Cellular respiration is a multi-step process that converts glucose into usable energy (ATP). This process involves several pathways distributed in different parts of the cell. Understanding which steps occur exclusively inside mitochondria versus those in the cytoplasm is essential for grasping cellular energy metabolism.

This article clarifies which metabolic processes occur only inside mitochondria and which take place in the cytoplasm.

Location of Key Metabolic Processes in Respiration

Glycolysis

• Location: Cytoplasm

• Function: Breaks down glucose into two molecules of pyruvate, producing a small amount of ATP and NADH.

• Significance: Glycolysis is the first step in respiration and occurs in the cytosol of all cells, including those with mitochondria.

Citric Acid Cycle (Krebs Cycle)

• Location: Mitochondrial matrix

• Function: Oxidizes acetyl-CoA derived from pyruvate to CO₂, producing NADH, FADH₂, and GTP (or ATP).

• Significance: This cycle is central to aerobic respiration and occurs only inside mitochondria.

Oxidative Phosphorylation

• Location: Inner mitochondrial membrane (cristae)

• Function: Uses electrons from NADH and FADH₂ to create a proton gradient that drives ATP synthesis via ATP synthase.

• Significance: This is the major ATP-generating step of aerobic respiration and is confined to mitochondria.

Pentose Phosphate Pathway

• Location: Cytoplasm

• Function: Produces NADPH and ribose-5-phosphate for biosynthesis; not directly involved in ATP production.

• Significance: Occurs in the cytosol, separate from mitochondrial respiration.

Which Processes Occur Only Inside Mitochondria?

• The citric acid cycle and oxidative phosphorylation are strictly mitochondrial processes.

• Glycolysis and the pentose phosphate pathway occur in the cytoplasm.

Answer to the Query

Among the options provided:

1. Glycolysis and the pentose phosphate pathway — both cytoplasmic processes.

2. Glycolysis and the citric acid cycle — glycolysis is cytoplasmic; citric acid cycle is mitochondrial.

3. The citric acid cycle and oxidative phosphorylation — both occur exclusively inside mitochondria.

4. Glycolysis and oxidative phosphorylation — glycolysis is cytoplasmic; oxidative phosphorylation is mitochondrial.

Correct answer: (3) The citric acid cycle and oxidative phosphorylation.

Summary Table

Conclusion

The citric acid cycle and oxidative phosphorylation are the two key processes of cellular respiration that occur exclusively inside mitochondria. Glycolysis and the pentose phosphate pathway take place in the cytoplasm. Recognizing these locations is fundamental to understanding cellular energy metabolism.