In cellular respiration, which pathway is common to both aerobic respiration and anaerobic respiration in living cells?

Introduction / Context:
Cells obtain energy by breaking down glucose and other molecules through respiration. There are two main types of respiration: aerobic, which uses oxygen, and anaerobic, which does not. This question asks which part of the respiration process is shared by both types.

Given Data / Assumptions:

• We are comparing aerobic and anaerobic respiration.
• Options include Krebs cycle, glycolysis, electron transport chain and related terms.
• We assume standard textbook descriptions of these pathways.

Concept / Approach:
Glycolysis is the first step in the breakdown of glucose and occurs in the cytoplasm of cells. It does not require oxygen and is therefore common to both aerobic and anaerobic respiration. In aerobic respiration, pyruvate from glycolysis enters the mitochondrion for the Krebs cycle and electron transport chain. In anaerobic respiration or fermentation, pyruvate is converted to lactate or ethanol and carbon dioxide in the cytoplasm without using the Krebs cycle or electron transport chain. Thus, glycolysis is the pathway shared by both modes.

Step-by-Step Solution:
Step 1: Recall that glycolysis splits one glucose molecule into two molecules of pyruvate, producing a small amount of ATP. Step 2: Note that glycolysis takes place in the cytoplasm and does not require oxygen. Step 3: Understand that in aerobic conditions, pyruvate enters the mitochondrion for further oxidation in the Krebs cycle and electron transport chain. Step 4: Recognise that in anaerobic conditions, pyruvate is converted into lactate or ethanol without using Krebs cycle or electron transport chain. Step 5: Conclude that glycolysis is the only pathway listed that occurs in both aerobic and anaerobic respiration.

Verification / Alternative check:
Check standard diagrams of respiration. All of them begin with glycolysis in the cytoplasm. Pathways then branch depending on whether oxygen is present. Aerobic respiration shows additional steps inside mitochondria, while anaerobic routes remain in the cytoplasm. None of the later steps are common to both. This confirms glycolysis as the shared pathway.

Why Other Options Are Wrong:
Option A, Krebs cycle, occurs only in aerobic respiration inside mitochondria and stops in the absence of oxygen. Option C, electron transport chain, also operates only in aerobic respiration to produce large amounts of ATP. Option D, all of the above, is wrong because Krebs cycle and electron transport chain are not present in anaerobic respiration. Option E, oxidative phosphorylation, refers to ATP production using the electron transport chain and oxygen, so it is also limited to aerobic respiration.

Common Pitfalls:
Students sometimes think that all parts of respiration simply slow down in anaerobic conditions but still occur. In reality, only glycolysis continues, and it links to fermentation pathways instead of the mitochondrial steps. Keeping a clear picture of where each pathway occurs and whether it needs oxygen helps you avoid mixing them up.

Final Answer:
The pathway common to both aerobic and anaerobic respiration is Glycolysis.