Problem 2 - Campbell Biology

Question

Which metabolic pathway is common to both fermentation and cellular respiration of a glucose molecule? (A) the citric acid cycle (B) the electron transport chain (C) glycolysis (D) reduction of pyruvate to lactate

Step-by-Step Solution

Answer

C) glycolysis

Step 1: Identify the Question

Determine which metabolic pathway is shared by both fermentation and cellular respiration when a glucose molecule is involved.

Step 2: Analyze Each Option

Evaluate each given option to find the common pathway.

Step 3: Option A - Citric Acid Cycle

The citric acid cycle occurs in cellular respiration but not in fermentation.

Step 4: Option B - Electron Transport Chain

The electron transport chain is part of cellular respiration's final stage but is not involved in fermentation.

Step 5: Option C - Glycolysis

Glycolysis is the first stage of glucose metabolism, occurring both in fermentation and cellular respiration.

Step 6: Option D - Reduction of Pyruvate to Lactate

The reduction of pyruvate to lactate occurs only in fermentation, not in cellular respiration.

Step 7: Select the Correct Option

Based on the analysis, determine that glycolysis is the common pathway for both fermentation and cellular respiration.

Key Concepts

glycolysis

Glycolysis is the first step in the metabolism of glucose. It occurs in the cytoplasm of the cell, and it does not require oxygen to proceed. During glycolysis, one glucose molecule is broken down into two molecules of pyruvate. This process generates a small net gain of ATP (adenosine triphosphate), which cells use for energy. The key stages in glycolysis are:

• Glucose Activation: Glucose is phosphorylated using two ATP molecules, transforming it into fructose-1,6-bisphosphate.
• Cleavage: Fructose-1,6-bisphosphate is split into two three-carbon molecules - glyceraldehyde-3-phosphate (G3P) and dihydroxyacetone phosphate (DHAP).
• Energy Harvesting: Both G3P molecules are converted into pyruvate, generating four ATP molecules and two NADH (nicotinamide adenine dinucleotide) in the process.

As glycolysis is an anaerobic process, it can be found in both fermentation and cellular respiration pathways. Both pathways rely on glycolysis to initially break down glucose and harvest energy.

fermentation

Fermentation is a metabolic pathway that allows cells to generate energy without the need for oxygen. This process happens after glycolysis if oxygen is not available. There are different types of fermentation, including lactic acid fermentation and alcoholic fermentation.

• Lactic Acid Fermentation: Pyruvate from glycolysis is reduced to lactate. This process is used by muscle cells during intense activity when oxygen is scarce and by certain bacteria.
• Alcoholic Fermentation: Pyruvate is converted into ethanol and carbon dioxide. This type of fermentation is carried out by yeast and some types of bacteria.

Both pathways regenerate NAD+ from NADH, ensuring glycolysis can continue to produce ATP. Although fermentation generates less energy compared to cellular respiration, it is essential for organisms in oxygen-poor environments.

cellular respiration

Cellular respiration is a highly efficient process used by cells to convert glucose into ATP, using oxygen. This pathway consists of three main stages:

• Glycolysis: As in fermentation, glucose is broken down into pyruvate, yielding a small amount of ATP and NADH.
• Citric Acid Cycle: Also known as the Krebs cycle, pyruvate is further broken down, generating more NADH, FADH2 (flavin adenine dinucleotide), and a small amount of ATP.
• Electron Transport Chain (ETC): NADH and FADH2 produced in the previous stages donate electrons to the ETC, located in the mitochondria's inner membrane. As electrons travel down the chain, they release energy used to pump protons across the membrane, creating a proton gradient. This gradient drives the production of a large amount of ATP through oxidative phosphorylation.

Oxygen is the final electron acceptor in the ETC, forming water as a byproduct. Cellular respiration produces significantly more ATP than fermentation, making it the preferred pathway for energy production in cells with access to oxygen.