cycle.

The Krebs Cycle is central to energy production, as it provides necessary electron carriers that drive the electron transport chain. Through these reactions, the cell efficiently captures energy by transferring electrons between molecules.

Mitochondria

Mitochondria are known as the powerhouse of the cell because they generate most of the cell's supply of adenosine triphosphate (ATP), used as a source of chemical energy. These organelles are essential for cellular respiration, including the Krebs Cycle and oxidative phosphorylation. Key features of mitochondria include:

• They have a double membrane; the inner membrane folds over many times and creates layered structures called cristae.
• The space between the membranes is known as the intermembrane space, while the interior is called the mitochondrial matrix.
• Their own DNA, distinct from the cell's nuclear DNA, suggesting they originated as independent prokaryotic organisms.

The mitochondria's structure is optimized to produce energy, supporting various vital functions and maintaining the health of the cell. The products of earlier pathways like glycolysis and the Krebs Cycle serve as inputs for the electron transport chain, located in the inner mitochondrial membrane, ultimately leading to ATP production.