Question

Which molecule does not form during glycolysis? a. NADH b. Pyruvate c. \(\mathrm{FADH}_{2}\) d. ATP

Short answer

FADH2 does not form during glycolysis.

Step-by-step solution

Understand Glycolysis

Glycolysis is a metabolic pathway that converts glucose into pyruvate. The process occurs in the cytoplasm of cells and is the first step in cellular respiration.

Identify Molecules Produced in Glycolysis

During glycolysis, certain molecules are produced as part of the process. These include ATP, NADH, and pyruvate, which are crucial for energy production and continuation to the next steps of cellular respiration.

Recognize Molecules Not Involved in Glycolysis

FADH2 is a molecule involved in cellular respiration but not produced during glycolysis. It is formed in the Krebs cycle, which takes place in the mitochondria after glycolysis.

Determine the Molecule That Does Not Form in Glycolysis

Given the options: a. NADH, b. Pyruvate, c. FADH2, and d. ATP, we identify that FADH2 is the molecule not formed during glycolysis.

Key concepts

NADH

NADH is a vital coenzyme in cellular metabolism. It acts as an electron carrier, playing a crucial role in the electron transport chain, which is a part of cellular respiration. During glycolysis, glucose is broken down into two molecules of pyruvate, and in this process, electrons are transferred to NAD+, reducing it to NADH.
This transformation helps to store energy that can be used later in the mitochondrial processes. Unlike FADH2, NADH is indeed produced during glycolysis. The significance of NADH lies in its ability to shuttle these high-energy electrons into the mitochondria, where they can be used to generate additional ATP.
NADH not only aids in energy production but also plays a role in preserving the balance of NAD+/NADH ratio in cells, which is critical for various biochemical processes.

• Produced through glycolysis.
• Essential for electron transport chain.
• Helps in generating energy (ATP).

Pyruvate

Pyruvate is the end product of glycolysis and serves as a key junction in cellular respiration. Each glucose molecule is split into two molecules of pyruvate through a series of enzymatic reactions. This step is crucial because it links glycolysis to the Krebs cycle.
Once formed, pyruvate can follow several pathways depending on the cell's needs, including:

• Entering the mitochondria for aerobic cellular respiration (when oxygen is present).
• Being converted to lactate during anaerobic conditions.

Pyruvate's versatility is essential for the cell, as it can adapt to different energy requirements and states of oxygen availability.

• Acts as a critical intermediary in metabolism.
• Bridge between glycolysis and the Krebs cycle.
• Versatile pathways depending on oxygen levels.

ATP

ATP, or adenosine triphosphate, is often termed the "energy currency" of the cell. They are primarily produced during glycolysis, where a small amount of ATP is generated directly. The overall breakdown of one glucose molecule during glycolysis results in a net gain of two ATP molecules.
This process is called substrate-level phosphorylation and is different from the more significant ATP production that occurs later through oxidative phosphorylation in the mitochondria.

• Direct energy source for many cellular processes.
• Produced in small quantities during glycolysis.
• Substrate-level phosphorylation unlike in mitochondria.

The role of ATP extends beyond energy supply; it also helps in the regulation of metabolic pathways as an allosteric regulator in various enzymes ensuring efficient energy use and production.

FADH2

FADH2 is similar to NADH in that it is also involved in carrying electrons. However, one key difference is that FADH2 is not produced during glycolysis but rather during the Krebs cycle, which follows glycolysis in the cellular respiration pathway.
In the Krebs cycle, FAD accepts electrons to form FADH2, which then transfers these electrons to the electron transport chain. Although not generated in glycolysis, FADH2 contributes significantly to the production of ATP during oxidative phosphorylation.

• Produced during the Krebs cycle, not glycolysis.
• Plays a role in the electron transport chain.
• Contributes to ATP production via oxidative phosphorylation.

The presence of FADH2 highlights the intricate coordination of metabolic pathways beyond glycolysis to ensure efficient energy production.

Cellular Respiration

Cellular respiration is a comprehensive process that breaks down glucose molecules to produce energy in the form of ATP. It involves multiple stages, starting with glycolysis, followed by the Krebs cycle, and finally the electron transport chain. Each stage takes place in different parts of the cell and contributes to overall energy production in unique ways.
Glycolysis is the first step and occurs in the cytoplasm, leading to the production of pyruvate. The Krebs cycle occurs in the mitochondria and is responsible for the production of NADH and FADH2. These molecules then carry electrons to the electron transport chain, which is the final stage of cellular respiration and occurs within the inner mitochondrial membrane.

• Involves glycolysis, the Krebs cycle, and the electron transport chain.
• Produces energy efficiently.
• Occurs across different cell compartments.

This multi-step process ensures that cells can meet their energy demands through the efficient use of biochemical pathways.